Found 44 papers in cond-mat

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A space-time gauge theory for modeling ductile damage and its NOSB peridynamic implementation
Sanjeev Kumar
arXiv:2403.09685v1 Announce Type: new Abstract: Local translational and scaling symmetries in space-time is exploited for modelling ductile damage in metals and alloys over wide ranges of strain rate and temperature. The invariant energy density corresponding to the ductile deformation is constructed through the gauge invariant curvature tensor by imposing the Weyl like condition. The energetics of the plastic deformation is brought in through the gauge compensating field emerged due to local translation. Invariance of the energy density under the local action of translation and scaling is preserved through minimally replaced space-time gauge covariant operators. Minimal replacement introduces two non-trivial gauge compensating fields pertaining to local translation and scaling. These are used to describe ductile damage, including plastic flow and micro-crack evolution in the material. A space-time pseudo-Riemannian metric is used to lay out the kinematics in a finite-deformation setting. Recognizing the available insights in classical theories of viscoplasticity, we also establish a correspondence of the gauge compensating field due to spatial translation with Kr\"{o}ner's multiplicative decomposition of the deformation gradient. Thermodynamically consistent coupling between viscoplasticity and ductile damage is ensured through an appropriate degradation function. Non-ordinary state-based (NOSB) peridynamics (PD) discretization of the model is used for numerical implementation. The model's viability is tested in reproducing a few experimentally known facts, viz., strain rate locking in the stress-strain response, whose origin is traced to a nonlinear microscopic inertia term arising out of the space-time translation symmetry. Finally, we solved 2D and axisymmetric deformation problems for qualitatively validating the model's viability. NOSB peridynamics axisymmetric formulation in finite deformation setup is also presented.

Phonon Linewidths in Twisted Bilayer Graphene near Magic Angle
Shinjan Mandal, Indrajit Maity, H. R. Krishnamurthy, Manish Jain
arXiv:2403.09692v1 Announce Type: new Abstract: We present a computational study of the phonon linewidths in twisted bilayer graphene arising from electron-phonon interactions and anharmonic effects. The electronic structure is calculated using distance-dependent transfer integrals based on the atomistic Slater-Koster tight-binding formalism. Furthermore, electron-electron interactions are treated at the Hartree level. The force constants for the phonon calculations are generated from classical force fields. These ingredients are used to calculate the phonon linewidths arising from electron-phonon interactions. Additionally, we account for the effects of phonon-phonon interactions on the linewidths by computing the mode-projected velocity autocorrelation function from classical molecular dynamics. Our findings show that both electron-phonon and anharmonic effects have a significant impact on the linewidth of the Raman active G mode near the magic angle. We predict a moir\'e potential induced splitting of this mode, which arises due to contributions from high symmetry stacking regions.

Kitaev physics in the two-dimensional magnet NiPSe$_3$
Cheng Peng, Sougata Mardanya, Alexander N. Petsch, Vineet Kumar Sharma, Shuyi Li, Chunjing Jia, Arun Bansil, Sugata Chowdhury, Joshua J. Turner
arXiv:2403.09831v1 Announce Type: new Abstract: The Kitaev interaction, found in candidate materials such as $\alpha$-RuCl$_3$, occurs through the metal ($M$)-ligand ($X$)-metal ($M$) paths of the edge-sharing octahedra because the large spin-orbit coupling (SOC) on the metal atoms activates directional spin interactions. Here, we show that even in $3d$ transition-metal compounds, where the SOC of the metal atom is negligible, heavy ligands can induce bond-dependent Kitaev interactions. In this work, we take as an example the $3d$ transition-metal chalcogenophosphate NiPSe$_3$ and show that the key is found in the presence of a sizable SOC on the Se $p$ orbital, one which mediates the super-exchange between the nearest-neighbor Ni sites. Our study provides a pathway for engineering enhanced Kitaev interactions through the interplay of SOC strength, lattice distortions, and chemical substitutions.

Fermi Liquid near a q=0 Charge Quantum Critical Point
R. David Mayrhofer, Peter W\"olfle, Andrey V. Chubukov
arXiv:2403.09835v1 Announce Type: new Abstract: We analyze the quasiparticle interaction function (the fully dressed and antisymmetrized interaction between fermions) for a two-dimensional Fermi liquid at zero temperature close to a q=0 charge quantum critical point (QCP) in the $s-$wave channel. By the Ward identities, this vertex function must be related to quasiparticle residue $Z$, which can be obtained independently from the fermionic self-energy. We show that to satisfy these Ward identities, one needs to go beyond the standard diagrammatic formulation of Fermi liquid theory and include series of additional contributions to the vertex function. These contributions are forbidden in a conventional Fermi liquid, but do emerge near a QCP, where the effective 4-fermion interaction is mediated by a soft dynamical boson. We demonstrate explicitly that including these terms restores the Ward identity. We also discuss the role of Aslamazov-Larkin-type diagrams for the vertex function. Our analysis is built on works on the vertex function near an antiferromagnetic QCP [Phys. Rev. B 89, 045108 (2014)] and a d-wave charge-nematic QCP [Phys. Rev. B 81, 045110 (2010)]. We show that for $s-$wave charge QCP (the one leading to phase separation), the analysis is more straightforward. We also discuss the structure of the Landau function in a critical Fermi liquid near a QCP.

Spiral to stripe transition in the two-dimensional Hubbard model
Robin Scholle, Walter Metzner, Demetrio Vilardi, Pietro M. Bonetti
arXiv:2403.09862v1 Announce Type: new Abstract: We obtain an almost complete understanding of the mean-field phase diagram of the two-dimensional Hubbard model on a square lattice with a sizable next-nearest neighbor hopping and a moderate interaction strength. In particular, we clarify the nature of the transition region between the spiral and the stripe phase. Complementing previous [Phys. Rev. B 108, 035139 (2023)] real-space Hartree-Fock calculations on large finite lattices, we solve the mean-field equations for coplanar unidirectional magnetic order directly in the thermodynamic limit, and we determine the nature of the magnetic states right below the mean-field critical temperature $T^*$ by a Landau free energy analysis. While the magnetic order for filling factors $n \geq 1$ is always of N\'eel type, for $n \leq 1$ the following sequence of magnetic states is found as a function of increasing hole-doping: N\'eel, planar circular spiral, multi-spiral, and collinear spin-charge stripe states. Multi-spiral states are superpositions of several spirals with distinct wave vectors, and lead to concomitant charge order. We finally point out that nematic and charge orders inherited from the magnetic order can survive even in the presence of fluctuations, and we present a corresponding qualitative phase diagram.

Berry curvature derived negative magnetoconductivity observed in type-II magnetic Weyl semimetal films
Ayano Nakamura, Shinichi Nishihaya, Hiroaki Ishizuka, Markus Kriener, Mizuki Ohno, Yuto Watanabe, Masashi Kawasaki, Masaki Uchida
arXiv:2403.09924v1 Announce Type: new Abstract: Here we study nonmonotonic features which appear both in magnetoresistivity and anomalous Hall resistivity during the simple magnetization process, by systematically measuring type-II magnetic Weyl semimetal EuCd$_2$Sb$_2$ films over a wide carrier density range. We find that a positive magnetoresistivity hump can be explained as manifestation of a field-linear term in the generalized magnetoconductivity formula including the Berry curvature. As also confirmed by model calculation, the term can be negative and pronounced near the Weyl point energy in the case that the Weyl cones are heavily tilted. Our findings demonstrate extensive effects of the Berry curvature on various magnetotransport in magnetic Weyl semimetals beyond the anomalous Hall effect.

Topological frequency conversion in rhombohedral multilayer graphene
\'Etienne Lantagne-Hurtubise, Iliya Esin, Gil Refael, Frederik Nathan
arXiv:2403.09935v1 Announce Type: new Abstract: We show that rhombohedral multilayer graphene supports topological frequency conversion, whereby a fraction of electrons transfer energy between two monochromatic light sources at a quantized rate. The pristine nature and gate tunability of these materials, along with a Berry curvature that directly couples to electric fields, make them ideal platforms for the experimental realization of topological frequency conversion. Among the rhombohedral family, we find that Bernal bilayer graphene appears most promising for THz-scale applications due to lower dissipation. We discuss strategies to circumvent cancellations between the two valleys of graphene and to minimize dissipative losses using commensurate frequencies, thus opening a potential pathway for net amplification.

Anomalous Raman Response in 2D Magnetic FeTe under Uniaxial Strain: Tetragonal and Hexagonal Polymorphs
Wuxiao Han, Tiansong Zhang, Pengcheng Zhao, Longfei Yang, Mo Cheng, Jianping Shi, Yabin Chen
arXiv:2403.09936v1 Announce Type: new Abstract: Two-dimensional (2D) Fe-chalcogenides have emerged with rich structures, magnetisms and superconductivities, which sparked the growing research interests in the torturous transition mechanism and tunable properties for their potential applications in nanoelectronics. Uniaxial strain can produce a lattice distortion to study symmetry breaking induced exotic properties in 2D magnets. Herein, the anomalous Raman spectrum of 2D tetragonal (t-) and hexagonal (h-) FeTe were systematically investigated via uniaxial strain engineering strategy. We found that both t- and h-FeTe keep the structural stability under different uniaxial tensile or compressive strain up to +/- 0.4%. Intriguingly, the lattice vibrations along both in-plane and out-of-plane directions exceptionally hardened (softened) under tensile (compressive) strain, distinguished from the behaviors of many conventional 2D systems. Furthermore, the difference in thickness-dependent strain effect can be well explained by their structural discrepancy between two polymorphs of FeTe. Our results could provide a unique platform to elaborate the vibrational properties of many novel 2D materials.

Polarized Charge Dynamics of a Novel Charge Density Wave in Kagome FeGe
Shaohui Yi, Zhiyu Liao, Qi Wang, Haiyang Ma, Jianpeng Liu, Xiaokun Teng, Pengcheng Dai, Yaomin Dai, Jianzhou Zhao, Yanpeng Qi, Bing Xu, Xianggang Qiu
arXiv:2403.09950v1 Announce Type: new Abstract: We report on the charge dynamics of kagome FeGe, an antiferromagnet with a charge density wave (CDW) transition at $T_{\mathrm{CDW}} \simeq 105$ K, using polarized infrared spectroscopy and band structure calculations. We reveal a pronounced optical anisotropy, various excitations associated with flat bands and van Hove singularities (VHSs), and a moderate level of electronic correlations. Notably, there are two types of remarkable spectral weight (SW) redistributions for above and below $T_{\mathrm{CDW}}$. The former involves a transfer between incoherent and coherent excitations driven by the magnetic splitting-induced elevation of flat bands. The latter manifests itself as a sudden change of SW from low to high energies for both $a$ and $c$ directions, suggesting a first-order transition and the three-dimensional nature of CDW. These anomalies in SW significantly differ from those observed in other kagome metals like CsV$_3$Sb$_5$, where the nesting of VHSs results in a pronounced CDW gap feature. Instead, our findings can be accounted for by the jump of VHSs relative to the Fermi energy via a first-order structural transition involving large partial Ge1-dimerization. Our study thus unveils a complex interplay among structure, magnetism, electronic correlations, and charge order in FeGe, offering valuable insights for a comprehensive understanding of CDW order in kagome systems.

Suppression of shear ionic motions in bismuth by coupling with large-amplitude internal displacement
Kunie Ishioka, Oleg V. Misochko
arXiv:2403.10046v1 Announce Type: new Abstract: Bismuth, with its rhombohedral crystalline structure and two Raman active phonon modes corresponding to the internal displacement ($A_{1g}$) and shear ($E_{g}$) ionic motions, offers an ideal target for the investigation of the phonon-phonon and electron-phonon couplings under photoexcitation. We perform transient reflectivity measurements of bismuth single crystal at 11 K over wide range of absorbed laser fluence up to $F_\text{abs}=9$ mJ/cm$^2$, at which a sign of an irreversible surface damage is observed. At the minimum fluence examined (0.1 mJ/cm$^2$) the coherent $A_{1g}$ and $E_g$ oscillations are a cosine and a sine functions of time, as are consistent with their generations in the displacive and impulsive limits, respectively. With increasing fluence the initial phases of the both modes deviate from their low-fluence values, indicating a finite time required for the transition from the ground-state potential energy surface (PES) to the excited-state one. Surprisingly, the $E_g$ amplitude increases with increasing fluence up to 3 mJ/cm$^2$ and then turns to an apparent decrease, in contrast to the monotonic increase of the $A_{1g}$ amplitude up to 6 mJ/cm$^2$. The contrasted behaviors can be understood by considering a two-dimensional PES, where the strongly driven $A_{1g}$ oscillation leads to a temporal fluctuation of the PES along the $E_g$ coordinate and thereby to a loss in the $E_g$ oscillation coherence at high fluences.

Anisotropic magneto-photothermal voltage in Sb2Te3 topological insulator thin films
Subhadip Manna, Sambhu G Nath, Samrat Roy, Soumik Aon, Sayani Pal, Kanav Sharma, Dhananjaya Mahapatra, Partha Mitra, Sourin Das, Bipul Pal, Chiranjib Mitra
arXiv:2403.10141v1 Announce Type: new Abstract: We studied longitudinal and Hall photothermal voltages under a planar magnetic field scan in epitaxial thin films of the Topological Insulator (TI) Sb2Te3, grown using pulsed laser deposition (PLD). Unlike prior research that utilised polarised light-induced photocurrent to investigate the TI, our study introduces advancements based on unpolarized light-induced local heating. This method yields a thermoelectric response exhibiting a direct signature of strong spin-orbit coupling. Our analysis reveals three distinct contributions when fitting the photothermal voltage data to the angular dependence of the planar magnetic field. The interaction between the applied magnetic field and the thermal gradient on the bulk band orbitals enables the differentiation between the ordinary Nernst effect from the out-of-plane thermal gradient and an extraordinary magneto-thermal contribution from the planar thermal gradient. The fitting of our data to theoretical models indicates that these effects primarily arise from the bulk states of the TI rather than the surface states. These findings highlight PLD-grown epitaxial topological insulator thin films as promising candidates for optoelectronic devices, including sensors and actuators. Such devices offer controllable responses through position-dependent, non-invasive local heating via focused incident light and variations in the applied magnetic field direction.

Spontaneous spin chirality reversal and competing phases in the topological magnet EuAl$_4$
A. M. Vibhakar, D. D. Khalyavin, F. Orlandi, J. M. Moya, S. Lei, E. Morosan, A. Bombardi
arXiv:2403.10159v1 Announce Type: new Abstract: We demonstrate the spontaneous reversal of spin chirality in a single crystal sample of the intermetallic magnet EuAl$_4$. We solve the nanoscopic nature of each of the four magnetically phases of EuAl$_4$ using resonant magnetic x-ray scattering, and demonstrate all four phases order with single-k incommensurate magnetic modulation vectors. Below 15.4 K the system forms a spin density modulated spin structure where the spins are orientated in the ab plane perpendicular to the orientation of the magnetic propagation vector. Below 13.2 K a second spin density wave orders with moments aligned parallel to the c-axis, such that the two spin density wave orders coexist. Below 12.2 K a magnetic helix of a single chirality is stabilised across the entire sample. Below 10 K the chirality of the magnetic helix reverses, and the sample remains a single chiral domain. Concomitant with the establishment of the helical magnetic ordering is the lowering of the crystal symmetry to monoclinic, as evidenced the formation of uniaxial charge and spin strip domains. A group theoretical analysis demonstrates that below 12.2 K the symmetry lowers to polar monoclinic, which is necessary to explain the observed asymmetry in the chiral states of the magnetic helix and the spin chiral reversal. We find that in every magnetically ordered phase of EuAl4 the in-plane moment is perpendicular to the orientation of the magnetic propagation vector, which we demonstrate is favoured by magnetic dipolar interactions.

Effects of spin-orbit coupling in a valley chiral kagom\'e network
P. Wittig, F. Dominguez, P. Recher
arXiv:2403.10181v1 Announce Type: new Abstract: Valley chiral kagom\'e networks can arise in various situations, like for example, in double-aligned graphene-hexagonal boron nitride and periodically strained graphene. Here, we construct a phenomenological scattering model based on the symmetries of the network to investigate the energy spectrum and magnetotransport in this system. Additionally, we consider the effects of a finite Rashba spin-orbit coupling on the transport properties of the kagom\'e network. We identify conditions where the interplay of the Rashba spin-orbit coupling and the geometry of the lattice results in a reduction of the periodicity of the magnetoconductance and characteristic sharp resonances. Moreover, we find a finite spin-polarization of the conductance, which could be exploited in spintronic devices.

Half-metallic transport and spin-polarized tunneling through the van der Waals ferromagnet Fe${_4}$GeTe$_{2}$
Anita Halder, Declan Nell, Antik Sihi, Akash Bajaj, Stefano Sanvito, Andrea Droghetti
arXiv:2403.10195v1 Announce Type: new Abstract: The recent emergence of van der Waals (vdW) ferromagnets has opened new opportunities for designing spintronic devices. We theoretically investigate the coherent spin-dependent transport properties of the vdW ferromagnet Fe$_4$GeTe$_2$, by using density functional theory combined with the non-equilibrium Green's functions method. We find that the conductance in the direction perpendicular to the layers is half-metallic, namely it is entirely spin-polarized, as a result of the material's electronic structure. This characteristic persists from bulk to single layer, even under significant bias voltages, and it is little affected by spin-orbit coupling and electron correlation. Motivated by this observation, we then investigate the tunnel magnetoresistance (TMR) effect in an magnetic tunnel junction, which comprises two Fe$_4$GeTe$_2$ layers separated by the vdW gap acting as insulating barrier. We predict a TMR ratio of almost 500\%, which can be further boosted by increasing the number of Fe$_4$GeTe$_2$ layers in the junction.

Steering internal and outgoing electron dynamics in bilayer graphene cavities by cavity design
Lukas Seemann, Angelika Knothe, Martina Hentschel
arXiv:2403.10201v1 Announce Type: new Abstract: Ballistic, gate-defined devices in two-dimensional materials offer a platform for electron optics phenomena influenced by the material's properties and gate control. We study the ray trajectory dynamics of all-electronic, gate-defined cavities in bilayer graphene to establish how distinct regimes of the internal and outgoing charge carrier dynamics can be tuned and optimized by the cavity shape, symmetry, and parameter choice, e.g., the band gap and the cavity orientation. In particular, we compare the dynamics of two cavity shapes, o'nigiri, and Lima\c{c}on cavities, which fall into different symmetry classes. We demonstrate that for stabilising regular, internal cavity modes, such as periodic and whispering gallery orbits, it is beneficial to match the cavity shape to the bilayer graphene Fermi line contour. Conversely, a cavity of a different symmetry than the material dispersion allows one to determine preferred emission directionalities in the emitted far-field.

Structure, control, and dynamics of altermagnetic textures
O. Gomonay, V. P. Kravchuk, R. Jaeschke-Ubiergo, K. V. Yershov, T. Jungwirth, L. \v{S}mejkal, J. van den Brink, J. Sinova
arXiv:2403.10218v1 Announce Type: new Abstract: We present a phenomenological theory of altermagnets, that captures their unique magnetization dynamics and allows modelling magnetic textures in this new magnetic phase. Focusing on the prototypical d-wave altermagnets, e.g. RuO$_2$, we can explain intuitively the characteristic lifted degeneracy of their magnon spectra, by the emergence of an effective sublattice-dependent anisotropic spin stiffness arising naturally from the phenomenological theory. We show that as a consequence the altermagnetic domain walls, in contrast to antiferromagnets, have a finite gradient of the magnetization, with its strength and gradient direction connected to the altermagnetic anisotropy, even for 180$^\circ$ domain walls. This gradient generates a ponderomotive force in the domain wall in the presence of a strongly inhomogeneous external magnetic field, which may be achieved through magnetic force microscopy techniques. The motion of these altermagentic domain walls is also characterized by an anisotropic Walker breakdown, with much higher speed limits of propagation than ferromagnets but lower than antiferromagnets.

A universal crack tip correction algorithm discovered by physical deep symbolic regression
David Melching, Florian Paysan, Tobias Strohmann, Eric Breitbarth
arXiv:2403.10320v1 Announce Type: new Abstract: Digital image correlation is a widely used technique in the field of experimental mechanics. In fracture mechanics, determining the precise location of the crack tip is crucial. In this paper, we introduce a universal crack tip detection algorithm based on displacement and strain fields obtained by digital image correlation. Iterative crack tip correction formulas are discovered by applying deep symbolic regression guided by physical unit constraints to a dataset of simulated cracks under mode I, II and mixed-mode conditions with variable T-stress. For the training dataset, we fit the Williams series expansion with super-singular terms to the simulated displacement fields at randomly chosen origins around the actual crack tip. We analyse the discovered formulas and apply the most promising one to digital image correlation data obtained from uniaxial and biaxial fatigue crack growth experiments of AA2024-T3 sheet material. Throughout the experiments, the crack tip positions are reliably detected leading to improved stability of the crack propagation curves.

One-dimensional Lieb superlattices: from the discrete to the continuum limit
Dylan Jones, Marcin Mucha-Kruczynski, Adelina Ilie, Lucian Covaci
arXiv:2403.10382v1 Announce Type: new Abstract: The Lieb lattice is one of the simplest lattices that exhibits both linear Dirac-like and flat topological electronic bands. We propose to further tailor its electronic properties through periodic 1D electrostatic superlattices (SLs), which, in the long wavelength limit, were predicted to give rise to novel transport signatures, such as the omnidirectional super-Klein tunnelling (SKT). By numerically modelling the electronic structure at tight-binding level, we uncover the evolution of the Lieb SL band structure from the discrete all the way to the continuum regime and build a comprehensive picture of the Lieb lattice under 1D potentials. This approach allows us to also take into consideration the discrete lattice symmetry-breaking that occurs at the well/barrier interfaces created by the 1D SL, whose consequences cannot be explored using the previous low energy and long wavelength approaches. We find novel features in the band structure, among which are intersections of quadratic and flat bands, tilted Dirac cones, or series of additional anisotropic Dirac cones at energies where the SKT is predicted. Such features are relevant to experimental realizations of electronic transport in Lieb 1D SL realized in artificial lattices or in real material systems like 2D covalent organic/metal-organic frameworks and inorganic 2D solids.

Gating single-molecule fluorescence with electrons
Katharina Kaiser, Michelangelo Romeo, Fabrice Scheurer, Guillaume Schull, Anna Ros{\l}awska
arXiv:2403.10410v1 Announce Type: new Abstract: Tip-enhanced photoluminescence (TEPL) measurements are performed with sub-nanometer spatial resolution on individual molecules decoupled from a metallic substrate by a thin NaCl layer. TEPL spectra reveal progressive fluorescence quenching with decreasing tip-molecule distance when electrons tunneling from the tip of a scanning tunneling microscope are injected at resonance with the molecular states. Rate equations based on a many-body model reveal that the luminescence quenching is due to a progressive population inversion between the ground neutral (S$_0$) and the ground charge ($D_0^-$) states of the molecule occurring when the current is raised. We demonstrate that both the bias voltage and the atomic-scale lateral position of the tip can be used to gate the molecular emission. Our approach can in principle be applied to any molecular system, providing unprecedented control over the fluorescence of a single molecule.

Ferroelectric phases and phase transitions in CsGeBr$_3$ induced by mechanical load
Joshua Townsend, Ravi Kashikar, S. Lisenkov, I. Ponomareva
arXiv:2403.10421v1 Announce Type: new Abstract: First-principles-based atomistic simulations are used to reveal ferroelectric phases and phase transitions induced in a semiconductor ferroelectric, CsGeBr$_3$, by external loads: hydrostatic pressure, uniaxial and biaxial stresses, and misfit strain. Hydrostatic pressure was found to suppress the Curie point at the rate -0.45$T_C(0)$ K/GPa, where $T_C(0)$ is the zero pressure Curie temperature. Stresses and misfit strains were found to induce additional ferroelectric phase transitions and phases not available under normal conditions. We find that tensile load significantly enhances both the Curie temperature and spontaneous polarization, while compressive load has the opposite effect but with the difference that the Curie temperature is only slightly suppressed. The isothermal dependencies of polarization on pressure and stresses are highly nonlinear, which could result in large nonlinear piezoelectric responses. The phase diagrams reveal the diversity of the phases accessible through mechanical load, which include tetragonal, orthorhombic and monoclinic symmetries in addition to the rhombohedral and cubic ones realizable under normal conditions. We believe that this work reveals the potential of Ge-based halide perovskites for applications in energy converting devices, which is especially significant in the current pursuit of environmental friendly lead-free technologies.

Variance sum rule: proofs and solvable models
Ivan Di Terlizzi, Marco Baiesi, Felix Ritort
arXiv:2403.10442v1 Announce Type: new Abstract: We derive, in more general conditions, a recently introduced variance sum rule (VSR) [I. Di Terlizzi et al., 2024 Science 383 971] involving variances of displacement and force impulse for overdamped Langevin systems in a nonequilibrium steady state (NESS). This formula allows visualising the effect of nonequilibrium as a deviation of the sum of variances from normal diffusion $2Dt$, with $D$ the diffusion constant and $t$ the time. From the VSR, we also derive formulas for the entropy production rate $\sigma$ that, differently from previous results, involve second-order time derivatives of position correlation functions. This novel feature gives a criterion for discriminating strong nonequilibrium regimes without measuring forces. We then apply and discuss our results to three analytically solved models: a stochastic switching trap, a Brownian vortex, and a Brownian gyrator. Finally, we compare the advantages and limitations of known and novel formulas for $\sigma$ in an overdamped NESS.

Microscopic understanding of NMR signals by dynamic mean-field theory for spins
Timo Gr\"a{\ss}er, Thomas Hahn, G\"otz S. Uhrig
arXiv:2403.10465v1 Announce Type: new Abstract: A recently developed dynamic mean-field theory for disordered spins (spinDMFT) is shown to capture the spin dynamics of nuclear spins very well. The key quantities are the spin autocorrelations. In order to compute the free induction decay (FID), pair correlations are needed in addition. They can be computed on spin clusters of moderate size which are coupled to the dynamic mean fields determined in a first step by spinDMFT. We dub this versatile approach non-local spinDMFT (nl-spinDMFT). It is a particular asset of nl-spinDMFT that one knows from where the contributions to the FID stem. We illustrate the strengths of nl-spinDMFT in comparison to experimental data for CaF$_2$. Furthermore, spinDMFT provides the dynamic mean-fields explaining the FID of the nuclear spins in $^{13}$C in adamantane up to some static noise. The spin Hahn echo in adamantane is free from effects of static noise and agrees excellently with the spinDMFT results without further fitting.

Tuneable band topology and optical conductivity in altermagnets
Peng Rao, Alexander Mook, Johannes Knolle
arXiv:2403.10509v1 Announce Type: new Abstract: We study two-dimensional $d$-wave altermagnetic metals taking into account the presence of substrate-induced Rashba spin-orbit coupling. We consider the altermagnet bandstructure using a 2D band Hamiltonian near the $\Gamma$ point under external magnetic field. It is shown that time-reversal-symmetry breaking due to altermagnetism, together with Rashba coupling and external magnetic field, can result in non-trivial band topology. The topological phases can be tuned by magnetic field strength and directions, and are classified by their Chern numbers. Furthermore, we investigate the charge response by computing the full optical conductivity tensor with and without magnetic field. In particular, we focus on magneto-optical responses, which are the finite-frequency analog of the Berry curvature-induced anomalous Hall conductivity. Finally, using experimentally realistic parameters for RuO$_2$, we estimate the Faraday angle in the absence of magnetic fields.

Quantum Synchronization in Nonconservative Electrical Circuits with Kirchhoff-Heisenberg Equations
Matteo Mariantoni, Noah Gorgichuk
arXiv:2403.10474v1 Announce Type: cross Abstract: We investigate quantum synchronization phenomena in electrical circuits that incorporate specifically designed nonconservative elements. A dissipative theory of classical and quantized electrical circuits is developed based on the Rayleigh dissipation function. The introduction of this framework enables the formulation of a generalized version of classical Poisson brackets, which are termed Poisson-Rayleigh brackets. By using these brackets, we are able to derive the equations of motion for a given circuit. Remarkably, these equations are found to correspond to Kirchhoff's current laws when Kirchhoff's voltage laws are employed to impose topological constraints, and vice versa. In the quantum setting, the equations of motion are referred to as the Kirchhoff-Heisenberg equations, as they represent Kirchhoff's laws within the Heisenberg picture. These Kirchhoff-Heisenberg equations, serving as the native equations for an electrical circuit, can be used in place of the more abstract master equations in Lindblad form. To validate our theoretical framework, we examine three distinct circuits. The first circuit consists of two resonators coupled via a nonconservative element. The second circuit extends the first to incorporate weakly nonlinear resonators, such as transmons. Lastly, we investigate a circuit involving two resonators connected through an inductor in series with a resistor. This last circuit, which incidentally represents a realistic implementation, allows for the study of a singular system, where the absence of a coordinate leads to an ill-defined system of Hamilton's equations. To analyze such a pathological circuit, we introduce the concept of auxiliary circuit element. After resolving the singularity, we demonstrate that this element can be effectively eliminated at the conclusion of the analysis, recuperating the original circuit.

Fragment-orbital-dependent spin fluctuations in the single-component molecular conductor [Ni(dmdt)$_2$]
Taiki Kawamura, Akito Kobayashi
arXiv:2205.04020v4 Announce Type: replace Abstract: Motivated by recent nuclear magnetic resonance experiments, we calculated the spin susceptibility, Knight shift, and spin-lattice relaxation rate ($1/T_{1}T$) of the single-component molecular conductor [Ni(dmdt)$_2$] using the random phase approximation in a multi-orbital Hubbard model describing the Dirac nodal line electronic system in this compound. This Hubbard model is composed of three fragment orbitals and on-site repulsive interactions obtained using ab initio many-body perturbation theory calculations. We found fragment-orbital-dependent spin fluctuations with the momentum $\textbf{q}$=$\textbf{0}$ and an incommensurate value of the wavenumber $\textbf{q}$=$\textbf{Q}$ at which a diagonal element of the spin susceptibility is maximum. The $\textbf{q}$=$\textbf{0}$ and $\textbf{Q}$ responses become dominant at low and high temperatures, respectively, with the Fermi-pocket energy scale as the boundary. We show that $1/T_{1}T$ decreases with decreasing temperature but starts to increase at low temperature owing to the $\textbf{q}$=$\textbf{0}$ spin fluctuations, while the Knight shift keeps monotonically decreasing. These properties are due to the intra-molecular antiferromagnetic fluctuations caused by the characteristic wave functions of this Dirac nodal line system, which is described by an $n$-band ($n\geq 3$) model. We show that the fragment orbitals play important roles in the magnetic properties of [Ni(dmdt)$_2$].

Higgs Condensates are Symmetry-Protected Topological Phases: I. Discrete Symmetries
Ruben Verresen, Umberto Borla, Ashvin Vishwanath, Sergej Moroz, Ryan Thorngren
arXiv:2211.01376v2 Announce Type: replace Abstract: Where in the landscape of many-body phases of matter do we place the Higgs condensate of a gauge theory? On the one hand, the Higgs phase is gapped, has no local order parameter, and for fundamental Higgs fields is adiabatically connected to the confined phase. On the other hand, Higgs phases such as superconductors display rich phenomenology. In this work, we propose a minimal description of the Higgs phase as a symmetry-protected topological (SPT) phase, utilizing conventional and higher-form symmetries. In this first part, we focus on 2+1D $\mathbb Z_2$ gauge theory and find that the Higgs phase is protected by a higher-form magnetic symmetry and a matter symmetry, whose meaning depends on the physical context. While this proposal captures known properties of Higgs phases, it also predicts that the Higgs phase of the Fradkin-Shenker model has SPT edge modes in the symmetric part of the phase diagram, which we confirm analytically. In addition, we argue that this SPT property is remarkably robust upon explicitly breaking the magnetic symmetry. Although the Higgs and confined phases are then connected without a bulk transition, they are separated by a boundary phase transition, which we confirm with tensor network simulations. More generally, the boundary anomaly of the Higgs SPT phase coincides with the emergent anomaly of symmetry-breaking phases, making precise the relation between Higgs phases and symmetry breaking. The SPT nature of the Higgs phase can also manifest in the bulk, e.g., at transitions between distinct Higgs condensates. Finally, we extract insights which are applicable to general SPT phases, such as a 'bulk-defect correspondence' generalizing discrete gauge group analogs of Superconductor-Insulator-Superconductor (SIS) junctions. The sequel to this work will generalize 'Higgs=SPT' to continuous symmetries, interpreting superconductivity as an SPT property.

Equivariant graph neural network interatomic potential for Green-Kubo thermal conductivity in phase change materials
Sung-Ho Lee, Jing Li, Valerio Olevano, Benoit Skl\'enard
arXiv:2307.02327v2 Announce Type: replace Abstract: Thermal conductivity is a fundamental material property that plays an essential role in technology, but its accurate evaluation presents a challenge for theory. In this work, we demonstrate the application of $E(3)$-equivariant neutral network interatomic potentials within Green-Kubo formalism to determine the lattice thermal conductivity in amorphous and crystalline materials. We apply this method to study the thermal conductivity of germanium telluride (GeTe) as a prototypical phase change material. A single deep learning interatomic potential is able to describe the phase transitions between the amorphous, rhombohedral and cubic phases, with critical temperatures in good agreement with experiments. Furthermore, this approach accurately captures the pronounced anharmonicity that is present in GeTe, enabling precise calculations of the thermal conductivity. In contrast, the Boltzmann transport equation including only three-phonon processes tends to overestimate the thermal conductivity by approximately a factor of 2 in the crystalline phases.

Physical properties of an Aperiodic monotile: Graphene-like features, chirality and zero-modes
Justin Schirmann, Selma Franca, Felix Flicker, Adolfo G. Grushin
arXiv:2307.11054v4 Announce Type: replace Abstract: The discovery of the Hat, an aperiodic monotile, has revealed novel mathematical aspects of aperiodic tilings. However, the physics of particles propagating in such a setting remains unexplored. In this work we study spectral and transport properties of a tight-binding model defined on the Hat. We find that (i) the spectral function displays striking similarities to that of graphene, including six-fold symmetry and Dirac-like features; (ii) unlike graphene, the monotile spectral function is chiral, differing for its two enantiomers; (iii) the spectrum has a macroscopic number of degenerate states at zero energy; (iv) when the magnetic flux per plaquette ($\phi$) is half of the flux quantum, zero-modes are found localized around the reflected `anti-hats'; and (v) its Hofstadter spectrum is periodic in $\phi$, unlike for other quasicrystals. Our work serves as a basis to study wave and electron propagation in possible experimental realizations of the Hat, which we suggest.

$W$ state is not the unique ground state of any local Hamiltonian
Lei Gioia, Ryan Thorngren
arXiv:2310.10716v2 Announce Type: replace Abstract: The characterization of ground states among all quantum states is an important problem in quantum many-body physics. For example, the celebrated entanglement area law for gapped Hamiltonians has allowed for efficient simulation of 1d and some 2d quantum systems using matrix product states. Among ground states, some types, such as cat states (like the GHZ state) or topologically ordered states, can only appear alongside their degenerate partners, as is understood from the theory of spontaneous symmetry breaking. In this work, we introduce a new class of simple states, including the $W$ state, that can only occur as a ground state alongside an exactly degenerate partner, even in gapless or disordered models. We show that these states are never an element of a stable gapped ground state manifold, which may provide a new method to discard a wide range of 'unstable' entanglement area law states in the numerical search of gapped phases. On the other hand when these degenerate states are the ground states of gapless systems they possess an excitation spectrum with $O(1/L^2)$ finite-size splitting. One familiar situation where this special kind of gaplessness occurs is at a Lifshitz transition due to a zero mode; a potential quantum state signature of such a critical point. We explore pathological parent Hamiltonians, and discuss generalizations to higher dimensions, other related states, and implications for understanding thermodynamic limits of many-body quantum systems.

Photo-induced electronic and spin topological phase transitions in monolayer bismuth
Bo Peng, Gunnar F. Lange, Daniel Bennett, Kang Wang, Robert-Jan Slager, Bartomeu Monserrat
arXiv:2310.16886v2 Announce Type: replace Abstract: Ultrathin bismuth exhibits rich physics including strong spin-orbit coupling, ferroelectricity, nontrivial topology, and light-induced structural dynamics. We use \textit{ab initio} calculations to show that light can induce structural transitions to four transient phases in bismuth monolayers. These light-induced phases exhibit nontrivial topological character, which we illustrate using the recently introduced concept of spin bands and spin-resolved Wilson loops. Specifically, we find that the topology changes via the closing of the electron and spin band gaps during photo-induced structural phase transitions, leading to distinct edge states. Our study provides strategies to tailor electronic and spin topology via ultrafast control of photo-excited carriers and associated structural dynamics.

Hole subband dispersions in a cylindrical Ge nanowire: exact results based on the axial Luttinger-Kohn Hamiltonian
Rui Li
arXiv:2311.03816v2 Announce Type: replace Abstract: Based on the Luttinger-Kohn Hamiltonian in the axial approximation, the transcendental equations determining the hole subband dispersions in a cylindrical Ge nanowire are analytically derived. These equations are more general than that derived using the spherical approximation, and are suitable to study the growth direction dependence of the subband dispersions. The axial approximation almost gives rise to the accurate low-energy subband dispersions for high-symmetry nanowire growth directions [001] and [111]. The perturbation correction from the non-axial term is negligible for these two directions. The lowest two subband dispersions can be regarded as two shifted parabolic curves with an energy gap at $k_{z}=0$ for both growth directions [001] and [111]. At the position of the energy gap, the eigenstates for growth direction [111] are inverted in comparison with the normal eigenstates for growth direction [001]. A nanowire growth direction where the energy gap closes at $k_{z}=0$ is predicted to exist between directions [001] and [111].

Ground states of one-dimensional dipolar lattice bosons at unit filling
Mateusz \L\k{a}cki, Henning Korbmacher, G. A. Dom\'inguez-Castro, Jakub Zakrzewski, Luis Santos
arXiv:2311.14606v2 Announce Type: replace Abstract: Recent experiments on ultracold dipoles in optical lattices open exciting possibilities for the quantum simulation of extended Hubbard models. When considered in one dimension, these models present at unit filling a particularly interesting ground-state physics, including a symmetry-protected topological phase known as Haldane insulator. We show that the tail of the dipolar interaction beyond nearest-neighbors, which may be tailored by means of the transversal confinement, does not only modify quantitatively the Haldane insulator regime and lead to density waves of larger periods, but results as well in unexpected insulating phases. These insulating phases may be topological or topologically trivial, and are characterized by peculiar correlations of the site occupations. These phases may be realized and observed in state-of-the-art experiments.

Macroscopic fluctuation theory of local time in lattice gases
Naftali R. Smith, Baruch Meerson
arXiv:2311.15286v2 Announce Type: replace Abstract: The local time in an ensemble of particles measures the amount of time the particles spend in the vicinity of a given point in space. Here we study fluctuations of the empirical time average $R= T^{-1}\int_{0}^{T}\rho\left(x=0,t\right)\,dt$ of the density $\rho\left(x=0,t\right)$ at the origin (so that $R$ is the local time spent at the origin, rescaled by $T$) for an initially uniform one-dimensional diffusive lattice gas. We consider both the quenched and annealed initial conditions and employ the Macroscopic Fluctuation Theory (MFT). For a gas of non-interacting random walkers (RWs) the MFT yields exact large-deviation functions of $R$, which are closely related to the ones recently obtained by Burenev \textit{et al.} (2023) using microscopic calculations for non-interacting Brownian particles. Our MFT calculations, however, additionally yield the most likely history of the gas density $\rho(x,t)$ conditioned on a given value of $R$. Furthermore, we calculate the variance of the local time fluctuations for arbitrary particle- or energy-conserving diffusive lattice gases. Better known examples of such systems include the simple symmetric exclusion process, the Kipnis-Marchioro-Presutti model and the symmetric zero-range process. Our results for the non-interacting RWs can be readily extended to a step-like initial condition for the density.

Electronic band structure of Sb2Te3
I. Mohelsky, J. Wyzula, F. Le Mardele, F. Abadizaman, O. Caha, A. Dubroka, X. D. Sun, C. W. Cho, B. A. Piot, M. F. Tanzim, I. Aguilera, G. Bauer, G. Springholz, M. Orlita
arXiv:2312.07402v2 Announce Type: replace Abstract: Here we report on Landau level spectroscopy of an epitaxially grown thin film of the topological insulator Sb2Te3, complemented by ellipsometry and magneto-transport measurements. The observed response suggests that Sb2Te3 is a direct-gap semiconductor with the fundamental band gap located at the \Gamma point, or along the trigonal axis, and its width reaches Eg = 190 meV at low temperatures. Our data also indicate the presence of other low-energy extrema with a higher multiplicity in both the conduction and valence bands. The conclusions based on our experimental data are confronted with and to a great extent corroborated by the electronic band structure calculated using the GW method.

Spontaneous gap opening and potential excitonic states in an ideal Dirac semimetal Ta$_2$Pd$_3$Te$_5$
Peng Zhang, Yuyang Dong, Dayu Yan, Bei Jiang, Tao Yang, Jun Li, Zhaopeng Guo, Yong Huang, Bo Hao, Qing Li, Yupeng Li, Kifu Kurokawa, Rui Wang, Yuefeng Nie, Makoto Hashimoto, Donghui Lu, Wen-He Jiao, Jie Shen, Tian Qian, Zhijun Wang, Youguo Shi, Takeshi Kondo
arXiv:2312.14456v2 Announce Type: replace Abstract: The opening of an energy gap in the electronic structure generally indicates the presence of interactions. In materials with low carrier density and short screening length, long-range Coulomb interaction favors the spontaneous formation of electron-hole pairs, so-called excitons, opening an excitonic gap at the Fermi level. Excitonic materials host unique phenomenons associated with pair excitations. However, there is still no generally recognized single-crystal material with excitonic order, which is, therefore, awaited in condensed matter physics. Here, we show that excitonic states may exist in the quasi-one-dimensional material Ta$_2$Pd$_3$Te$_5$, which has an almost ideal Dirac-like band structure, with Dirac point located exactly at Fermi level. We find that an energy gap appears at 350 K, and it grows with decreasing temperature. The spontaneous gap opening is absent in a similar material Ta$_2$Ni$_3$Te$_5$. Intriguingly, the gap is destroyed by the potassium deposition on the crystal, likely due to extra-doped carriers. Furthermore, we observe a pair of in-gap flat bands, which is an analog of the impurity states in a superconducting gap. All these observations can be properly explained by an excitonic order, providing Ta$_2$Pd$_3$Te$_5$ as a new and promising candidate realizing excitonic states.

From anti-Arrhenius to Arrhenius behavior in a dislocation-obstacle bypass: Atomistic Simulations and Theoretical Investigation
Mohammadhossein Nahavandian, Soumit Sarkar, Soumendu Bagchi, Danny Perez, Enrique Martinez
arXiv:2401.04100v2 Announce Type: replace Abstract: Dislocations are the primary carriers of plasticity in metallic material. Understanding the basic mechanisms for dislocation movement is paramount to predicting the material mechanical response. Relying on atomistic simulations, we observe a transition from non-Arrhenius to Arrhenius behavior in the rate for an edge dislocation to overcome the elastic interaction with a prismatic loop in tungsten. Beyond the critical resolved shear stress, the process shows a non-Arrhenius behavior at low temperatures. However, as the temperature increases, the activation entropy starts to dominate, leading to a traditional Arrhenius behavior. We have computed the activation entropy analytically along the minimum energy path following Schoeck's methods [1], which capture the cross-over between anti-Arrhenius and Arrhenius domains. Also, the Projected Average Force Integrator (PAFI) [2], another simulation method to compute free energies along an initial transition path, exhibits considerable concurrence with Schoeck's formalism. We conclude that entropic effects need to be considered to understand processes involving dislocations bypassing elastic barriers close to the critical resolved shear stress. More work needs to be performed to fully understand the discrepancies between Schoeck's and PAFI results compared to molecular dynamics.

Minimal Models for Altermagnetism
Merc\`e Roig, Andreas Kreisel, Yue Yu, Brian M. Andersen, Daniel F. Agterberg
arXiv:2402.15616v2 Announce Type: replace Abstract: Altermagnets feature vanishing net magnetization, like antiferromagnets, but exhibit time-reversal symmetry breaking and momentum-dependent spin-split band structures. Motivated by the prevalence of altermagnetic materials with non-symmorphic symmetry-dictated band degeneracies, we provide realistic minimal models for altermagnetism by constructing tight-binding models for nonsymmorphic space groups with a sublattice defined by two magnetic atoms. These models can be applied to monoclinic, orthorhombic, tetragonal, rhombohedral, hexagonal, and cubic materials and can describe d-wave, g-wave, and i-wave altermagnetism. By examining the altermagnetic susceptibility and mean field instabilities within a Hubbard model we reveal that these models have altermagnetic ground states and yield a Berry curvature that is linear in the spin-orbit coupling. We apply our models to RuO$_2$, MnF$_2$, FeSb$_2$, $\kappa$-Cl, CrSb, and MnTe.

Site Symmetry and Multiorbital Flat Bands on Kagome and Pyrochlore Lattices
Keyu Zeng, Ziqiang Wang
arXiv:2403.03201v2 Announce Type: replace Abstract: Flat bands in electronic band structures are intriguing platforms for strong correlation and topological physics, primarily due to the suppressed kinetic energy of electrons. Various methods have been developed to create flat bands, utilizing lattice geometry or finely tuned parameters. Despite this, the investigation of orbital symmetry in multiorbital materials is a relatively new area of focus. In this work, we propose a site symmetry based systematic approach to emerging multiorbital flat bands in lattices made of corner-connecting motifs such as the kagome and pyrochlore lattices. As a conceptual advance, the one-orbital flat bands are shown to originate as mutual eigenstates of isolated molecular motifs. Further developing the mutual eigenstate method for multiorbitals transforming differently under the site symmetries such as mirror and inversion, we derive multiorbital flat bands from the skew-symmetric interorbital Hamiltonian and introduce an isolated molecule enabled group-theoretic description of the flat band wavefunctions. Realizations of the multiorbital flatbands in relevant materials are shown to be possible under the Slater-Koster formalism. Our findings provide new directions for exploring flatband electronic structures for novel correlated and topological quantum states.

Dual Symmetry Classification of Non-Hermitian Systems and $\mathbb{Z}_2$ Point-Gap Topology of a Non-Unitary Quantum Walk
Zhiyu Jiang, Ryo Okamoto, Hideaki Obuse
arXiv:2403.04147v2 Announce Type: replace Abstract: Non-Hermitian systems exhibit richer topological properties compared to their Hermitian counterparts. It is well known that non-Hermitian systems have been classified based on either the symmetry relations for non-Hermitian Hamiltonians or the symmetry relations for non-unitary time-evolution operators in the context of Floquet topological phases. In this work, we propose that non-Hermitian systems can always be classified in two ways; a non-Hermitian system can be classified using the symmetry relations for non-Hermitian Hamiltonians or time-evolution operator regardless of the Floquet topological phases or not. We refer to this as dual symmetry classification. To demonstrate this, we successfully introduce a new non-unitary quantum walk that exhibits point gaps with a $\mathbb{Z}_2$ point-gap topological phase applying the dual symmetry classification and treating the time-evolution operator of this quantum walk as the non-Hermitian Hamiltonian.

Superconductivity in Two-Dimensional Systems with Unconventional Rashba Bands
Ran Wang, Jiayang Li, Xinliang Huang, Rui Song, Ning Hao
arXiv:2403.05051v2 Announce Type: replace Abstract: In two-dimensional system with Rashba spin-orbit coupling, it is well-known that superconductivity can have mixed spin-singlet and -triplet parity, and the $\boldsymbol{d}$-vector of spin-triplet pairing is parallel to $\boldsymbol{g}$-vector of Rashba spin-orbit coupling. Here, we propose a model to describe a two-dimensional system with unconventional Rashba bands and study its superconductivity. We show that the $\boldsymbol{d}$-vector of spin-triplet pairing can be either parallel or perpendicular to $\boldsymbol{g}$-vector of Rashba spin-orbit coupling depending on the different pairing interaction. We also propose a junction to generate tunneling spin current depending on the direction of $\boldsymbol{d}$-vector. It provides a detecable evidence to distinguish these two different but very similar pairing channels. Furthermore, we find this model can give arise to a subleading spin-singlet chiral $p$-wave topological superconducting state. More significantly, we find that such unconventional Rashba bands and unconventional superconudcting pairings can be realized on surface of some superconducting topological materials, such as trigonal layered PtBi$_{2}$.

CFT$_D$ from TQFT$_{D+1}$ via Holographic Tensor Network, and Precision Discretisation of CFT$_2$
Lin Chen, Haochen Zhang, Kaixin Ji, Ce Shen, Ruoshui Wang, Xiangdong Zeng, Ling-Yan Hung
arXiv:2210.12127v3 Announce Type: replace-cross Abstract: We show that the path-integral of conformal field theories in $D$ dimensions (CFT$_D$) can be constructed by solving for eigenstates of an RG operator following from the Turaev-Viro formulation of a topological field theory in $D+1$ dimensions (TQFT$_{D+1}$), explicitly realising the holographic sandwich relation between a symmetric theory and a TQFT. Generically, exact eigenstates corresponding to symmetric-TQFT$_D$ follow from Frobenius algebra in the TQFT$_{D+1}$. For $D=2$, we constructed eigenstates that produce 2D rational CFT path-integral exactly, which, curiously connects a continuous field theoretic path-integral with the Turaev-Viro state sum. We also devise and illustrate numerical methods for $D=2,3$ to search for CFT$_D$ as phase transition points between symmetric TQFT$_D$. Finally since the RG operator is in fact an exact analytic holographic tensor network, we compute ``bulk-boundary'' correlator and compare with the AdS/CFT dictionary at $D=2$. Promisingly, they are numerically compatible given our accuracy, although further works will be needed to explore the precise connection to the AdS/CFT correspondence.

Enhanced Hydrogen Evolution Catalysis of Pentlandite due to the Increases in Coordination Number and Sulfur Vacancy during Cubic-Hexagonal Phase Transition
Yuegao Liu, Chao Cai, Shengcai Zhu, Zhi Zheng, Guowu Li, Haiyan Chen, Chao Li, Haiyan Sun, I-Ming Chou, Yanan Yu, Shenghua Mei, Liping Wang
arXiv:2210.13348v4 Announce Type: replace-cross Abstract: The search for new phases is an important direction in materials science. The phase transition of sulfides results in significant changes in catalytic performance, such as MoS2 and WS2. Cubic pentlandite [cPn, (Fe, Ni)9S8] can be a functional material in batteries, solar cells, and catalytic fields. However, no report about the material properties of other phases of pentlandite exists. In this study, the unit-cell parameters of a new phase of pentlandite, sulfur-vacancy enriched hexagonal pentlandite (hPn), and the phase boundary between cPn and hPn were determined for the first time. Compared to cPn, the hPn shows a high coordination number, more sulfur vacancies, and high conductivity, which result in significantly higher hydrogen evolution performance of hPn than that of cPn and make the non-nano rock catalyst hPn superior to other most known nanosulfide catalysts. The increase of sulfur vacancies during phase transition provides a new approach to designing functional materials.

Assessment of error variation in high-fidelity two-qubit gates in silicon
Tuomo Tanttu, Wee Han Lim, Jonathan Y. Huang, Nard Dumoulin Stuyck, Will Gilbert, Rocky Y. Su, MengKe Feng, Jesus D. Cifuentes, Amanda E. Seedhouse, Stefan K. Seritan, Corey I. Ostrove, Kenneth M. Rudinger, Ross C. C. Leon, Wister Huang, Christopher C. Escott, Kohei M. Itoh, Nikolay V. Abrosimov, Hans-Joachim Pohl, Michael L. W. Thewalt, Fay E. Hudson, Robin Blume-Kohout, Stephen D. Bartlett, Andrea Morello, Arne Laucht, Chih Hwan Yang, Andre Saraiva, Andrew S. Dzurak
arXiv:2303.04090v3 Announce Type: replace-cross Abstract: Achieving high-fidelity entangling operations between qubits consistently is essential for the performance of multi-qubit systems and is a crucial factor in achieving fault-tolerant quantum processors. Solid-state platforms are particularly exposed to errors due to materials-induced variability between qubits, which leads to performance inconsistencies. Here we study the errors in a spin qubit processor, tying them to their physical origins. We leverage this knowledge to demonstrate consistent and repeatable operation with above 99% fidelity of two-qubit gates in the technologically important silicon metal-oxide-semiconductor (SiMOS) quantum dot platform. We undertake a detailed study of these operations by analysing the physical errors and fidelities in multiple devices through numerous trials and extended periods to ensure that we capture the variation and the most common error types. Physical error sources include the slow nuclear and electrical noise on single qubits and contextual noise. The identification of the noise sources can be used to maintain performance within tolerance as well as inform future device fabrication. Furthermore, we investigate the impact of qubit design, feedback systems, and robust gates on implementing scalable, high-fidelity control strategies. These results are achieved by using three different characterization methods, we measure entangling gate fidelities ranging from 96.8% to 99.8%. Our analysis tools identify the causes of qubit degradation and offer ways understand their physical mechanisms. These results highlight both the capabilities and challenges for the scaling up of silicon spin-based qubits into full-scale quantum processors.

Single-photon emitters in WSe$_2$: The critical role of phonons on excitation schemes and indistinguishability
Luca Vannucci, Jos\'e Ferreira Neto, Claudia Piccinini, Athanasios Paralikis, Niels Gregersen, Battulga Munkhbat
arXiv:2402.10897v2 Announce Type: replace-cross Abstract: Within optical quantum information processing, single-photon sources based on a two-level system in a semiconductor material allow for on-demand generation of single photons. To initiate the spontaneous emission process, it is necessary to efficiently populate the excited state. However, reconciling the requirement for on-demand excitation with both high efficiency and high photon indistinguishability remains a challenge due to the presence of charge noise and phonon-induced decoherence in the solid-state environment. Here, we propose a method for reconstructing the phonon spectral density experienced by WSe$_{2}$ quantum emitters in the emission process. Using the reconstructed phonon spectral density, we analyze the performance of the resonant, phonon-assisted, and SUPER swing-up excitation schemes. Under resonant excitation, we obtain an exciton preparation fidelity limited to $\sim$0.80 by the strong phonon coupling, which improves to 0.96 for the SUPER scheme (or 0.89, depending on the type of emitter considered). Under near-resonant phonon-assisted excitation, we observe near-unity excitation fidelity up to 0.976 (0.997). Additionally, we demonstrate that, assuming the suppression of the phonon sidebands, residual dephasing mechanisms such as charge/spin fluctuations are the dominating decoherence mechanisms undermining the photon indistinguishability.