Found 27 papers in cond-mat
Date of feed: Mon, 25 Sep 2023 00:30:00 GMT

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Grand canonical ensemble of a $d$-dimensional Reissner-Nordstr\"om black hole in a cavity. (arXiv:2309.12388v1 [hep-th])
Tiago V. Fernandes, José P. S. Lemos

The grand canonical ensemble of a $d$-dimensional Reissner-Nordstr\"om black hole space in a cavity is analyzed. The realization of this ensemble is made through the Euclidean path integral approach by giving the Euclidean action for the black hole with the correct topology, and boundary conditions corresponding to a cavity, where the fixed quantities are the temperature and the electric potential. One performs a zero loop approximation to find and analyze the stationary points of the reduced action. This yields two solutions for the electrically charged black hole, $r_{+1}$, which is the smaller and unstable, and $r_{+2}$, which is the larger and stable. One also analyzes the most probable configurations, which are either a stable charged black hole or hot flat space, mimicked by a nongravitating charged shell. Making the correspondence between the action and the grand potential, one can get the black hole thermodynamic quantities, such as the entropy, the mean charge, the mean energy, and the thermodynamic pressure, as well as the Smarr formula, shown to be valid only for the unstable black hole. We find that thermodynamic stability is related to the positivity of the heat capacity at constant electric potential and area of the cavity. We also comment on the most favorable thermodynamic phases and phase transitions. We then choose $d = 5$, which is singled out naturally from the other higher dimensions as it provides an exact solution for the problem, and apply all the results previously found. The case $d = 4$ is mentioned. We compare thermodynamic radii with the photonic orbit radius and the Buchdahl-Andr\'easson-Wright bound radius in $d$-dimensional Reissner-Nordstr\"om spacetimes and find they are unconnected, showing that the connections displayed in the Schwarzschild case are not generic, rather they are very restricted holding only in the pure gravitational situation.


Characterization and classification of interacting (2+1)D topological crystalline insulators with orientation-preserving wallpaper groups. (arXiv:2309.12389v1 [cond-mat.str-el])
Naren Manjunath, Vladimir Calvera, Maissam Barkeshli

While free fermion topological crystalline insulators have been largely classified, the analogous problem in the strongly interacting case has been only partially solved. In this paper, we develop a characterization and classification of interacting, invertible fermionic topological phases in (2+1) dimensions with charge conservation, discrete magnetic translation and $M$-fold point group rotation symmetries, which form the group $G_f = \text{U}(1)^f \times_{\phi} [\mathbb{Z}^2\rtimes \mathbb{Z}_M]$ for $M=1,2,3,4,6$. $\phi$ is the magnetic flux per unit cell. We derive a topological response theory in terms of background crystalline gauge fields, which gives a complete classification of different phases and a physical characterization in terms of quantized response to symmetry defects. We then derive the same classification in terms of a set of real space invariants $\{\Theta_{\text{o}}^\pm\}$ that can be obtained from ground state expectation values of suitable partial rotation operators. We explicitly relate these real space invariants to the quantized coefficients in the topological response theory, and find the dependence of the invariants on the chiral central charge $c_-$ of the invertible phase. Finally, when $\phi = 0$ we derive an explicit map between the free and interacting classifications.


The influence of film thickness and uniaxial anisotropy on in-plane skyrmions: Numerical investigations of the phase space of chiral magnets. (arXiv:2309.12419v1 [cond-mat.mtrl-sci])
Cameron Rudderham, Martin Plumer, Theodore Monchesky

The equilibrium phase space of magnetic textures in thin-films of cubic chiral ferromagnets, including skyrmions, is explored as a function of in-plane magnetic field strength, film thickness and uniaxial anisotropy. The interplay between these system parameters is found to give rise to a phase space with a rich structure, distinct from that of the nano-stripes that have been previously studied. For certain values of the anisotropy, the range of thicknesses supporting in-plane skyrmions and/or helicoids with an out-of-plane propagation vector is found to be disconnected, suggesting a possible direction for future experiments. We explain how this interesting phase space topology arises due to the geometric confinement of the thin-film system, and identify the optimal parameter ranges for future explorations of novel magnetic textures such as the oblique spiral phase.


Critical Field Anisotropy and Muon Spin Relaxation Study of Superconducting Dirac-Semimetal CaSb$_2$. (arXiv:2309.12457v1 [cond-mat.supr-con])
M. Oudah, Y. Cai, M. V. De Toro Sanchez, J. Bannies, M. C. Aronson, K. M. Kojima, D. A. Bonn

CaSb$_2$ has been identified as a bulk superconductor and a topological semimetal, which makes it a great platform for realizing topological superconductivity. In this work, we investigate the superconducting upper and lower critical field anisotropy using magnetic susceptibility, and study the superconducting state using muon spin-relaxation. The temperature dependence of transverse-field relaxation can be fitted with a single-gap model or two-gap model, consistent with previous tunnel-diode oscillator measurements. We highlight that the normal state of CaSb$_2$ shows a large diamagnetic signal, which is likely related to its Dirac semimetal nature. Zero-field relaxation shows little temperature dependence when the muon-spin is parallel to the $c*$-axis, while an increase in relaxation appears below 1~K when the muon-spin is parallel to the $ab$-plane. This may be related to a second superconducting phase appearing at low temperature below the bulk $T_c$ . However, we find no discernible anomaly in $\mu_{\rm{0}} H_{\rm{c1}}(0)$ around this temperature as has been seen in other superconductors with secondary superconducting states that appear at lower temperatures.


Spin Weyl Topological Insulators. (arXiv:2309.12470v1 [cond-mat.mtrl-sci])
Rafael Gonzalez-Hernandez, Bernardo Uribe

The quantum nature of electron spin is crucial for establishing topological invariants in real materials. Since the spin does not in general commute with the Hamiltonian, some of the topological features of the material can be extracted from its study. In insulating materials, the spin operator induces a projected operator on valence states called the spin valence operator. Its spectrum contains information with regard to the different phases of the spin Chern class. If the spin valence spectrum is gapped, the spin Chern numbers are constant along parallel planes thus defining spin Chern insulating materials. If the spin valence spectrum is not gapped, the changes in the spin Chern numbers occur whenever this spectrum is zero. Materials whose spin valence spectrum are gapless will be denoted spin Weyl topological insulators and its definition together with some of their properties will be presented in this work. The classification of materials from the properties of the spin valence operator provides a new characterization which complements the existing list of topological invariants.


Experimental and theoretical assessment of native oxide in the superconducting TaN. (arXiv:2309.12520v1 [cond-mat.supr-con])
V. Quintanar-Zamora, M. Cedillo-Rosillo, O. Contreras-López, C. Corona-García, A. Reyes-Serrato, R. Ponce-Pérez, J. Guerrero-Sánchez, J. A. Díaz

In this manuscript, we show through an experimental-computational proof of concept the native oxide formation into superconducting TaN films. First, TaN was synthesized at an ultra-high vacuum system by reactive pulsed laser deposition and characterized in situ by X-ray photoelectron spectroscopy. The material was also characterized ex situ by X-ray diffraction, transmission electron microscopy, and the four-point probe method. It was detected that TaN contained considerable oxygen impurities (up to 26 %O) even though it was grown in an ultra-high vacuum chamber. Furthermore, the impurified TaN evidence a face-centered cubic crystalline structure only and exhibits superconductivity at 2.99 K. To understand the feasibility of the native oxide in TaN, we study the effect of incorporating different amounts of O atoms in TaN using ab-initio calculations. A thermodynamic stability analysis shows that a TaOxN1-x model increases its stability as oxygen is added, demonstrating that oxygen may always be present in TaN, even when obtained at ultra-high vacuum conditions. All analyzed models exhibit metallic behavior. Charge density difference maps reveal that N and O atoms have a higher charge density redistribution than Ta atoms. The electron localization function maps and line profiles indicate that Ta-O and Ta-N bonds are mainly ionic. As expected, stronger ionic behavior is observed in the Ta-O bonds due to the electronegativity difference between O and N atoms. Recent evidence points to superconductivity in bulk TaO, confirming the asseverations of superconductivity in our samples. The results discussed here highlight the importance of considering native oxide when reporting superconductivity in TaN films since the TaO regions formed in the compound may be key to understanding the different critical temperatures reported in the literature.


Spatio-temporal correlations of noise in MOS spin qubits. (arXiv:2309.12542v1 [quant-ph])
Amanda E. Seedhouse, Nard Dumoulin Stuyck, Santiago Serrano, Tuomo Tanttu, Will Gilbert, Jonathan Yue Huang, Fay E. Hudson, Kohei M. Itoh, Arne Laucht, Wee Han Lim, Chih Hwan Yang, Andrew S. Dzurak, Andre Saraiva

In quantum computing, characterising the full noise profile of qubits can aid the efforts towards increasing coherence times and fidelities by creating error mitigating techniques specific to the type of noise in the system, or by completely removing the sources of noise. Spin qubits in MOS quantum dots are exposed to noise originated from the complex glassy behaviour of two-level fluctuators, leading to non-trivial correlations between qubit properties both in space and time. With recent engineering progress, large amounts of data are being collected in typical spin qubit device experiments, and it is beneficiary to explore data analysis options inspired from fields of research that are experienced in managing large data sets, examples include astrophysics, finance and climate science. Here, we propose and demonstrate wavelet-based analysis techniques to decompose signals into both frequency and time components to gain a deeper insight into the sources of noise in our systems. We apply the analysis to a long feedback experiment performed on a state-of-the-art two-qubit system in a pair of SiMOS quantum dots. The observed correlations serve to identify common microscopic causes of noise, as well as to elucidate pathways for multi-qubit operation with a more scalable feedback system.


Coalescence of immiscible sessile droplets on a partial wetting surface. (arXiv:2309.12561v1 [physics.flu-dyn])
Huadan Xu, Xinjin Ge, Tianyou Wang, Zhizhao Che

Droplet coalescence is a common phenomenon and plays an important role in multi-disciplinary applications. Previous studies mainly consider the coalescence of miscible liquid, even though the coalescence of immiscible droplets on a solid surface is a common process. In this study, we explore the coalescence of two immiscible droplets on a partial wetting surface experimentally and theoretically. We find that the coalescence process can be divided into three stages based on the timescales and force interactions involved, namely (I) the growth of the liquid bridge, (II) the oscillation of the coalescing sessile droplet, and (III) the formation of a partially-engulfed compound sessile droplet and the subsequent retraction. In stage I, the immiscible interface is found not to affect the scaling of the temporal evolution of the liquid bridge, which follows the same 2/3 power law as that of miscible droplets. In Stage II, by developing a new capillary timescale considering both surface and interfacial tensions, we show that the interfacial tension between the two immiscible liquids functions as a nonnegligible resistance to the oscillation which decreases the oscillation periods. In Stage III, a modified Ohnesorge number is developed to characterize the visco-capillary and inertia-capillary timescales involved during the displacement of water by oil; a new model based on energy balance is proposed to analyze the maximum retraction velocity, highlighting that the viscous resistance is concentrated in a region close to the contact line.


Dirac fermions on wires confined to the graphene Moebius strip. (arXiv:2309.12609v1 [cond-mat.mes-hall])
L. N. Monteiro, J. E. G. Silva, C. A. S. Almeida

We investigate the effects of the curved geometry on a massless relativistic electron constrained to a graphene strip with a Moebius strip shape. The anisotropic and parity-violating geometry of the Moebius band produces a geometric potential that inherits these features. By considering wires along the strip width and the strip length, we find exact solutions for the Dirac equation and the effects of the geometric potential on the electron were explored. In both cases, the geometric potential yields to a geometric phase on the wave function. Along the strip width, the density of states depends on the direction chosen for the wire, a consequence of the lack of axial symmetry. Moreover, the breaking of the parity symmetry enables the electronic states to be concentrated on the inner or on the outer portion of the strip. For wires along the strip length, the nontrivial topology influences the eigenfunctions by modifying their periodicity. It turns out that the ground state has a period of $4\pi$ whereas the first excited state is a $2\pi$ periodic function. Moreover, we found that the energy levels are half-integer multiples of the energy of the ground state.


Fingerprints for anisotropic Kondo lattice behavior in the quasiparticle dynamics of the kagome metal Ni$_3$In. (arXiv:2309.12648v1 [cond-mat.str-el])
Dong-Hyeon Gim, Dirk Wulferding, Chulwan Lee, Hengbo Cui, Kiwan Nam, Myung Joon Han, Kee Hoon Kim

We present a temperature- and polarization-resolved phononic and electronic Raman scattering study in combination with the first-principles calculations on the kagome metal Ni$_3$In with anisotropic transport properties and non-Fermi liquid behavior. At temperatures below 50 K and down to 2 K, several Raman phonon modes, including particularly an interlayer shear mode, exhibit appreciable frequency and linewidth renormalization, reminiscent of the onset of the Kondo screening without an accompanying structural or magnetic phase transition. In addition, a low-energy electronic continuum observed in polarization perpendicular to the kagome planes reveals strong temperature dependence below 50 K, implying thermal depletion of incoherent quasiparticles, while the in-plane continuum remains invariant. These concomitant electronic and phononic Raman signatures suggest that Ni$_3$In undergoes an anisotropic electronic crossover from an incoherent to a coherent Kondo lattice regime below 50 K. We discuss the origin of the anisotropic incoherent-coherent crossover in association with the possible anisotropic Kondo hybridization involving localized Ni-$3d_{xz}$ flat-band electrons.


Classification of Classical Spin Liquids: Topological Quantum Chemistry and Crystalline Symmetry. (arXiv:2309.12652v1 [cond-mat.str-el])
Yuan Fang, Jennifer Cano, Andriy H. Nevidomskyy, Han Yan

Frustrated magnetic systems can host highly interesting phases known as classical spin liquids (CSLs), which feature {extensive} ground state degeneracy and lack long-range magnetic order. Recently, Yan and Benton et al. proposed a classification scheme of CSLs in the large-$\mathcal{N}$ (soft spin) limit [arXiv.2305.00155, arXiv:2305.19189]. This scheme classifies CSLs into two categories: the algebraic CSLs and the fragile topological CSLs, each with their own correlation properties, low energy effective description, and finer classification frameworks. In this work, we further develop the classification scheme by considering the role of crystalline symmetry. We present a mathematical framework for computing the band representation of the flat bands in the spectrum of these CSLs, which extends beyond the conventional representation analysis. It allows one to determine whether the algebraic CSLs, which features gapless points on their bottom flat bands, are protected by symmetry or not. It also provides more information on the finer classifications of algebraic and fragile topological CSLs. We demonstrate this framework via concrete examples and showcase its power by constructing a pinch-line algebraic CSL protected by symmetry.


Engineering ferroelectricity in monoclinic hafnia. (arXiv:2309.12800v1 [cond-mat.mtrl-sci])
Hong Jian Zhao, Yuhao Fu, Longju Yu, Yanchao Wang, Yurong Yang, Laurent Bellaiche, Yanming Ma

Ferroelectricity in the complementary metal-oxide semiconductor (CMOS)-compatible hafnia (HfO$_2$) is crucial for the fabrication of high-integration nonvolatile memory devices. However, the capture of ferroelectricity in HfO$_2$ requires the stabilization of thermodynamically-metastable orthorhombic or rhombohedral phases, which entails the introduction of defects (e.g., dopants and vacancies) and pays the price of crystal imperfections, causing unpleasant wake-up and fatigue effects. Here, we report a theoretical strategy on the realization of robust ferroelectricity in HfO$_2$-based ferroelectrics by designing a series of epitaxial (HfO$_2$)$_1$/(CeO$_2$)$_1$ superlattices. The advantages of the designated ferroelectric superlattices are defects free, and most importantly, on the base of the thermodynamically stable monoclinic phase of HfO$_2$. Consequently, this allows the creation of superior ferroelectric properties with an electric polarization $>$25 $\mu$C/cm$^2$ and an ultralow polarization-switching energy barrier at $\sim$2.5 meV/atom. Our work may open an entirely new route towards the fabrication of high-performance HfO$_2$ based ferroelectric devices.


Fractional quantum Hall states with variational Projected Entangled-Pair States: a study of the bosonic Harper-Hofstadter model. (arXiv:2309.12811v1 [cond-mat.str-el])
Erik Lennart Weerda, Matteo Rizzi

An important class of model Hamiltonians for investigation of topological phases of matter consists of mobile, interacting particles on a lattice subject to a semi-classical gauge field, as exemplified by the bosonic Harper-Hofstadter model. A unique method for investigations of two-dimensional quantum systems are the infinite projected-entangled pair states (iPEPS), as they avoid spurious finite size effects that can alter the phase structure. However, due to no-go theorems in related cases this was often conjectured to be impossible in the past. In this letter, we show that upon variational optimization the infinite projected-entangled pair states can be used to this end, by identifying fractional Hall states in the bosonic Harper-Hofstadter model. The obtained states are characterized by showing exponential decay of bulk correlations, as dictated by a bulk gap, as well as chiral edge modes via the entanglement spectrum.


Matching the photocurrent of perovskite/organic tandem solar modules by varying the cell width. (arXiv:2309.12890v1 [cond-mat.mtrl-sci])
Jose Garcia Cerrillo (1), Andreas Distler (1), Fabio Matteocci (2), Karen Forberich (3), Michael Wagner (3), Robin Basu (1), Luigi Angelo Castriotta (2), Farshad Jafarzadeh (2), Francesca Brunetti (2), Fu Yang (4), Ning Li (1, 3, and 5), Asiel Neftali Corpus Mendoza (6), Aldo Di Carlo (2 and 7), Christoph J. Brabec (1 and 3), Hans-Joachim Egelhaaf (1 and 3) ((1) Institute of Materials for Electronics and Energy Technology (i-MEET) of the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), (2) Center for Hybrid and Organic Solar Energy (CHOSE) of the University of Rome Tor Vergata, (3) Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (HI-ERN), (4) Laboratory of Advanced Optoelectronic Materials of the Soochow University, (5) State Key Laboratory of Luminescent Materials and Devices of the South China University of Technology, (6) Instituto de Energias Renovables (IER) of the Universidad Nacional Autonoma de Mexico, (7) Institute of Structure of Matter (ISM) of the National Research Council (CNR))

Photocurrent matching in conventional monolithic tandem solar cells is achieved by choosing semiconductors with complementary absorption spectra and by carefully adjusting the optical properties of the complete top and bottom stacks. However, for thin film photovoltaic technologies at the module level, another design variable significantly alleviates the task of photocurrent matching, namely the cell width, whose modification can be readily realized by the adjustment of the module layout. Herein we demonstrate this concept at the experimental level for the first time for a 2T-mechanically stacked perovskite (FAPbBr3)/organic (PM6:Y6:PCBM) tandem mini-module, an unprecedented approach for these emergent photovoltaic technologies fabricated in an independent manner. An excellent Isc matching is achieved by tuning the cell widths of the perovskite and organic modules to 7.22 mm (PCEPVKT-mod= 6.69%) and 3.19 mm (PCEOPV-mod= 12.46%), respectively, leading to a champion efficiency of 14.94% for the tandem module interconnected in series with an aperture area of 20.25 cm2. Rather than demonstrating high efficiencies at the level of small lab cells, our successful experimental proof-of-concept at the module level proves to be particularly useful to couple devices with non-complementary semiconductors, either in series or in parallel electrical connection, hence overcoming the limitations imposed by the monolithic structure.


Non-Abelian dynamical gauge field and topological superfluids in optical Raman lattice. (arXiv:2309.12923v1 [cond-mat.quant-gas])
Xin-Chi Zhou, Tian-Hua Yang, Zhi-Yuan Wang, Xiong-Jun Liu

We propose an experimental scheme to realize non-Abelian dynamical gauge field for ultracold fermions, which induces a novel pairing mechanism of topological superfluidity. The dynamical gauge fields arise from nontrivial interplay effect between the strong Zeeman splitting and Hubbard interaction in a two-dimensional (2D) optical Raman lattice. The spin-flip transitions are forbidden by the large Zeeman detuning, but are restored when the Zeeman splitting is compensated by Hubbard interaction. This scheme allows to generate a dynamical non-Abelian gauge field that leads to a Dirac type correlated 2D spin-orbit interaction depending on local state configurations. The topological superfluid from a novel pairing driven by 2D dynamical gauge fields is reached, with analytic and numerical results being obtained. Our work may open up a door to emulate non-Abelian dynamical gauge fields and correlated topological phases with experimental feasibility.


Laser-induced real-space topology control of spin wave resonances. (arXiv:2309.12956v1 [cond-mat.mtrl-sci])
Tim Titze, Sabri Koraltan, Timo Schmidt, Marcel Möller, Florian Bruckner, Claas Abert, Dieter Suess, Claus Ropers, Daniel Steil, Manfred Albrecht, Stefan Mathias

Femtosecond laser excitation of materials that exhibit magnetic spin textures promises advanced magnetic control via the generation of ultrafast and non-equilibrium spin dynamics. We explore such possibilities in ferrimagnetic [Fe(0.35 nm)/Gd(0.40 nm)]$_{160}$ multilayers, which host a rich diversity of magnetic textures from stripe domains at low magnetic fields, a dense bubble/skyrmion lattice at intermediate fields, and a single domain state for high magnetic fields. Using femtosecond magneto-optics, we observe distinct coherent spin wave dynamics in response to a weak laser excitation allowing us to unambiguously identify the different magnetic spin textures. Moreover, employing strong laser excitation we show that we achieve versatile control of the coherent spin dynamics via non-equilibrium and ultrafast transformation of magnetic spin textures by both creating and annihilating bubbles/skyrmions. We corroborate our findings by micromagnetic simulations and by Lorentz transmission electron microscopy before and after laser exposure.


Higher-order Graph Convolutional Network with Flower-Petals Laplacians on Simplicial Complexes. (arXiv:2309.12971v1 [cs.LG])
Yiming Huang, Yujie Zeng, Qiang Wu, Linyuan Lü

Despite the recent successes of vanilla Graph Neural Networks (GNNs) on many tasks, their foundation on pairwise interaction networks inherently limits their capacity to discern latent higher-order interactions in complex systems. To bridge this capability gap, we propose a novel approach exploiting the rich mathematical theory of simplicial complexes (SCs) - a robust tool for modeling higher-order interactions. Current SC-based GNNs are burdened by high complexity and rigidity, and quantifying higher-order interaction strengths remains challenging. Innovatively, we present a higher-order Flower-Petals (FP) model, incorporating FP Laplacians into SCs. Further, we introduce a Higher-order Graph Convolutional Network (HiGCN) grounded in FP Laplacians, capable of discerning intrinsic features across varying topological scales. By employing learnable graph filters, a parameter group within each FP Laplacian domain, we can identify diverse patterns where the filters' weights serve as a quantifiable measure of higher-order interaction strengths. The theoretical underpinnings of HiGCN's advanced expressiveness are rigorously demonstrated. Additionally, our empirical investigations reveal that the proposed model accomplishes state-of-the-art (SOTA) performance on a range of graph tasks and provides a scalable and flexible solution to explore higher-order interactions in graphs.


Deep learning probability flows and entropy production rates in active matter. (arXiv:2309.12991v1 [cond-mat.stat-mech])
Nicholas M. Boffi, Eric Vanden-Eijnden

Active matter systems, from self-propelled colloids to motile bacteria, are characterized by the conversion of free energy into useful work at the microscopic scale. These systems generically involve physics beyond the reach of equilibrium statistical mechanics, and a persistent challenge has been to understand the nature of their nonequilibrium states. The entropy production rate and the magnitude of the steady-state probability current provide quantitative ways to do so by measuring the breakdown of time-reversal symmetry and the strength of nonequilibrium transport of measure. Yet, their efficient computation has remained elusive, as they depend on the system's unknown and high-dimensional probability density. Here, building upon recent advances in generative modeling, we develop a deep learning framework that estimates the score of this density. We show that the score, together with the microscopic equations of motion, gives direct access to the entropy production rate, the probability current, and their decomposition into local contributions from individual particles, spatial regions, and degrees of freedom. To represent the score, we introduce a novel, spatially-local transformer-based network architecture that learns high-order interactions between particles while respecting their underlying permutation symmetry. We demonstrate the broad utility and scalability of the method by applying it to several high-dimensional systems of interacting active particles undergoing motility-induced phase separation (MIPS). We show that a single instance of our network trained on a system of 4096 particles at one packing fraction can generalize to other regions of the phase diagram, including systems with as many as 32768 particles. We use this observation to quantify the spatial structure of the departure from equilibrium in MIPS as a function of the number of particles and the packing fraction.


Signatures of a Majorana-Fermi surface in the Kitaev magnet Ag$_3$LiIr$_2$O$_6$. (arXiv:2108.03246v2 [cond-mat.str-el] UPDATED)
Joshuah T. Heath, Faranak Bahrami, Sangyun Lee, Roman Movshovich, Xiao Chen, Fazel Tafti, Kevin S. Bedell

Detecting Majorana fermions in experimental realizations of the Kitaev honeycomb model is often complicated by non-trivial interactions inherent to potential spin liquid candidates. In this work, we identify several distinct thermodynamic signatures of massive, itinerant Majorana fermions within the well-established analytical paradigm of Landau-Fermi liquid theory. We find a qualitative and quantitative agreement between the salient features of our Landau-Majorana liquid theory and the Kitaev spin liquid candidate Ag$_3$LiIr$_2$O$_6$. Our study presents strong evidence for a Fermi liquid-like ground state in the fundamental excitations of a honeycomb iridate, and opens new experimental avenues to detect itinerant Majorana fermions in condensed matter systems.


Anisotropic Quantum Hall Droplets. (arXiv:2301.01726v3 [cond-mat.mes-hall] UPDATED)
Blagoje Oblak, Bastien Lapierre, Per Moosavi, Jean-Marie Stéphan, Benoit Estienne

We study two-dimensional (2D) droplets of noninteracting electrons in a strong magnetic field, placed in a confining potential with arbitrary shape. Using semiclassical methods adapted to the lowest Landau level, we obtain near-Gaussian energy eigenstates that are localized on level curves of the potential and have a position-dependent height. This one-particle insight allows us to deduce explicit formulas for expectation values of local many-body observables, such as density and current, in the thermodynamic limit. In particular, correlations along the edge are long-ranged and inhomogeneous. As we show, this is consistent with the system's universal low-energy description as a free 1D chiral conformal field theory of edge modes, known from earlier works in simple geometries. A delicate interplay between radial and angular dependencies of eigenfunctions ultimately ensures that the theory is homogeneous in terms of the canonical angle variable of the potential, despite its apparent inhomogeneity in terms of more na\"ive angular coordinates. Finally, we propose a scheme to measure the anisotropy by subjecting the droplet to microwave radiation; we compute the corresponding absorption rate and show that it depends on the droplet's shape and the waves' polarization. These results, both local and global, are likely to be observable in solid-state systems or quantum simulators of 2D electron gases with a high degree of control on the confining potential.


Two elementary band representation model, Fermi surface nesting, and surface topological superconductivity in $A$V$_{3}$Sb$_ {5}$ ($A = \text{K, Rb, Cs}$). (arXiv:2302.06211v2 [cond-mat.mtrl-sci] UPDATED)
Junze Deng, Ruihan Zhang, Yue Xie, Xianxin Wu, Zhijun Wang

The recently discovered vanadium-based Kagome metals $A$V$_{3}$Sb$_{5}$ ($A = \text{K, Rb, Cs}$) are of great interest with the interplay of charge density wave (CDW) order, band topology and superconductivity. In this paper, by identifying elementary band representations (EBRs), we construct a two-EBR graphene-Kagome model to capture the two low-energy van-Hove-singularity dispersions and, more importantly, the nontrivial band topology in these Kagome metals. This model consists of $A_g@3g$ (V-$d_{x^2-y^2/z^2}$, Kagome sites) and $A_2''@2d$ EBRs (Sb1-$p_z$, honeycomb sites). We have investigated the Fermi surface instability by calculating the electronic susceptibility $\chi(\mathbf{q})$. Prominent Fermi-surface nesting peaks are obtained at three L points, where the $z$ component of the nesting vector shows intimate relationship with the anticrossing point along M--L. The nesting peaks at L are consistent with the $2\times 2\times 2$ CDW reconstruction in these compounds. In addition, the sublattice-resolved bare susceptibility is calculated and similar sharp peaks are observed at the L points, indicating a strong antiferromagnetic fluctuation. Assuming a bulk $s$-wave superconducting pairing, helical surface states and nontrivial superconducting gap are obtained on the (001) surface. In analogous to FeTe$_{1-x}$Se$_{x}$ superconductor, our results establish another material realization of a stoichiometric superconductor with nontrivial band topology, providing a promising platform for studying exotic Majorana physics in condensed matter


Hybrid Topological Superconductivity and Hinge Majorana Flat Band in Type-II Dirac Semimetals. (arXiv:2303.11729v2 [cond-mat.supr-con] UPDATED)
Yue Xie, Xianxin Wu, Zhong Fang, Zhijun Wang

Type-II Dirac semimetals (DSMs) have a distinct Fermi surface topology, which allows them to host novel topological superconductivity (TSC) different from type-I DSMs. Depending on the relationship between intra- and inter-orbital electron-electron interactions, the phase diagram of superconductivity is obtained in type-II DSMs. We find that when the inter-orbital attraction is dominant, an unconventional inter-orbital intra-spin superconducting (SC) state ($B_{1u}$ and $B_{2u}$ pairing channels of $D_{4h}$ point group) is realized, yielding hybrid TSC, i.e., first- and second-order TSC exists at the same time. Further analysis reveals the Majorana flat bands on the $z$-directed hinges, which penetrate through the whole hinge Brillouin zone and link the projections of the surface helical Majorana cones at time-reversal-invariant momenta. These higher-order hinge modes are symmetry-protected and can even host strong stability against finite $C_{4z}$ rotation symmetry-breaking order. We suggest that experimental realization of these findings can be explored in transition metal dichalcogenides.


Differentiable graph-structured models for inverse design of lattice materials. (arXiv:2304.05422v2 [cond-mat.mtrl-sci] UPDATED)
Dominik Dold, Derek Aranguren van Egmond

Architected materials possessing physico-chemical properties adaptable to disparate environmental conditions embody a disruptive new domain of materials science. Fueled by advances in digital design and fabrication, materials shaped into lattice topologies enable a degree of property customization not afforded to bulk materials. A promising venue for inspiration toward their design is in the irregular micro-architectures of nature. However, the immense design variability unlocked by such irregularity is challenging to probe analytically. Here, we propose a new computational approach using graph-based representation for regular and irregular lattice materials. Our method uses differentiable message passing algorithms to calculate mechanical properties, therefore allowing automatic differentiation with surrogate derivatives to adjust both geometric structure and local attributes of individual lattice elements to achieve inversely designed materials with desired properties. We further introduce a graph neural network surrogate model for structural analysis at scale. The methodology is generalizable to any system representable as heterogeneous graphs.


Vorticity phase separation and defect lattices in the isotropic phase of active liquid crystals. (arXiv:2306.04526v2 [cond-mat.soft] UPDATED)
Fernando Caballero, Zhihong You, M. Cristina Marchetti

We use numerical simulations and linear stability analysis to study the dynamics of an active liquid crystal film on a substrate in the regime where the passive system would be isotropic. Extensile activity builds up local orientational order and destabilizes the quiescent isotropic state above a critical activity value, eventually resulting in spatiotemporal chaotic dynamics akin to the one observed ubiquitously in the nematic state. Here we show that tuning substrate friction yields a variety of emergent structures at intermediate activity, including lattices of flow vortices with associated regular arrangements of topological defects and a new state where flow vortices trap pairs of $+1/2$ defect that chase each other tail. These chiral units spontaneously pick the sense of rotation and organize in a hexagonal lattice, surrounded by a diffuse flow of opposite rotation to maintain zero net vorticity. The length scale of these emergent structures is set by the screening length $l_\eta=\sqrt{\eta/\Gamma}$ of the flow, controlled by the shear viscosity $\eta$ and the substrate friction $\Gamma$, and can be captured by simple mode selection of the vortical flows. We demonstrate that the emergence of coherent structures can be interpreted as a phase separation of vorticity, where friction plays a role akin to that of birth/death processes in breaking conservation of the phase separating species and selecting a characteristic scale for the patterns. Our work shows that friction provides an experimentally accessible tuning parameter for designing controlled active flows.


Trion resonance in polariton-electron scattering. (arXiv:2307.08244v2 [cond-mat.mes-hall] UPDATED)
Sangeet S. Kumar, Brendan C. Mulkerin, Meera M. Parish, Jesper Levinsen

Strong interactions between charges and light-matter coupled quasiparticles offer an intriguing prospect with applications from optoelectronics to light-induced superconductivity. Here, we investigate how the interactions between electrons and exciton-polaritons in a two-dimensional semiconductor microcavity can be resonantly enhanced due to a strong coupling to a trion, i.e., an electron-exciton bound state. We develop a microscopic theory that uses a strongly screened interaction between charges to enable the summation of all possible diagrams in the polariton-electron scattering process. The position and magnitude of the resonance is found to vary depending on the values of the light-matter coupling and detuning, thus indicating a large degree of tunability. We furthermore derive an analytic approximation of the interaction strength based on universal lowenergy scattering theory. This is found to match extremely well with our full calculation, indicating that the trion resonance is near universal, depending more on the strength of the light-matter coupling relative to the trion binding energy rather than on the details of the electronic interactions. Thus, we expect the trion resonance in polariton-electron scattering to appear in a broad range of microcavity systems with few semiconductor layers, such as doped monolayer MoSe2 where such resonances have recently been observed experimentally [Sidler et al., Nature Physics 13, 255 (2017)].


On the self-consistent Landauer-B\"uttiker formalism. (arXiv:2309.01564v2 [math-ph] UPDATED)
Horia D. Cornean, Giovanna Marcelli

We provide sufficient conditions such that the time evolution of a mesoscopic tight-binding open system with a local Hartree-Fock non-linearity converges to a self-consistent non-equilibrium steady state, which is independent of the initial condition from the "small sample". We also show that the steady charge current intensities are given by Landauer-B\"uttiker-like formulas, and make the connection with the case of weakly self-interacting many-body systems.


Electrostatic environment and Majorana bound states in full-shell topological insulator nanowires. (arXiv:2309.11149v2 [cond-mat.mes-hall] UPDATED)
Li Chen, Xiao-Hong Pan, Zhan Cao, Dong E. Liu, Xin Liu

The combination of a superconductor (SC) and a topological insulator (TI) nanowire was proposed as a potential candidate for realizing Majorana zero modes (MZMs). In this study, we adopt the Schr\"odinger-Poisson formalism to incorporate the electrostatic environment inside the nanowire and systematically explore its topological properties. Our calculations reveal that the proximity to the SC induces a band bending effect, leading to a non-uniform potential across the TI nanowire. As a consequence, there is an upward shift of the Fermi level within the conduction band. This gives rise to the coexistence of surface and bulk states, localized in an accumulation layer adjacent to the TI-SC interface. When magnetic flux is applied, these occupied states have different flux-penetration areas, suppressing the superconducting gap. However, this impact can be mitigated by increasing the radius of the nanowire. Finally, We demonstrate that MZMs can be achieved across a wide range of parameters centered around one applied flux quantum, $\phi_0 = h/2e$. Within this regime, MZMs can be realized even in the presence of conduction bands, which are not affected by the band bending effect. These findings provide valuable insights into the practical realization of MZMs in TI nanowire-based devices, especially in the presence of a complicated electrostatic environment.


Found 1 papers in sci-rep


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X-ray dynamical diffraction by quasi-monolayer graphene
Vyacheslav V. Lizunov

Scientific Reports, Published online: 24 September 2023; doi:10.1038/s41598-023-43269-6

X-ray dynamical diffraction by quasi-monolayer graphene