Found 32 papers in cond-mat
Date of feed: Mon, 11 Dec 2023 01:30:00 GMT

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Phonon topology and winding of spectral weight in graphite. (arXiv:2312.04575v1 [cond-mat.mes-hall])
N. D. Andriushin, A. S. Sukhanov, A. N. Korshunov, M. S. Pavlovskii, M. C. Rahn, S. E. Nikitin

The topology of electronic and phonon band structures of graphene is well studied and known to exhibit a Dirac cone at the K point of the Brillouin zone. Here, we applied inelastic x-ray scattering (IXS) along with $\textit{ab initio}$ calculations to investigate phonon topology in graphite, the 3D analogue of graphene. We identified a pair of modes that form a very weakly gapped linear anticrossing at the K point that can be essentially viewed as a Dirac cone approximant. The IXS intensity in the vicinity of the quasi-Dirac point reveals a harmonic modulation of the phonon spectral weight above and below the Dirac energy, which was previously proposed as an experimental fingerprint of the nontrivial topology. We illustrate how the topological winding of IXS intensity can be understood in terms of atomic displacements, and highlight that the intensity winding is not in fact sensitive in telling quasi- and true Dirac points apart.


Haldane Bundles: A Dataset for Learning to Predict the Chern Number of Line Bundles on the Torus. (arXiv:2312.04600v1 [cond-mat.mes-hall])
Cody Tipton, Elizabeth Coda, Davis Brown, Alyson Bittner, Jung Lee, Grayson Jorgenson, Tegan Emerson, Henry Kvinge

Characteristic classes, which are abstract topological invariants associated with vector bundles, have become an important notion in modern physics with surprising real-world consequences. As a representative example, the incredible properties of topological insulators, which are insulators in their bulk but conductors on their surface, can be completely characterized by a specific characteristic class associated with their electronic band structure, the first Chern class. Given their importance to next generation computing and the computational challenge of calculating them using first-principles approaches, there is a need to develop machine learning approaches to predict the characteristic classes associated with a material system. To aid in this program we introduce the {\emph{Haldane bundle dataset}}, which consists of synthetically generated complex line bundles on the $2$-torus. We envision this dataset, which is not as challenging as noisy and sparsely measured real-world datasets but (as we show) still difficult for off-the-shelf architectures, to be a testing ground for architectures that incorporate the rich topological and geometric priors underlying characteristic classes.


A holographic view of topological stabilizer codes. (arXiv:2312.04617v1 [cond-mat.str-el])
Thomas Schuster, Nathanan Tantivasadakarn, Ashvin Vishwanath, Norman Y. Yao

The bulk-boundary correspondence is a hallmark feature of topological phases of matter. Nonetheless, our understanding of the correspondence remains incomplete for phases with intrinsic topological order, and is nearly entirely lacking for more exotic phases, such as fractons. Intriguingly, for the former, recent work suggests that bulk topological order manifests in a non-local structure in the boundary Hilbert space; however, a concrete understanding of how and where this perspective applies remains limited. Here, we provide an explicit and general framework for understanding the bulk-boundary correspondence in Pauli topological stabilizer codes. We show -- for any boundary termination of any two-dimensional topological stabilizer code -- that the boundary Hilbert space cannot be realized via local degrees of freedom, in a manner precisely determined by the anyon data of the bulk topological order. We provide a simple method to compute this "obstruction" using a well-known mapping to polynomials over finite fields. Leveraging this mapping, we generalize our framework to fracton models in three-dimensions, including both the X-Cube model and Haah's code. An important consequence of our results is that the boundaries of topological phases can exhibit emergent symmetries that are impossible to otherwise achieve without an unrealistic degree of fine tuning. For instance, we show how linear and fractal subsystem symmetries naturally arise at the boundaries of fracton phases.


Decoherence through Ancilla Anyon Reservoirs. (arXiv:2312.04638v1 [cond-mat.str-el])
Nayan Myerson-Jain, Taylor L. Hughes, Cenke Xu

We explore the decoherence of the gapless/critical boundary of a topological order, through interactions with the bulk reservoir of "ancilla anyons." We take the critical boundary of the $2d$ toric code as an example. The intrinsic nonlocal nature of the anyons demands the strong and weak symmetry condition for the ordinary decoherence problem be extended to the strong or weak gauge invariance conditions. We demonstrate that in the $\textit{doubled}$ Hilbert space, the partition function of the boundary is mapped to two layers of the $2d$ critical Ising model with an inter-layer line defect that depends on the species of the anyons causing the decoherence. The line defects associated with the tunneling of bosonic $e$ and $m$ anyons are relevant, and result in long-range correlations for either the $e$ or $m$ anyon respectively on the boundary in the doubled Hilbert space. In contrast, the defect of the $f$ anyon is marginal and leads to a line of fixed points with varying effective central charges, and power-law correlations having continuously varying scaling dimensions. We also demonstrate that decoherence-analogues of Majorana zero modes are localized at the spatial interface of the relevant $e$ and $m$ anyon decoherence channels, which leads to a universal logarithmic scaling of the R\'enyi entropy of the boundary.


Analytical model and dynamical phase-field simulation of terahertz transmission across ferroelectrics. (arXiv:2312.04824v1 [cond-mat.mes-hall])
Taorui Chen, Bo Wang, Yujie Zhu, Shihao Zhuang, Long-Qing Chen, Jia-Mian Hu

We theoretically investigate the steady-state transmission of continuous terahertz (THz) wave across a freestanding ferroelectric slab. Based on the Landau-Ginzburg-Devonshire theory of ferroelectrics and the coupled equations of motion for polarization and electromagnetic (EM) waves, we derive the analytical expressions of the frequency- and thickness-dependent dielectric susceptibility and transmission coefficient at the thin slab limit in the harmonic excitation regime. When the slab thickness is much smaller than the THz wavelength in the ferroelectric, the analytical predictions agree well with the numerical simulations from a dynamical phase-field model that incorporates the coupled dynamics of strain, polarization, and EM wave in multiphase systems. At larger thicknesses, the transmission is mainly determined by the frequency-dependent attenuation of THz waves in the ferroelectric and the formation of a standing polarization/THz wave. Our results advance the understanding of the interaction between THz wave and ferroelectrics and suggest the potential of exploiting ferroelectrics to achieve low-heat-dissipation, nonvolatile voltage modulation of THz transmission for high-data-rate wireless communication.


Low Resistance Ohmic Contact to P-type Monolayer WSe2. (arXiv:2312.04849v1 [cond-mat.mes-hall])
Jingxu Xie, Zuocheng Zhang, Haodong Zhang, Vikram Nagarajan, Wenyu Zhao, Haleem Kim, Collin Sanborn, Ruishi Qi, Sudi Chen, Salman Kahn, Kenji Watanabe, Takashi Taniguchi, Alex Zettl, Michael Crommie, James Analytis, Feng Wang

Advanced microelectronics in the future may require semiconducting channel materials beyond silicon. Two-dimensional (2D) semiconductors, characterized by their atomically thin thickness, hold immense promise for high-performance electronic devices at the nanometer scale with lower heat dissipation. One challenge for achieving high-performance 2D semiconductor field effect transistors (FET), especially for p-type materials, is the high electrical contact resistance present at the metal-semiconductor interface. In conventional bulk semiconductors, low resistance ohmic contact is realized through heavy substitutional doping with acceptor or donor impurities at the contact region. The strategy of substitutional doping, however, does not work for p-type 2D semiconductors such as monolayer tungsten diselenide (WSe$_2$).In this study, we developed highly efficient charge-transfer doping with WSe$_2$/$\alpha$-RuCl$_3$ heterostructures to achieve low-resistance ohmic contact for p-type WSe$_2$ transistors. We show that a hole doping as high as 3$\times$10$^{13}$ cm$^{-2}$ can be achieved in the WSe$_2/\alpha$-RuCl$_3$ heterostructure due to its type-III band alignment. It results in an Ohmic contact with resistance lower than 4 k Ohm $\mu$m at the p-type monolayer WSe$_2$/metal junction. at room temperature. Using this low-resistance contact, we demonstrate high-performance p-type WSe$_2$ transistors with a saturation current of 35 $\mu$A$\cdot$ $\mu$m$^{-1}$ and an I$_{ON}$/I$_{OFF}$ ratio exceeding 10$^9$ It could enable future microelectronic devices based on 2D semiconductors and contribute to the extension of Moore's law.


Enhancement of Goos-H\"{a}nchen shifts in graphene through Fizeau drag effect. (arXiv:2312.04850v1 [cond-mat.mes-hall])
Rafi Ud Din, Muzamil Shah, Reza Asgari, Gao Xianlong

We investigate the Goos-H\"{a}nchen shifts in reflection for a light beam within a graphene structure, utilizing the Fizeau drag effect induced by its massless Dirac electrons in incident light. The magnitudes of spatial and angular shifts for a light beam propagating against the direction of drifting electrons are significantly enhanced, while shifts for a beam co-propagating with the drifting electrons are suppressed. The Goos-H\"{a}nchen shifts exhibit augmentation with increasing drift velocities of electrons in graphene. The impact of incident wavelength on the angular and spatial shifts in reflection is discussed. Furthermore, the study highlights the crucial roles of the density of charged particles in graphene, the particle relaxation time, and the thickness of the graphene in manipulating the drag-affected Goos-H\"{a}nchen shifts. This investigation offers valuable insights for efficiently guiding light in graphene structures under the influence of the Fizeau drag effect.


Electron Magneto-Hydrodynamics in Graphene. (arXiv:2312.04896v1 [cond-mat.mes-hall])
Jack N. Engdahl, Aydın Cem Keser, Thomas Schmidt, Oleg P. Sushkov

We consider the hydrodynamic flow of an electron fluid in a channel formed in a two-dimensional electron gas (2DEG) with no-slip boundary conditions. To generate vorticity in the fluid the flow is influenced by an array of micromagnets on the top of 2DEG. We analyse the viscous boundary layer and demonstrate anti-Poiseuille behaviour in this region. Furthermore we predict a longitudinal Hall effect, where a periodic magnetic field generates a voltage modulation in the direction of transport. From the experimental point of view we propose a method for a precise measurements of the properties of different electron fluids. The results are applicable to graphene away from the charge neutrality point and to semiconductors.


Gate-controlled neuromorphic functional transition in an electrochemical graphene transistor. (arXiv:2312.04934v1 [physics.app-ph])
Chenglin Yu, Shaorui Li, Yongchao Wang, Siyi Zhou, Zhiting Gao, Kaili Jiang, Yayu Wang, Jinsong Zhang

Neuromorphic devices have gained significant attention as potential building blocks for the next generation of computing technologies owing to their ability to emulate the functionalities of biological nervous systems. The essential components in artificial neural network such as synapses and neurons are predominantly implemented by dedicated devices with specific functionalities. In this work, we present a gate-controlled transition of neuromorphic functions between artificial neurons and synapses in monolayer graphene transistors that can be employed as memtransistors or synaptic transistors as required. By harnessing the reliability of reversible electrochemical reactions between C atoms and hydrogen ions, the electric conductivity of graphene transistors can be effectively manipulated, resulting in high on/off resistance ratio, well-defined set/reset voltage, and prolonged retention time. Furthermore, the gate-controlled linear response of set/reset voltage in memtransistors facilitates a more versatile way to regulate the synaptic weight and neural spiking. Overall, the on-demand switching of neuromorphic functions in a single graphene transistor provides a promising opportunity to develop adaptive neural networks for the upcoming era of artificial intelligence and machine learning.


Beyond spin models in orbitally-degenerate open-shell nanographenes. (arXiv:2312.04938v1 [cond-mat.mes-hall])
J. C. G. Henriques, D. Jacob, A. Molina-Sánchez, G. Catarina, A. T. Costa, J. Fernández-Rossier

The study of open-shell nanographenes has relied on a paradigm where spins are the only low-energy degrees of freedom. Here we show that some nanographenes can host low-energy excitations that include strongly coupled spin and orbital degrees of freedom. The key ingredient is the existence of orbital degeneracy, as a consequence of leaving the benzenoid/half-filling scenario. We analyze the case of nitrogen-doped triangulenes, using both density-functional theory and Hubbard model multiconfigurational and random-phase approximation calculations. We find a rich interplay between orbital and spin degrees of freedom that confirms the need to go beyond the spin-only paradigm, opening a new venue in this field of research.


Dirac plasmon polaritons and magnetic modes in topological-insulator nanoparticles. (arXiv:2312.04958v1 [cond-mat.mes-hall])
Nikolaos Kyvelos, Vassilios Yannopapas, N. Asger Mortensen, Christos Tserkezis

We report the existence of previously unreported magnetic modes with record-high magnetic Purcell factors in topological-insulator nanospheres. Focusing on Bi$_{2}$Se$_{3}$, and based on full electromagnetic Mie theory, we find magnetic modes arising from the surface current on the conductive surface of the topological insulator due to the existence of delocalized surface states. These currents are induced by electrons in the topologically protected states within the Dirac cone. Furthermore, we demonstrate that the Dirac plasmon polaritons resulting from the interaction between THz photons and Dirac electrons dramatically influence both the electric and the magnetic transitions of quantum emitters placed near Bi$_2$Se$_3$ nanospheres, providing significantly enhanced Purcell factors and entering the strong-coupling regime. These findings indicate that Bi$_{2}$Se$_{3}$ nanospheres exhibit a rich optical response, stemming from both bulk and topologically protected surface states, making them promising candidates for enhancing strong light--matter interactions in the fields of nanophotonics and THz technologies.


Detecting defect dynamics in relativistic field theories far from equilibrium using topological data analysis. (arXiv:2312.04959v1 [hep-ph])
Viktoria Noel, Daniel Spitz

We study nonequilibrium dynamics of relativistic $N$-component scalar field theories in Minkowski space-time in a classical-statistical regime, where typical occupation numbers of modes are much larger than unity. In this strongly correlated system far from equilibrium, the role of different phenomena such as nonlinear wave propagation and defect dynamics remains to be clarified. We employ persistent homology to infer topological features of the nonequilibrium many-body system for different numbers of field components $N$ via a hierarchy of cubical complexes. Specifically, we show that the persistent homology of local energy density fluctuations can give rise to signatures of self-similar scaling associated with topological defects, distinct from the scaling behaviour of nonlinear wave modes. This contributes to the systematic understanding of the role of topological defects for far-from-equilibrium time evolutions of nonlinear many-body systems.


Toward Modeling the Structure of Electrolytes at Charged Mineral Interfaces using Classical Density Functional Theory. (arXiv:2312.04976v1 [cond-mat.soft])
Thomas Petersen

The organization of water molecules and ions between charged mineral surfaces determines the stability of colloidal suspensions and the strength of phase-separated particulate gels. In this article we assemble a density functional that measures the free energy due to the interaction of water molecules and ions near electric double layers. The model accounts for the finite size of the particles using fundamental measure theory, hydrogen-bonding between water molecules using Wertheim's statistical association theory, long-range dispersion interactions using Barker and Henderson's high temperature expansion, electrostatic correlations using a functionalized mean-spherical approximation, and Coulomb forces through the Poisson equation. These contributions are shown to produce highly correlated structures, aptly rendering the layering of counter-ions and co-ions at highly charged surfaces, and permitting the solvation of ions and surfaces to be measured by a combination of short-ranged association and long-ranged attraction. The model is tested in a planar geometry near soft, charged surfaces to reproduce the structure of water near graphene and mica. For mica surfaces, explicitly representing the density of the outer oxygen layer of the exposed silica tetrahedra in the domain permits water molecules to hydrogen-bind to the surface. When electrostatic interactions are included, water molecules assume a hybrid character, being accounted for implicitly in the dielectric constant but explicitly otherwise. The disjoining pressure between approaching like-charged surfaces is calculated, demonstrating the model's ability to probe pressure oscillations that arise during the expulsion of ions and water layers from the interfacial gap, and predict the strong inter-attractive stresses that form at narrow interfacial spacing when the surface charge is overscreened.


Quasiparticles-mediated thermal diode effect in Weyl Josephson junctions. (arXiv:2312.05008v1 [cond-mat.supr-con])
Pritam Chatterjee, Paramita Dutta

We theoretically show quasiparticles-driven thermal diode effect (TDE) in an inversion symmetry-broken (ISB) Weyl superconductor (WSC)-Weyl semimetal (WSM)-WSC Josephson junction. A Zeeman field perpendicular to the WSM region of the thermally-biased Weyl Josephson junction (WJJ) induces an asymmetry between the forward and reverse thermal currents, which is responsible for the TDE. Most interestingly, we show that the sign and magnitude of the thermal diode rectification coefficient is highly tunable by the superconducting phase difference and external Zeeman field, and also strongly depends on the junction length. The tunability of the rectification, particularly, the sign changing behavior associated with higher rectification enhances the potential of our WJJ thermal diode to use as functional switching components in thermal devices.


Proximity-induced nonlinear magnetoresistances on topological insulators. (arXiv:2312.05035v1 [cond-mat.mes-hall])
M. Mehraeen, Steven S.-L. Zhang

We employ quadratic-response Kubo formulas to investigate the nonlinear magnetotransport in bilayers composed of a topological insulator and a magnetic insulator, and predict both unidirectional magnetoresistance and nonlinear planar Hall effects driven by interfacial disorder and spin-orbit scattering. These effects exhibit strong dependencies on the Fermi energy relative to the strength of the exchange interaction between the spins of Dirac electrons and the interfacial magnetization. In particular, as the Fermi energy becomes comparable to the exchange energy, the nonlinear magnetotransport coefficients can be greatly amplified and their dependencies on the magnetization orientation deviate significantly from conventional sinusoidal behavior. These findings may not only deepen our understanding of the origin of nonlinear magnetotransport in magnetic topological systems but also open new pathways to probe the Fermi and exchange energies via transport measurements.


Electron-hole collision-limited resistance of gapped graphene. (arXiv:2312.05066v1 [cond-mat.mes-hall])
Arseny Gribachov, Vladimir Vyurkov, Dmitry Svintsov

Collisions between electrons and holes can dominate the carrier scattering in clean graphene samples in the vicinity of charge neutrality point. While electron-hole limited resistance in pristine gapless graphene is well-studied, its evolution with induction of band gap $E_g$ is less explored. Here, we derive the functional dependence of electron-hole limited resistance of gapped graphene $\rho_{eh}$ on the ratio of gap and thermal energy $E_g/kT$. At low temperatures and large band gaps, the resistance grows linearly with $E_g/kT$, and possesses a minimum at $E_g \approx 2.5 kT$. This contrast to the Arrhenius activation-type behaviour for intrinsic semiconductors. Introduction of impurities restores the Arrhenius law for resistivity at low temperatures and/or high doping densities. The hallmark of electron-hole collision effects in graphene resistivity at charge neutrality is the crossover between exponential and power-law resistivity scalings with temperature.


Scaling of Hybrid QDs-Graphene Photodetectors to Subwavelength Dimension. (arXiv:2312.05083v1 [cond-mat.mtrl-sci])
Gökhan Kara, Patrik Rohner, Erfu Wu, Dmitry N. Dirin, Roman Furrer, Dimos Poulikakos, Maksym V. Kovalenko, Michel Calame, Ivan Shorubalko

Emerging colloidal quantum dot (cQD) photodetectors currently challenge established state-of-the-art infrared photodetectors in response speed, spectral tunability, simplicity of solution processable fabrication, and integration onto curved or flexible substrates. Hybrid phototransistors based on 2D materials and cQDs, in particular, are promising due to their inherent photogain enabling direct photosignal enhancement. The photogain is sensitive to both, measurement conditions and photodetector geometry. This makes the cross-comparison of devices reported in the literature rather involved. Here, the effect of device length L and width W scaling to subwavelength dimensions (sizes down to 500 nm) on the photoresponse of graphene-PbS cQD phototransistors was experimentally investigated. Photogain and responsivity were found to scale with 1/LW, whereas the photocurrent and specific detectivity were independent of geometrical parameters. The measurements were performed at scaled bias voltage conditions for comparable currents. Contact effects were found to limit the photoresponse for devices with L < 3 {\mu}m. The relation of gate voltage, bias current, light intensity, and frequency on the photoresponse was investigated in detail, and a photogating efficiency to assess the cQD-graphene interface is presented. In particular, the specific detectivity values in the range between 10^8 to 10^9 Jones (wavelength of 1550 nm, frequency 6 Hz, room temperature) were found to be limited by the charge transfer across the photoactive interface.


Manipulating Topological Properties in Bi$_2$Se$_3$/BiSe/TMDC Heterostructures with Interface Charge Transfer. (arXiv:2312.05112v1 [cond-mat.mtrl-sci])
Xuance Jiang, Turgut Yilmaz, Elio Vescovo, Deyu Lu

Heterostructures of topological insulator Bi$_2$Se$_3$ on transition metal dichalcogenides (TMDCs) offer a new materials platform for studying novel quantum states by exploiting the interplay among topological orders, charge orders and magnetic orders. The diverse interface attributes, such as material combination, charge re-arrangement, defect and strain, can be utilized to manipulate the quantum properties of this class of materials. Recent experiments of Bi$_2$Se$_3$/NbSe$_2$ heterostructures show signatures of strong Rashba band splitting due to the presence of a BiSe buffer layer, but the atomic level mechanism is not fully understood. We conduct first-principles studies of the Bi$_2$Se$_3$/BiSe/TMDC heterostructures with five different TMDC substrates (1T phase VSe$_2$, MoSe$_2$, TiSe$_2$, and 2H phase NbSe$_2$, MoSe$_2$). We find significant charge transfer at both BiSe/TMDC and Bi$_2$Se$_3$/BiSe interfaces driven by the work function difference, which stabilizes the BiSe layer as an electron donor and creates interface dipole. The electric field of the interface dipole breaks the inversion symmetry in the Bi$_2$Se$_3$ layer, leading to the giant Rashba band splitting in two quintuple layers and the recovery of the Dirac point in three quintuple layers, with the latter otherwise only occurring in thicker samples with at least six Bi$_2$Se$_3$ quintuple layers. Besides, we find that strain can significantly affect the charge transfer at the interfaces. Our study presents a promising avenue for tuning topological properties in heterostructures of two-dimensional materials, with potential applications in quantum devices.


Superconducting Penetration Depth Through a Van Hove Singularity: Sr$_2$RuO$_4$ Under Uniaxial Stress. (arXiv:2312.05130v1 [cond-mat.supr-con])
Eli Mueller, Yusuke Iguchi, Fabian Jerzembeck, Jorge O. Rodriguez, Marisa Romanelli, Edgar Abarca-Morales, Anastasios Markou, Naoki Kikugawa, Dmitry A. Sokolov, Gwansuk Oh, Clifford W. Hicks, Andrew P. Mackenzie, Yoshiteru Maeno, Vidya Madhavan, Kathryn A. Moler

In the unconventional superconductor Sr$_2$RuO$_4$, uniaxial stress along the $[100]$ direction tunes the Fermi level through a Van Hove singularity (VHS) in the density of states, causing a strong enhancement of the superconducting critical temperature $T_\textrm{c}$. Here, we report measurements of the London penetration depth $\lambda$ as this tuning is performed. We find that the zero-temperature superfluid density, here defined as $\lambda(0)^{-2}$, increases by $\sim$15%, with a peak that coincides with the peak in $T_\textrm{c}$. We also find that the low temperature form of $\lambda(T)$ is quadratic over the entire strain range. Using scanning tunneling microscopy, we find that the gap increases from $\Delta_0 \approx 350~\mu$eV in unstressed Sr$_2$RuO$_4$ to $\Delta_0 \approx 600~\mu$eV in a sample strained to near the peak in $T_c$. With a nodal order parameter, an increase in the superconducting gap could bring about an increase in the superfluid density through reduced sensitivity to defects and through reduced non-local effects in the Meissner screening. Our data indicate that tuning to the VHS increases the gap throughout the Brillouin zone, and that non-local effects are likely more important than reduced scattering.


Twist driven deep-ultraviolet-wavelength exciton funnel effect in bilayer boron nitride. (arXiv:2312.05135v1 [cond-mat.mtrl-sci])
Linghan Zhu, Yizhou Wang, Li Yang

Realizing direct-bandgap quantum dots working within the deep-ultraviolet frequency is highly desired for electro-optical and biomedical applications while remaining challenging. In this work, we combine the first-principles many-body perturbation theory and effective Hamiltonian approximation to propose the realization of arrays of deep-ultraviolet excitonic quantum dots in twisted bilayer hexagonal boron nitride. The effective quantum confinement of excitons can reach ~400 meV within small twisting angles, which is about four times larger than those observed in twisted semiconducting transitional metal dichalcogenides. Especially because of enhanced electron-hole attraction, those excitons will accumulate via the so-call exciton funnel effect to the direct-bandgap regime, giving the possibility to better luminescence performance and manipulating coherent arrays of deep-ultraviolet quantum dots.


Detecting Atomic Scale Surface Defects in STM of TMDs with Ensemble Deep Learning. (arXiv:2312.05160v1 [cond-mat.mtrl-sci])
Darian Smalley (1 and 2), Stephanie D. Lough (1 and 2), Luke Holtzman (3), Kaikui Xu (4), Madisen Holbrook (3), Matthew R. Rosenberger (4), J.C. Hone (3), Katayun Barmak (3), Masahiro Ishigami (1 and 2) ((1) Department of Physics, University of Central Florida, (2) NanoScience Technology Center, University of Central Florida, (3) Department of Applied Physics and Applied Mathematics, University of Columbia, (4) Department of Aerospace and Mechanical Engineering, University of Notre Dame)

Atomic-scale defect detection is shown in scanning tunneling microscopy images of single crystal WSe2 using an ensemble of U-Net-like convolutional neural networks. Standard deep learning test metrics indicated good detection performance with an average F1 score of 0.66 and demonstrated ensemble generalization to C-AFM images of WSe2 and STM images of MoSe2. Defect coordinates were automatically extracted from defect detections maps showing that STM image analysis enhanced by machine learning can be used to dramatically increase sample characterization throughput.


Dynamical Signatures of Symmetry Broken and Liquid Phases in an $S=1/2$ Heisenberg Antiferromagnet on the Triangular Lattice. (arXiv:2209.03344v2 [cond-mat.str-el] UPDATED)
Markus Drescher, Laurens Vanderstraeten, Roderich Moessner, Frank Pollmann

We present the dynamical spin structure factor of the antiferromagnetic spin-$\frac{1}{2}$ $J_1-J_2$ Heisenberg model on a triangular lattice obtained from large-scale matrix-product state simulations. The high frustration due to the combination of antiferromagnetic nearest and next-to-nearest neighbour interactions yields a rich phase diagram. We resolve the low-energy excitations both in the $120^{\circ}$-ordered phase and in the putative spin liquid phase at $J_2/J_1 = 0.125$. In the ordered phase, we observe an avoided decay of the lowest magnon-branch, demonstrating the robustness of this phenomenon in the presence of gapless excitations. Our findings in the spin-liquid phase chime with the field-theoretical predictions for a gapless Dirac spin liquid, in particular the picture of low-lying monopole excitations at the corners of the Brillouin zone. We comment on possible practical difficulties of distinguishing proximate liquid and solid phases based on the dynamical structure factor.


Revisiting Bloch electrons in magnetic field: Hofstadter physics via hybrid Wannier states. (arXiv:2303.16347v4 [cond-mat.mes-hall] UPDATED)
Xiaoyu Wang, Oskar Vafek

We revisit the Hofstadter butterfly for a subset of topologically trivial Bloch bands arising from a continuum free electron Hamiltonian in a periodic lattice potential. We employ the recently developed procedure -- which was previously used to analyze the case of topologically non-trivial bands [\href{https://journals.aps.org/prb/abstract/10.1103/PhysRevB.106.L121111}{Phys. Rev. B \textbf{106}, L121111 (2022)}] -- to construct the finite field Hilbert space from the zero-field hybrid Wannier basis states. Such states are Bloch extended along one direction and exponentially localized along the other. The method is illustrated for square and triangular lattice potentials and is shown to reproduce all the main features of the Hofstadter spectrum obtained from a numerically exact Landau level expansion method.

In the regime when magnetic length is much longer than the spatial extent of the hybrid Wannier state in the localized direction we recover the well known Harper equation. Because the method applies to both topologically trivial and non-trivial bands, it provides an alternative and efficient approach to moir\'e materials in magnetic field.


Inversion symmetry breaking in the probability density by surface-bulk hybridization in topological insulators. (arXiv:2306.09601v2 [cond-mat.mes-hall] UPDATED)
Jorge David Castaño-Yepes, Enrique Muñoz

We analyze the probability density distribution in a topological insulator slab of finite thickness, where the bulk and surface states are allowed to hybridize. By using an effective continuum Hamiltonian approach as a theoretical framework, we analytically obtained the wave functions for each state near the $\Gamma$-point. Our results reveal that, under particular combinations of the hybridized bulk and surface states, the spatial symmetry of the electronic probability density with respect to the center of the slab can be spontaneously broken. This symmetry breaking arises as a combination of the parity of the solutions, their spin projection, and the material constants.


Nontrivial Aharonov-Bohm effect and alternating dispersion of magnons in cone-state ferromagnetic rings. (arXiv:2308.08486v3 [cond-mat.mes-hall] UPDATED)
Vera Uzunova, Lukas Körber, Agapi Kavvadia, Gwendolyn Quasebarth, Helmut Schultheiss, Attila Kákay, Boris Ivanov

Soft magnetic dots in the form of thin rings have unique topological properties. They can be in a vortex state with no vortex core. Here, we study the magnon modes of such systems both analytically and numerically. In an external magnetic field, magnetic rings are characterized by easy-cone magnetization and shows a giant splitting of doublets for modes with the opposite value of the azimuthal mode quantum number. The effect of the splitting can be refereed as a magnon analog of the topology-induced Aharonov-Bohm effect. For this we develop an analytical theory to describe the non-monotonic dependence of the mode frequencies on the azimuthal mode number, influenced by the balance between the local exchange and non-local dipole interactions.


Topological analog of the magnetic bit within the Su-Schrieffer-Heeger-Holstein model. (arXiv:2308.11099v2 [cond-mat.mtrl-sci] UPDATED)
Xinyuan Xu, David Sénéchal, Ion Garate

In magnetic memories, the state of a ferromagnet is encoded in the orientation of its magnetization. The energy of the system is minimized when the magnetization is parallel or antiparallel to a preferred (easy) axis. These two stable directions define the logical bit. Under an external perturbation, the direction of magnetization can be controllably reversed and thus the bit flipped. Here, we theoretically design a topological analogue of the magnetic bit in the Su-Schrieffer-Heeger (SSH)-Holstein model, where we show that a transient external perturbation can lead to a permanent change in the electronic band topology.


Steering-induced phase transition in measurement-only quantum circuits. (arXiv:2309.01315v3 [quant-ph] UPDATED)
Dongheng Qian, Jing Wang

Competing measurements alone can give rise to distinct phases characterized by entanglement entropy$\unicode{x2013}$such as the volume law phase, symmetry-breaking (SB) phase, and symmetry-protected topological (SPT) phase$\unicode{x2013}$that can only be discerned through quantum trajectories, making them challenging to observe experimentally. In another burgeoning area of research, recent studies have demonstrated that steering can give rise to additional phases within quantum circuits. In this work, we show that new phases can appear in measurement-only quantum circuit with steering. Unlike conventional steering methods that rely solely on local information, the steering scheme we introduce requires the circuit's structure as an additional input. These steering induced phases are termed as "informative" phases. They are distinguished by the intrinsic dimension of the bitstrings measured in each circuit run, making them substantially easier to detect in experimental setups. We explicitly show this phase transition by numerical simulation in three circuit models that are previously well-studied: projective transverse field Ising model, lattice gauge-Higgs model and XZZX model. When the informative phase coincides with the SB phase, our steering mechanism effectively serves as a "pre-selection" routine, making the SB phase more experimentally accessible. Additionally, an intermediate phase may manifest, where a discrepancy arises between the quantum information captured by entanglement entropy and the classical information conveyed by bitstrings. Our findings demonstrate that steering not only adds theoretical richness but also offers practical advantages in the study of measurement-only quantum circuits.


Engineering rich two-dimensional higher-order topological phases by flux and periodic driving. (arXiv:2309.01499v4 [cond-mat.mes-hall] UPDATED)
Ming-Jian Gao, Jun-Hong An

Nodal-line semimetals are commonly believed to exist in $\mathcal{PT}$ symmetric or mirror-rotation symmetric systems. Here, we find a flux-induced parameter-dimensional second-order nodal-line semimetal (SONLS) in a two-dimensional system without $\mathcal{PT}$ and mirror-rotation symmetries. It has coexisting hinge Fermi arcs and drumhead surface states. Meanwhile, we discover a flux-induced second-order topological insulator (SOTI). We then propose a Floquet engineering scheme to create exotic parameter-dimensional hybrid-order nodal-line semimetals with abundant nodal-line structures and widely tunable numbers of corner states in a SONLS and SOTI, respectively. Our results break the perception of SONLSs and supply a convenient way to artificially synthesize exotic topological phases by periodic driving.


Predicting the mechanical properties of spring networks. (arXiv:2309.07844v4 [cond-mat.soft] UPDATED)
Doron Grossman, Arezki Boudaoud

The elastic response of mechanical, chemical, and biological systems is often modeled using a discrete arrangement of Hookean springs, either representing finite material elements or even the molecular bonds of a system. However, to date, there is no direct derivation of the relation between a general discrete spring network and it's corresponding elastic continuum. Furthermore, understanding the network's mechanical response requires simulations that may be expensive computationally. Here we report a method to derive the exact elastic continuum model of any discrete network of springs, requiring network geometry and topology only. We identify and calculate the so-called "non-affine" displacements. Explicit comparison of our calculations to simulations of different crystalline and disordered configurations, shows we successfully capture the mechanics even of auxetic materials. Our method is valid for residually stressed systems with non-trivial geometries, is easily generalizable to other discrete models, and opens the possibility of a rational design of elastic systems.


Theory of correlated Chern insulators in twisted bilayer graphene. (arXiv:2310.15982v2 [cond-mat.mes-hall] UPDATED)
Xiaoyu Wang, Oskar Vafek

Magic-angle twisted bilayer graphene is the best studied physical platform featuring moire potential induced narrow bands with non-trivial topology and strong electronic correlations. Despite their significance, the Chern insulating states observed at a finite magnetic field -- and extrapolating to a band filling, $s$, at zero field -- remain poorly understood. Unraveling their nature is among the most important open problems in the province of moir\'e materials. Here we present the first comprehensive study of interacting electrons in finite magnetic field while varying the electron density, twist angle and heterostrain. Within a panoply of correlated Chern phases emerging at a range of twist angles, we uncover a unified description for the ubiquitous sequence of states with the Chern number $t$ for $(s,t)=\pm (0,4), \pm(1,3),\pm(2,2)$ and $\pm(3,1)$. We also find correlated Chern insulators at unconventional sequences with $s+t\neq \pm 4$, as well as with fractional $s$, and elucidate their nature.


Harnessing excitons at the nanoscale -- photoelectrical platform for quantitative sensing and imaging. (arXiv:2311.04211v2 [cond-mat.mes-hall] UPDATED)
Zhurun Ji, Mark E. Barber, Ziyan Zhu, Carlos R. Kometter, Jiachen Yu, Kenji Watanabe, Takashi Taniguchi, Mengkun Liu, Thomas P. Devereaux, Benjamin E. Feldman, Zhixun Shen

Excitons -- quasiparticles formed by the binding of an electron and a hole through electrostatic attraction -- hold promise in the fields of quantum light confinement and optoelectronic sensing. Atomically thin transition metal dichalcogenides (TMDs) provide a versatile platform for hosting and manipulating excitons, given their robust Coulomb interactions and exceptional sensitivity to dielectric environments. In this study, we introduce a cryogenic scanning probe photoelectrical sensing platform, termed exciton-resonant microwave impedance microscopy (ER-MIM). ER-MIM enables ultra-sensitive probing of exciton polarons and their Rydberg states at the nanoscale. Utilizing this technique, we explore the interplay between excitons and material properties, including carrier density, in-plane electric field, and dielectric screening. Furthermore, we employ deep learning for automated data analysis and quantitative extraction of electrical information, unveiling the potential of exciton-assisted nano-electrometry. Our findings establish an invaluable sensing platform and readout mechanism, advancing our understanding of exciton excitations and their applications in the quantum realm.


Chiral gauge theory at the boundary between topological phases. (arXiv:2312.01494v2 [hep-lat] UPDATED)
David B. Kaplan

I show how chiral fermions with an exact gauge symmetry in any representation can appear on the d-dimensional boundary of a finite volume (d + 1)-dimensional manifold, without any light mirror partners. The condition for it to look like a local d-dimensional theory is that gauge anomalies cancel, and that the volume be large. This provides a new paradigm for the lattice regularization of chiral gauge theories.