Found 33 papers in cond-mat
Date of feed: Mon, 28 Aug 2023 00:30:00 GMT

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Stability of topological superconducting qubits with number conservation. (arXiv:2308.12995v1 [cond-mat.supr-con])
Matthew F. Lapa, Michael Levin

The study of topological superconductivity is largely based on the analysis of simple mean-field models that do not conserve particle number. A major open question in the field is whether the remarkable properties of these mean-field models persist in more realistic models with a conserved total particle number and long-range interactions. For applications to quantum computation, two key properties that one would like to verify in more realistic models are (i) the existence of a set of low-energy states (the qubit states) that are separated from the rest of the spectrum by a finite energy gap, and (ii) an exponentially small (in system size) bound on the splitting of the energies of the qubit states. It is well known that these properties hold for mean-field models, but so far only property (i) has been verified in a number-conserving model. In this work we fill this gap by rigorously establishing both properties (i) and (ii) for a number-conserving toy model of two topological superconducting wires coupled to a single bulk superconductor. Our result holds in a broad region of the parameter space of this model, suggesting that properties (i) and (ii) are robust properties of number-conserving models, and not just artifacts of the mean-field approximation.

Anomaly Enforced Gaplessness and Symmetry Fractionalization for $Spin_G$ Symmetries. (arXiv:2308.12999v1 [hep-th])
T. Daniel Brennan

Symmetries and their anomalies give strong constraints on renormalization group (RG) flows of quantum field theories. Recently, the identification of a theory's global symmetries with its topological sector has provided additional constraints on RG flows to symmetry preserving gapped phases due to mathematical results in category and topological quantum field theory. In this paper, we derive constraints on RG flows from $\mathbb{Z}_2$-valued pure- and mixed-gravitational anomalies that can only be activated on non-spin manifolds. We show that such anomalies cannot be matched by a unitary, symmetry preserving gapped phase without symmetry fractionalization. In particular, we discuss examples that commonly arise in $4d$ gauge theories with fermions.

Magneto-optical anisotropies of 2D antiferromagnetic MPX$_3$ from first principles. (arXiv:2308.13109v1 [cond-mat.mtrl-sci])
Miłosz Rybak, Paulo E. Faria Junior, Tomasz Woźniak, Paweł Scharoch, Jaroslav Fabian, Magdalena Birowska

Here we systematically investigate the impact of the spin direction on the electronic and optical properties of transition metal phosphorus trichalcogenides (MPX$_3$, M=Mn, Ni, Fe; X=S, Se) exhibiting various antiferromagnetic arrangement within the 2D limit. Our analysis based on the density functional theory and versatile formalism of Bethe-Salpeter equation reveals larger exciton binding energies for MPS$_3$ (up to 1.1 eV in air) than MPSe$_3$(up to 0.8 eV in air), exceeding the values of transition metal dichalcogenides (TMDs). For the (Mn,Fe)PX$_3$ we determine the optically active band edge transitions, revealing that they are sensitive to in-plane magnetic order, irrespective of the type of chalcogen atom. We predict the anistropic effective masses and the type of linear polarization as an important fingerprints for sensing the type of magnetic AFM arrangements. Furthermore, we identify the spin-orientation-dependent features such as the valley splitting, the effective mass of holes, and the exciton binding energy. In particular, we demonstrate that for MnPX$_3$ (X=S, Se) a pair of non equivalent K+ and K- points exists yielding the valley splittings that strongly depend on the direction of AFM aligned spins. Notably, for the out-of-plane direction of spins, two distinct peaks are expected to be visible below the absorption onset, whereas one peak should emerge for the in-plane configuration of spins. These spin-dependent features provide an insight into spin flop transitions of 2D materials. Finally, we propose a strategy how the spin valley polarization can be realized in 2D AFM within honeycomb lattice.

Predicting Psi-BN: computational insights into its mechanical, electronic, and optical characteristics. (arXiv:2308.13112v1 [cond-mat.mtrl-sci])
F. F. Monteiro, K. A. L. Lima, L. A. Ribeiro Junior

Computational materials are pivotal in advancing our understanding of distinct material classes and their properties, offering valuable insights in predicting novel structures and complementing experimental approaches. In this context, Psi-graphene is a stable two-dimensional carbon allotrope composed of 5-6-7 carbon rings theoretically predicted recently. Using density functional theory (DFT) calculations, we explored its boron nitride counterpart's mechanical, electronic, and optical characteristics (Psi-BN). Our results indicate that Psi-BN possesses a band gap of 4.59 eV at the HSE06 level. Phonon calculations and ab initio molecular dynamics simulations demonstrated that this material has excellent structural and dynamic stability. Moreover, its formation energy is -7.48 eV. Psi-BN exhibited strong ultraviolet activity, suggesting its potential as an efficient UV collector. Furthermore, we determined critical mechanical properties of Psi-BN, such as the elastic stiffness constants, Young's modulus (250-300 GPa), and Poisson ratio (0.7), providing valuable insights into its mechanical behavior.

Thermal effect on microwave pulse driven magnetization switching of Stoner particle. (arXiv:2308.13124v1 [cond-mat.mes-hall])
S. Chowdhury, M. A. S. Akanda, M. A. J. Pikul, M.T. Islam, Tai Min

Recently it has been demonstrated that the cosine chirp microwave pulse (CCMP) is capable of achieving fast and energy-efficient magnetization-reversal of a nanoparticle with zero-Temperature. However, we investigate the finite temperature, $T$ effect on the CCMP-driven magnetization reversal using the framework of the stochastic Landau Lifshitz Gilbert equation. At finite Temperature, we obtain the CCMP-driven fast and energy-efficient reversal and hence estimate the maximal temperature, $T_{max}$ at which the magnetization reversal is valid. $T_{max}$ increases with increasing the nanoparticle cross-sectional area/shape anisotropy up to a certain value, and afterward $T_{max}$ decreases with the further increment of nanoparticle cross-sectional area/shape anisotropy. This is because of demagnetization/shape anisotropy field opposes the magnetocrystalline anisotropy, i.e., reduces the energy barrier which separates the two stable states. For smaller cross-sectional area/shape anisotropy, the controlling parameters of CCMP show decreasing trend with temperature. We also find that with the increment easy-plane shape-anisotropy, the required initial frequency of CCMP significantly reduces. For the larger volume of nanoparticles, the parameters of CCMP remains constant for a wide range of temperature which are desired for the device application. Therefore, The above findings might be useful to realize the CCMP-driven fast and energy-efficient magnetization reversal in realistic conditions.

The Acoustophotoelectric Effect: Efficient Phonon-Photon-Electron Coupling in Zero-Voltage-Biased 2D SnS$_2$ for Broadband Photodetection. (arXiv:2308.13143v1 [])
Hossein Alijani, Philipp Reineck, Robert Komljenovic, Salvy Russo, Mei Xian Low, Sivacarendran Balendhran, Kenneth Crozier, Sumeet Walia, Geoff. R. Nash, Leslie Y. Yeo, Amgad R. Rezk

Two-dimensional (2D) layered metal dichalcogenides constitute a promising class of materials for photodetector applications due to their excellent optoelectronic properties. The most common photodetectors, which work on the principle of photoconductive or photovoltaic effects, however, require either the application of external voltage biases or built-in electric fields, which makes it challenging to simultaneously achieve high responsivities across broadband wavelength excitation - especially beyond the material's nominal band gap - while producing low dark currents. In this work, we report the discovery of an intricate phonon-photon-electron coupling - which we term the acoustophotoelectric effect - in SnS$_2$ that facilitates efficient photodetection through the application of 100-MHz-order propagating surface acoustic waves (SAWs). This effect not only reduces the band gap of SnS$_2$, but also provides the requisite momentum for indirect band gap transition of the photoexcited charge carriers, to enable broadband photodetection beyond the visible light range, whilst maintaining pA-order dark currents - remarkably without the need for any external voltage bias. More specifically, we show in the infrared excitation range that it is possible to achieve up to eight orders of magnitude improvement in the material's photoresponsivity compared to that previously reported for SnS$_2$-based photodetectors, in addition to exhibiting superior performance compared to most other 2D materials reported to date for photodetection.

Robust Luttinger liquid state of 1D Dirac fermions in a van der Waals system Nb$_9$Si$_4$Te$_{18}$. (arXiv:2308.13199v1 [cond-mat.str-el])
Qirong Yao, Hyunjin Jung, Kijeong Kong, Chandan De, Jaeyoung Kim, Jonathan D. Denlinger, Han Woong Yeom

We report on the Tomonaga-Luttinger liquid (TLL) behavior in fully degenerate 1D Dirac fermions. A ternary van der Waals material Nb$_9$Si$_4$Te$_{18}$ incorporates in-plane NbTe$_2$ chains, which produce a 1D Dirac band crossing Fermi energy. Tunneling conductance of electrons confined within NbTe2 chains is found to be substantially suppressed at Fermi energy, which follows a power law with a universal temperature scaling, hallmarking a TLL state. The obtained Luttinger parameter of ~0.15 indicates strong electron-electron interaction. The TLL behavior is found to be robust against atomic-scale defects, which might be related to the Dirac electron nature. These findings, as combined with the tunability of the compound and the merit of a van der Waals material, offer a robust, tunable, and integrable platform to exploit non-Fermi liquid physics.

Flexo-electricity of the dowser texture. (arXiv:2308.13226v1 [cond-mat.soft])
Pawel Pieranski (LPS), Maria Helena Godinho

The persistent quasi-planar nematic texture known also as the dowser texture is characterized by a 2D unitary vector field d. We show here that the dowser texture is sensitive, in first order, to electric fields. This property is due to the flexo-electric polarisation P collinear with d expected from R.B. Meyer's considerations on flexo-electricity in nematics. It is pointed out that due to the flexo-electric polarisation nematic monopoles can be manipulated by electric fields of appropriated geometry.

Giant atomic swirl in graphene bilayers with biaxial heterostrain. (arXiv:2308.13230v1 [cond-mat.mes-hall])
F. Mesple, N. R. Walet, G. Trambly de Laissardière, F. Guinea, D. Dosenovic, H. Okuno, C. Paillet, A. Michon, C. Chapelier, V. T. Renard

The study of moir\'e engineering started with the advent of van der Waals heterostructures in which stacking two-dimensional layers with different lattice constants leads to a moir\'e pattern controlling their electronic properties. The field entered a new era when it was found that adjusting the twist between two graphene layers led to strongly-correlated-electron physics and topological effects associated with atomic relaxation. Twist is now used routinely to adjust the properties of two-dimensional materials. Here, we investigate a new type of moir\'e superlattice in bilayer graphene when one layer is biaxially strained with respect to the other - so-called biaxial heterostrain. Scanning tunneling microscopy measurements uncover spiraling electronic states associated with a novel symmetry-breaking atomic reconstruction at small biaxial heterostrain. Atomistic calculations using experimental parameters as inputs reveal that a giant atomic swirl forms around regions of aligned stacking to reduce the mechanical energy of the bilayer. Tight-binding calculations performed on the relaxed structure show that the observed electronic states decorate spiraling domain wall solitons as required by topology. This study establishes biaxial heterostrain as an important parameter to be harnessed for the next step of moir\'e engineering in van der Waals multilayers.

WSTac: Interactive Surface Perception based on Whisker-Inspired and Self-Illuminated Vision-Based Tactile Sensor. (arXiv:2308.13241v1 [cs.RO])
Kai Chong Lei, Kit Wa Sou, Wang Sing Chan, Jiayi Yan, Siqi Ping, Dengfeng Peng, Wenbo Ding, Xiao-Ping Zhang

Modern Visual-Based Tactile Sensors (VBTSs) use cost-effective cameras to track elastomer deformation, but struggle with ambient light interference. Solutions typically involve using internal LEDs and blocking external light, thus adding complexity. Creating a VBTS resistant to ambient light with just a camera and an elastomer remains a challenge. In this work, we introduce WStac, a self-illuminating VBTS comprising a mechanoluminescence (ML) whisker elastomer, camera, and 3D printed parts. The ML whisker elastomer, inspired by the touch sensitivity of vibrissae, offers both light isolation and high ML intensity under stress, thereby removing the necessity for additional LED modules. With the incorporation of machine learning, the sensor effectively utilizes the dynamic contact variations of 25 whiskers to successfully perform tasks like speed regression, directional identification, and texture classification. Videos are available at:

Effective tight-binding Hamiltonian for the low-energy electronic structure of the Cu-doped lead apatite and the parent compound. (arXiv:2308.13275v1 [cond-mat.supr-con])
Mayank Gupta, S. Satpathy, B. R. K. Nanda

We examine the origin of the formation of narrow bands in LK-99 (Pb$_{9}$Cu(PO$_4$)$_6$O) and the parent compound without the Cu doping using density functional theory calculations and model Hamiltonian studies. Explicit analytical expressions are given for a nearest-neighbor tight-binding (TB) Hamiltonian in the momentum space for both the parent and the LK-99 compound, which can serve as an effective model to study various quantum phenomena including superconductivity. The parent material is an insulator with the buckle oxygen atom on the stacked triangular lattice forming the topmost bands, well-separated from the remaining oxygen band manifold. The $C_3$ symmetry-driven two-band TB model describes these two bands quite well. These bands survive in the Cu-doped, LK-99, though with drastically altered band dispersion due to the Cu-O interaction. A similar two-band model involving the Cu $xz$ and $yz$ orbitals broadly describes the top two valence bands of LK-99. However, the band dispersions of both the Cu and O bands are much better described by the four-band TB model incorporating the Cu-O interactions on the buckled honeycomb lattice. We comment on the possible mechanisms of superconductivity in LK-99. even though the actual T$_c$ may be much smaller than reported, and suggest that interstitial Cu clusters leading to broad bands might have a role to play

Training normalizing flows with computationally intensive target probability distributions. (arXiv:2308.13294v1 [cs.LG])
Piotr Bialas, Piotr Korcyl, Tomasz Stebel

Machine learning techniques, in particular the so-called normalizing flows, are becoming increasingly popular in the context of Monte Carlo simulations as they can effectively approximate target probability distributions. In the case of lattice field theories (LFT) the target distribution is given by the exponential of the action. The common loss function's gradient estimator based on the "reparametrization trick" requires the calculation of the derivative of the action with respect to the fields. This can present a significant computational cost for complicated, non-local actions like e.g. fermionic action in QCD. In this contribution, we propose an estimator for normalizing flows based on the REINFORCE algorithm that avoids this issue. We apply it to two dimensional Schwinger model with Wilson fermions at criticality and show that it is up to ten times faster in terms of the wall-clock time as well as requiring up to $30\%$ less memory than the reparameterization trick estimator. It is also more numerically stable allowing for single precision calculations and the use of half-float tensor cores. We present an in-depth analysis of the origins of those improvements. We believe that these benefits will appear also outside the realm of the LFT, in each case where the target probability distribution is computationally intensive.

Is the topological surface state floating on top of a thick lead layer? The case of the Pb/Bi2Se3 interface. (arXiv:2308.13316v1 [cond-mat.mtrl-sci])
Oreste De Luca, Igor A. Shvets, Sergey V. Eremeev, Ziya S. Aliev, Marek Kopciuszynski, Alexey Barinov, Fabio Ronci, Stefano Colonna, Evgueni V. Chulkov, Raffaele G. Agostino, Marco Papagno, Roberto Flammini

The puzzling question about the floating of the topological surface state on top of a thick Pb layer, has now possibly been answered. A study of the interface made by Pb on Bi2Se3 for different temperature and adsorbate coverage condition, allowed us to demonstrate that the evidence reported in the literature can be related to the surface diffusion phenomenon exhibited by the Pb atoms, which leaves the substrate partially uncovered. Comprehensive density functional theory calculations show that despite the specific arrangement of the atoms at the interface, the topological surface state cannot float on top of the adlayer but rather tends to move inward within the substrate.

Evidence of the Coulomb gap in the density of states of MoS$_2$. (arXiv:2308.13337v1 [cond-mat.mes-hall])
Michele Masseroni, Tingyu Qu, Takashi Taniguchi, Kenji Watanabe, Thomas Ihn, Klaus Ensslin

$\mathrm{MoS_2}$ is an emergent van der Waals material that shows promising prospects in semiconductor industry and optoelectronic applications. However, its electronic properties are not yet fully understood. In particular, the nature of the insulating state at low carrier density deserves further investigation, as it is important for fundamental research and applications. In this study, we investigate the insulating state of a dual-gated exfoliated bilayer $\mathrm{MoS_2}$ field-effect transistor by performing magnetotransport experiments. We observe positive and non-saturating magnetoresistance, in a regime where only one band contributes to electron transport. At low electron density ($\sim 1.4\times 10^{12}~\mathrm{cm^{-2}}$) and a perpendicular magnetic field of 7 Tesla, the resistance exceeds by more than one order of magnitude the zero field resistance and exponentially drops with increasing temperature. We attribute this observation to strong electron localization. Both temperature and magnetic field dependence can, at least qualitatively, be described by the Efros-Shklovskii law, predicting the formation of a Coulomb gap in the density of states due to Coulomb interactions. However, the localization length obtained from fitting the temperature dependence exceeds by more than one order of magnitude the one obtained from the magnetic field dependence. We attribute this discrepancy to the presence of a nearby metallic gate, which provides electrostatic screening and thus reduces long-range Coulomb interactions. The result of our study suggests that the insulating state of $\mathrm{MoS_2}$ originates from a combination of disorder-driven electron localization and Coulomb interactions.

Work statistics for Quantum Spin Chains: characterizing quantum phase transitions, benchmarking time evolution, and examining passivity of quantum states. (arXiv:2308.13366v1 [cond-mat.stat-mech])
Feng-Li Lin, Ching-Yu Huang

We study three aspects of work statistics in the context of the fluctuation theorem for the quantum spin chains by numerical methods based on matrix-product states. First, we elaborate that the work done on the spin-chain by a sudden quench can be used to characterize the quantum phase transitions (QPT). We further obtain the numerical results to demonstrate its capability of characterizing the QPT of both Landau-Ginzbrug types, such as the Ising chain, or topological types, such as the Haldane chain. Second, we propose to use the fluctuation theorem, such as Jarzynski's equality, which relates the real-time correlator to the ratio of the thermal partition functions, as a benchmark indicator for the numerical real-time evolving methods. Third, we study the passivity of ground and thermal states of quantum spin chains under some cyclic impulse processes. We verify the passivity of thermal states. Furthermore, we find that some ground states in the Ising-like chain, with less overall spin order from spontaneous or explicit symmetry breaking, can be active so that they can be exploited for quantum engines.

Stabilization of Hubbard-Thouless pumps through nonlocal fermionic repulsion. (arXiv:2308.13375v1 [cond-mat.quant-gas])
Javier Argüello-Luengo, Manfred J. Mark, Francesca Ferlaino, Maciej Lewenstein, Luca Barbiero, Sergi Julià-Farré

Thouless pumping represents a powerful concept to probe quantized topological invariants in quantum systems. We explore this mechanism in a generalized Rice-Mele Fermi-Hubbard model characterized by the presence of competing onsite and intersite interactions. Contrary to recent experimental and theoretical results, showing a breakdown of quantized pumping induced by the onsite repulsion, we prove that sufficiently large intersite interactions allow for an interaction-induced recovery of Thouless pumps. Our analysis further reveals that the occurrence of stable topological transport at large interactions is connected to the presence of a spontaneous bond-order-wave in the ground-state phase diagram of the model. Finally, we discuss a concrete experimental setup based on ultracold magnetic atoms in an optical lattice to realize the newly introduced Thouless pump. Our results provide a new mechanism to stabilize Thouless pumps in interacting quantum systems.

Geometric and conventional contributions to superfluid weight in the minimal models for superconducting copper-doped lead apatite. (arXiv:2308.13400v1 [cond-mat.supr-con])
Wojciech Brzezicki, Timo Hyart

The density functional theory calculations and tight-binding models for the copper-doped lead apatite support flat bands, which could be susceptible to the emergence of high-temperature superconductivity. We develop theory for the geometric contribution of the superfluid weight arising from the momentum-space topology of the Bloch wave functions of these flat bands, and we compare our results to the paradigmatic case of $s$-wave superconductivity on an isolated topological flat band. We show that, in contrast to the standard paradigm of flat-band superconductivity, there does not exist any lower bound for the superfluid weight in these models. Moreover, although the nontrivial quantum geometries of the normal state bands are the same when the superconductivity appears in the ferromagnetic and paramagnetic phases, the emerging superconducting phases have very different superfluid weights. In the case of superconductivity appearing on the spin-polarized bands the superfluid weight varies a lot as a function of model parameters. On the other hand, if the superconductivity emerges in the paramagnetic phase the superfluid weight is robustly large and it contains a significant geometric component.

Ionic liquid gating induced self-intercalation of transition metal chalcogenides. (arXiv:2308.13402v1 [cond-mat.mtrl-sci])
Fei Wang, Yang Zhang, Zhijie Wang, Haoxiong Zhang, Xi Wu, Changhua Bao, Jia Li, Pu Yu, Shuyun Zhou

Ionic liquids provide versatile pathways for controlling the structures and properties of quantum materials. Previous studies have reported electrostatic gating of nanometre-thick flakes leading to emergent superconductivity, insertion or extraction of protons and oxygen ions in perovskite oxide films enabling the control of different phases and material properties, and intercalation of large-sized organic cations into layered crystals giving access to tailored superconductivity. Here, we report an ionic-liquid gating method to form three-dimensional transition metal monochalcogenides (TMMCs) by driving the metals dissolved from layered transition metal dichalcogenides (TMDCs) into the van der Waals gap. We demonstrate the successful self-intercalation of PdTe$_2$ and NiTe$_2$, turning them into high-quality PdTe and NiTe single crystals, respectively. Moreover, the monochalcogenides exhibit distinctive properties from dichalcogenides. For instance, the self-intercalation of PdTe$_2$ leads to the emergence of superconductivity in PdTe. Our work provides a synthesis pathway for TMMCs by means of ionic liquid gating driven self-intercalation.

Disorder-induced phase transitions in double HgTe quantum wells. (arXiv:2308.13440v1 [cond-mat.mes-hall])
S. S. Krishtopenko, A. V. Ikonnikov, B. Jouault, F. Teppe

By using the self-consistent Born approximation, we investigate disorder effect induced by short-range impurities on the band-gap of a seminal two-dimensional (2D) system, whose phase diagram contains trivial, single-band-inverted and double-band-inverted states. Following the density-of-states (DOS) evolution, we demonstrate multiple closings and openings of the band-gap with the increase of the disorder strength. Calculations of the spectral function describing the quasiparticles at the $\Gamma$ point of the Brillouin zone evidence that the observed band-gap behavior is unambiguously caused by the topological phase transitions due to the mutual inversions between the first and second electron-like and hole-like subbands. We also find that an increase in the disorder strength in the double-inverted state always leads to the band-gap closing due to the overlap of the tails of DOS from conduction and valence subbands.

Ultra-clean assembly of van der Waals heterostructures. (arXiv:2308.13484v1 [])
Wendong Wang, Nicholas Clark, Matthew Hamer, Amy Carl, Endre Tovari, Sam Sullivan-Allsop, Evan Tillotson, Yunze Gao, Hugo de Latour, Francisco Selles, James Howarth, Eli G. Castanon, Mingwei Zhou, Haoyu Bai, Xiao Li, Astrid Weston, Kenji Watanabe, Takashi Taniguchi, Cecilia Mattevi, Thomas H. Bointon, Paul V. Wiper, Andrew J. Strudwick, Leonid A. Ponomarenko, Andrey Kretinin, Sarah J. Haigh, Alex Summerfield, Roman Gorbachev

Layer-by-layer assembly of van der Waals (vdW) heterostructures underpins new discoveries in solid state physics, material science and chemistry. Despite the successes, all current 2D material (2DM) transfer techniques rely on the use of polymers which limit the cleanliness, ultimate electronic performance, and potential for optoelectronic applications of the heterostructures. In this article, we present a novel polymer-free platform for rapid and facile heterostructure assembly which utilises re-usable flexible silicon nitride membranes. We demonstrate that this allows fast and reproducible production of 2D heterostructures using both exfoliated and CVD-grown materials with perfect interfaces free from interlayer contamination and correspondingly excellent electronic behaviour, limited only by the size and intrinsic quality of the crystals used. Furthermore, removing the need for polymeric carriers allows new possibilities for vdW heterostructure fabrication: assembly at high temperatures up to 600{\deg}C, and in different environments including ultra-high vacuum (UHV) and when the materials are fully submerged in liquids. We demonstrate UHV heterostructure assembly for the first time, and show the reliable creation of graphene moir\'e superlattices with more than an order of magnitude improvement in their structural homogeneity. We believe that broad adaptation of our novel inorganic 2D materials assembly strategy will allow realisation of the full potential of vdW heterostructures as a platform for new physics and advanced optoelectronic technologies.

Anyon braiding on a fractal lattice with a local Hamiltonian. (arXiv:2106.13816v3 [cond-mat.quant-gas] UPDATED)
Sourav Manna, Callum W. Duncan, Carrie A. Weidner, Jacob F. Sherson, Anne E. B. Nielsen

There is a growing interest in searching for topology in fractal dimensions with the aim of finding different properties and advantages compared to the integer dimensional case. It has previously been shown that the Laughlin state can be adapted to fractal lattices. A key element in doing so is to replace the uniform background charge by a background charge that resides only on the lattice sites. This motivates the study of Hofstadter type models on fractal lattices, in which the magnetic field is present only at the lattice sites. Here, we study such models for hardcore bosons on finite lattices derived from the Sierpinski carpet and on square lattices with open boundary conditions. We find that the system sizes that we can investigate with exact diagonalization are generally too small to judge whether these local models are topological or not. Studying the particle densities on the lattices derived from the Sierpinski carpet, we find that the densities tend to accumulate in the regions that are locally similar to a square lattice. Such accumulation seems to be incompatible with the uniform densities in fractional quantum Hall systems, which might suggest that the models are not topological. Our computations provide guidance for future searches for topology in finite systems. We also propose a scheme to implement both fractal lattices and our proposed local Hamiltonian with ultracold atoms in optical lattices, which could allow for quantum simulators to go beyond the numerical results presented here.

Injection and nucleation of topological defects in the quench dynamics of the Frenkel-Kontorova model. (arXiv:2210.14904v4 [cond-mat.stat-mech] UPDATED)
Oksana Chelpanova, Shane P. Kelly, Giovanna Morigi, Ferdinand Schmidt-Kaler, Jamir Marino

Topological defects have strong impact on both elastic and inelastic properties of materials. In this article, we investigate the possibility to controllably inject topological defects in quantum simulators of solid state lattice structures. We investigate the quench dynamics of a Frenkel-Kontorova chain, which is used to model discommensurations of particles in cold atoms and trapped ionic crystals. The interplay between an external periodic potential and the inter-particle interaction makes lattice discommensurations, the topological defects of the model, energetically favorable and can tune a commensurate-incommensurate structural transition. Our key finding is that a quench from the commensurate to incommensurate phase causes a controllable injection of topological defects at periodic time intervals. We employ this mechanism to generate quantum states which are a superposition of lattice structures with and without topological defects. We conclude by presenting concrete perspectives for the observation and control of topological defects in trapped ion experiments.

Symmetry induced selective excitation of topological states in SSH waveguide arrays. (arXiv:2211.06228v2 [physics.optics] UPDATED)
Min Tang, Jiawei Wang, Sreeramulu Valligatla, Christian N. Saggau, Haiyun Dong, Ehsan Saei Ghareh Naz, Sebastian Klembt, Ching Hua Lee, Ronny Thomale, Jeroen van den Brink, Ion Cosma Fulga, Oliver G. Schmidt, Libo Ma

The investigation of topological state transition in carefully designed photonic lattices is of high interest for fundamental research, as well as for applied studies such as manipulating light flow in on-chip photonic systems. Here, we report on topological phase transition between symmetric topological zero modes (TZM) and antisymmetric TZMs in Su-Schrieffer-Heeger (SSH) mirror symmetric waveguides. The transition of TZMs is realized by adjusting the coupling ratio between neighboring waveguide pairs, which is enabled by selective modulation of the refractive index in the waveguide gaps. Bi-directional topological transitions between symmetric and antisymmetric TZMs can be achieved with our proposed switching strategy. Selective excitation of topological edge mode is demonstrated owing to the symmetry characteristics of the TZMs. The flexible manipulation of topological states is promising for on-chip light flow control and may spark further investigations on symmetric/antisymmetric TZM transitions in other photonic topological frameworks.

Topological inverse band theory in waveguide quantum electrodynamics. (arXiv:2301.05481v3 [physics.optics] UPDATED)
Yongguan Ke, Jiaxuan Huang, Wenjie Liu, Yuri Kivshar, Chaohong Lee

Topological phases play a crucial role in the fundamental physics of light-matter interaction and emerging applications of quantum technologies. However, the topological band theory of waveguide QED systems is known to break down, because the energy bands become disconnected. Here, we introduce a concept of the inverse energy band and explore analytically topological scattering in a waveguide with an array of quantum emitters. We uncover a rich structure of topological phase transitions, symmetric scale-free localization, completely flat bands, and the corresponding dark Wannier states. Although bulk-edge correspondence is partially broken because of radiative decay, we prove analytically that the scale-free localized states are distributed in a single inverse energy band in the topological phase and in two inverse bands in the trivial phase. Surprisingly, the winding number of the scattering textures depends on both the topological phase of inverse subradiant band and the odevity of the cell number. Our work uncovers the field of the topological inverse bands, and it brings a novel vision to topological phases in light-matter interactions.

Polarization Jumps across Topological Phase Transitions in Two-dimensional Systems. (arXiv:2304.12742v2 [cond-mat.mes-hall] UPDATED)
Hiroki Yoshida, Tiantian Zhang, Shuichi Murakami

In topological phase transitions involving a change in topological invariants such as the Chern number and the $\mathbb{Z}_2$ topological invariant, the gap closes, and the electric polarization becomes undefined at the transition. In this paper, we show that the jump of polarization across such topological phase transitions in two dimensions is described in terms of positions and monopole charges of Weyl points in the intermediate Weyl semimetal phase. We find that the jump of polarization is described by the Weyl dipole at $\mathbb{Z}_2$ topological phase transitions and at phase transitions without any change in the value of the Chern number. Meanwhile, when the Chern number changes at the phase transition, the jump is expressed in terms of the relative positions of Weyl points measured from a reference point in the reciprocal space.

Impact of competing energy scales on the shell-filling sequence in elliptic bilayer graphene quantum dots. (arXiv:2305.09284v2 [cond-mat.mes-hall] UPDATED)
Samuel Möller, Luca Banszerus, Angelika Knothe, Lucca Valerius, Katrin Hecker, Eike Icking, Kenji Watanabe, Takashi Taniguchi, Christian Volk, Christoph Stampfer

We report on a detailed investigation of the shell-filling sequence in electrostatically defined elliptic bilayer graphene quantum dots (QDs) in the regime of low charge carrier occupation, $N \leq 12$, by means of magnetotransport spectroscopy and numerical calculations. We show the necessity of including both short-range electron-electron interaction and wavefunction-dependent valley g-factors for understanding the overall fourfold shell-filling sequence. These factors lead to an additional energy splitting at half-filling of each orbital state and different energy shifts in out-of-plane magnetic fields. Analysis of 31 different BLG QDs reveals that both valley g-factor and electron-electron interaction induced energy splitting increase with decreasing QD size, validating theory. However, we find that the electrostatic charging energy of such gate-defined QDs does not correlate consistently with their size, indicating complex electrostatics. These findings offer significant insights for future BLG QD devices and circuit designs.

Twofold Symmetry Observed in Bi$_{2}$Te$_{3}$/FeTe Interfacial Superconductor. (arXiv:2307.05904v2 [cond-mat.supr-con] UPDATED)
Xinru Han, Hailang Qin, Tianluo Pan, Bin Guo, Kaige Shi, Zijin Huang, Jie Jiang, Hangyu Yin, Hongtao He, Fei Ye, Wei-Qiang Chen, Jia-Wei Mei, Gan Wang

Superconducting pairing symmetry are crucial in understanding the microscopic superconducting mechanism of a superconductor. Here we report the observation of a twofold superconducting gap symmetry in an interfacial superconductor Bi$_{2}$Te$_{3}$/FeTe, by employing quasiparticle interference (QPI) technique in scanning tunneling microscopy and macroscopic magnetoresistance measurements. The QPI patterns corresponding to energies inside and outside the gap reveal a clear anisotropic superconducting gap. Furthermore, both the in-plane angle-dependent magnetoresistance and in-plane upper critical field exhibit a clear twofold symmetry. This twofold symmetry align with the Te-Te direction in FeTe, which weakens the possible generation by bi-collinear antiferromagnetism order. Our finding provides key information in further understanding of the topological properties in Bi$_{2}$Te$_{3}$/FeTe superconducting system and propels further theoretical interests in the paring mechanism in the system.

Electrostatic shielding effect and Binding energy shift of MoS$_2$, MoSeS$_2$ and MoTeS$_2$ materials. (arXiv:2307.08035v2 [cond-mat.mtrl-sci] UPDATED)
Yaorui Tan, Maolin Bo

In this paper, the electronic structure and bond properties of MoSS$_2$, MoSeS$_2$ and MoTeS$_2$ are studied. Density functional theory (DFT) calculates combined with the binding energy and bond-charge (BBC) model to obtain electronic structure, binding energy shift and bond properties. It is found that electrostatic shielding by electron exchange is the main cause of density fluctuation. A method for calculating the density of Green's function with energy level shift is established. It provides new methods and ideas for the further study of the binding energy, bond states and electronic properties of nanomaterials.

Large circular photogalvanic effect in non-centrosymmetric magnetic Weyl semimetal CeAlSi. (arXiv:2308.00045v2 [cond-mat.mes-hall] UPDATED)
Abhirup Roy Karmakar, A. Taraphder, G. P. Das

The recent discovery of the Weyl semimetal CeAlSi with simultaneous breaking of inversion and time-reversal symmetries has opened up new avenues for research into the interaction between light and topologically protected bands. In this work, we present a comprehensive examination of shift current and injection current responsible for the circular photogalvanic effect in CeAlSi using first-principles calculations. Our investigation identifies a significant injection current of 1.2 mA/V$^2$ over a broad range in the near-infrared region of the electromagnetic spectrum, exceeding previously reported findings. In addition, we explored several externally controllable parameters to further enhance the photocurrent. A substantial boost in the injection current is observed when applying uniaxial strain along the $c$-axis of the crystal $-$ a 5% strain results in a remarkable 64% increment. The exceptional photocurrent response in CeAlSi suggests that magnetic non-centrosymmetric Weyl semimetals may provide promising opportunities for novel photogalvanic applications.

A Platform for Far-Infrared Spectroscopy of Quantum Materials at Millikelvin Temperatures. (arXiv:2308.00610v3 [cond-mat.mes-hall] UPDATED)
Michael Onyszczak, Ayelet J. Uzan, Yue Tang, Pengjie Wang, Yanyu Jia, Guo Yu, Tiancheng Song, Ratnadwip Singha, Jason F. Khoury, Leslie M. Schoop, Sanfeng Wu

Optical spectroscopy of quantum materials at ultralow temperatures is rarely explored, yet it may provide critical characterizations of quantum phases not possible using other approaches. We describe the development of a novel experimental platform that enables optical spectroscopic studies, together with standard electronic transport, of materials at millikelvin temperatures inside a dilution refrigerator. The instrument is capable of measuring both bulk crystals and micron-sized two-dimensional van der Waals materials and devices. We demonstrate the performance by implementing photocurrent-based Fourier transform infrared spectroscopy on a monolayer WTe$_2$ device and a multilayer 1T-TaS$_2$ crystal, with a spectral range available from the near-infrared to the terahertz regime and in magnetic fields up to 5 T. In the far-infrared regime, we achieve spectroscopic measurements at a base temperature as low as ~ 43 mK and a sample electron temperature of ~ 450 mK. Possible experiments and potential future upgrades of this versatile instrumental platform are envisioned.

Strong-coupling phases of trions and excitons in electron-hole bilayers at commensurate densities. (arXiv:2308.00825v2 [cond-mat.str-el] UPDATED)
David D. Dai, Liang Fu

We introduce density imbalanced electron-hole bilayers at a commensurate 2 : 1 density ratio as a platform for realizing novel phases involving electrons, excitons and trions. Three length scales are identified which characterize the interplay between kinetic energy, intralayer repulsion, and interlayer attraction. By a combination of theoretical analysis and numerical calculation, we find a variety of strong-coupling phases in different parameter regions, including quantum crystals of electrons, excitons, and trions. We also propose an "excitonic supersolid" phase that features electron crystallization and exciton superfluidity simultaneously. The material realization and experimental signature of these phases are discussed in the context of semiconductor transition metal dichalcogenide bilayers.

Excitonic interplay between surface polar III-nitride quantum wells and MoS$_2$ monolayer. (arXiv:2308.10687v2 [cond-mat.mes-hall] UPDATED)
Danxuan Chen, Jin Jiang, Thomas F. K. Weatherley, Jean-François Carlin, Mitali Banerjee, Nicolas Grandjean

III-nitride wide bandgap semiconductors exhibit large exciton binding energies, preserving strong excitonic effects at room temperature. On the other hand, semiconducting two-dimensional (2D) materials, including MoS$_2$, also exhibit strong excitonic effects, attributed to enhanced Coulomb interactions. This study investigates excitonic interactions between surface GaN quantum well (QW) and 2D MoS$_2$ in van der Waals heterostructures by varying the spacing between these two excitonic systems. Optical property investigation first demonstrates the effective passivation of defect states at the GaN surface through MoS$_2$ coating. Furthermore, a strong interplay is observed between MoS$_2$ monolayers and GaN QW excitonic transitions. This highlights the interest of the 2D material/III-nitride QW system to study near-field interactions, such as F\"orster resonance energy transfer, which could open up novel optoelectronic devices based on such hybrid excitonic structures.

Valley-polarized Exitonic Mott Insulator in WS2/WSe2 Moir\'e Superlattice. (arXiv:2308.10799v2 [cond-mat.mes-hall] UPDATED)
Zhen Lian, Yuze Meng, Lei Ma, Indrajit Maity, Li Yan, Qiran Wu, Xiong Huang, Dongxue Chen, Xiaotong Chen, Xinyue Chen, Mark Blei, Takashi Taniguchi, Kenji Watanabe, Sefaattin Tongay, Johannes Lischner, Yong-Tao Cui, Su-Fei Shi

Strongly enhanced electron-electron interaction in semiconducting moir\'e superlattices formed by transition metal dichalcogenides (TMDCs) heterobilayers has led to a plethora of intriguing fermionic correlated states. Meanwhile, interlayer excitons in a type-II aligned TMDC heterobilayer moir\'e superlattice, with electrons and holes separated in different layers, inherit this enhanced interaction and strongly interact with each other, promising for realizing tunable correlated bosonic quasiparticles with valley degree of freedom. We employ photoluminescence spectroscopy to investigate the strong repulsion between interlayer excitons and correlated electrons in a WS2/WSe2 moir\'e superlattice and combine with theoretical calculations to reveal the spatial extent of interlayer excitons and the band hierarchy of correlated states. We further find that an excitonic Mott insulator state emerges when one interlayer exciton occupies one moir\'e cell, evidenced by emerging photoluminescence peaks under increased optical excitation power. Double occupancy of excitons in one unit cell requires overcoming the energy cost of exciton-exciton repulsion of about 30-40 meV, depending on the stacking configuration of the WS2/WSe2 heterobilayer. Further, the valley polarization of the excitonic Mott insulator state is enhanced by nearly one order of magnitude. Our study demonstrates the WS2/WSe2 moir\'e superlattice as a promising platform for engineering and exploring new correlated states of fermion, bosons, and a mixture of both.