Found 38 papers in cond-mat
Date of feed: Mon, 25 Dec 2023 01:30:00 GMT

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Reversal of Orbital Hall Conductivity and Emergence of Tunable Topological Quantum States in Orbital Hall Insulator. (arXiv:2312.14181v1 [cond-mat.mes-hall])
Shilei Ji, Chuye Quan, Ruijia Yao, Jianping Yang, Xing'ao Li

Recent findings indicate that orbital angular momentum (OAM) has the capability to induce the intrinsic orbital Hall effect (OHE), which is characterized by orbital Chern number in the orbital Hall insulator. Unlike the spin-polarized channel in Quantum anomalous Hall insulator, the OAM is valley-locked, posing challenges in manipulating the corresponding edge state. Here we demonstrate the sign-reversal orbital Chern number through strain engineering by combing the $k \cdot p$ model and first-principles calculation. Under the manipulation of strain, we observe the transfer of non-zero OAM from the valence band to the conduction band, aligning with the orbital contribution in the electronic structure. Our investigation reveals that electrons and holes with OAM exhibit opposing trajectories, resulting in a reversal of the orbital Hall conductivity. Furthermore, we explore the topological quantum state between the sign-reversible OHE.

Hofstadter-Toda spectral duality and quantum groups. (arXiv:2312.14242v1 [hep-th])
Pasquale Marra, Valerio Proietti, Xiaobing Sheng

The Hofstadter model allows to describe and understand several phenomena in condensed matter such as the quantum Hall effect, Anderson localization, charge pumping, and flat-bands in quasiperiodic structures, and is a rare example of fractality in the quantum world. An apparently unrelated system, the relativistic Toda lattice, has been extensively studied in the context of complex nonlinear dynamics, and more recently for its connection to supersymmetric Yang-Mills theories and topological string theories on Calabi-Yau manifolds in high-energy physics. Here we discuss a recently discovered spectral relationship between the Hofstadter model and the relativistic Toda lattice which has been later conjectured to be related to the Langlands duality of quantum groups. Moreover, by employing similarity transformations compatible with the quantum group structure, we establish a formula parametrizing the energy spectrum of the Hofstadter model in terms of elementary symmetric polynomials and Chebyshev polynomials. The main tools used are the spectral duality of tridiagonal matrices and the representation theory of the elementary quantum group.

Absence of quantization in the circular photogalvanic effect in disordered chiral Weyl semimetals. (arXiv:2312.14244v1 [cond-mat.mes-hall])
Ang-Kun Wu, Daniele Guerci, Yixing Fu, Justin H. Wilson, J. H. Pixley

The circularly polarized photogalvanic effect (CPGE) is studied in chiral Weyl semimetals with short-ranged quenched disorder. Without disorder, the topological properties of chiral Weyl semimetals lead to the quantization of the CPGE, which is a second-order optical response. Using a combination of diagrammatic perturbation theory in the continuum and exact numerical calculations via the kernel polynomial method on a lattice model we show that disorder perturbatively destabilizes the quantization of the CPGE.

40 Years of SCES at Los Alamos. (arXiv:2312.14283v1 [cond-mat.str-el])
Z. Fisk, J. L. Smith, J. D. Thompson

Reports of unconventional superconductivity in UBe13 in 1983 and soon thereafter of the possible coexistence of bulk superconductivity and spin fluctuations in UPt3 marked the beginning of a 40-year adventure in the study of strongly correlated quantum materials and phenomena at Los Alamos. The subsequent discovery and exploration of heavy-fermion magnetism, cuprates, Kondo insulators, Ce- and Pu-115 superconductors and, more broadly, quantum states of narrow-band systems provided challenges for the next 30 years. Progress was not made in a vacuum but benefitted from significant advances in the Americas, Asia and Europe as well as from essential collaborations, visitors and Los Alamos students and postdocs, many subsequently setting their own course in SCES. As often the case, serendipity played a role in shaping this history.

Enhancing Transport Barriers with Swimming Microorganisms in Chaotic Flows. (arXiv:2312.14284v1 [physics.flu-dyn])
Ranjiangshang Ran, Paulo E. Arratia

We investigate the effects of bacterial activity on the mixing and transport properties of two-dimensional, time-periodic flows in experiments and in a simple model. We focus on the interactions between swimming E. coli and the flow Lagrangian Coherent Structure (LCS), which are computed from experimentally measured velocity fields. Experiments show that such interactions are non-trivial and can lead to transport barriers through which the tracer flux is significantly reduced. Using the Poincar\'e map, we show that these transport barriers coincide with the outermost members of elliptic LCSs known as Lagrangian vortex boundaries. Numerical simulations further show that elliptic LCSs can repel elongated swimmers and lead to swimmer depletion within Lagrangian coherent vortices. A simple mechanism shows that such depletion is due to the preferential alignment of elongated swimmers with the tangents of elliptic LCSs. Our results provide insights into understanding the transport of microorganisms in complex flows with dynamical topological features from a Lagrangian viewpoint.

Crystal Growth Characterization of WSe$_2$ Thin Film Using Machine Learning. (arXiv:2312.14311v1 [cond-mat.mtrl-sci])
Isaiah A. Moses, Chengyin Wu, Wesley F. Reinhart

Materials characterization remains a labor-intensive process, with a large amount of expert time required to post-process and analyze micrographs. As a result, machine learning has become an essential tool in materials science, including for materials characterization. In this study, we perform an in-depth analysis of the prediction of crystal coverage in WSe$_2$ thin film atomic force microscopy (AFM) height maps with supervised regression and segmentation models. Regression models were trained from scratch and through transfer learning from a ResNet pretrained on ImageNet and MicroNet to predict monolayer crystal coverage. Models trained from scratch outperformed those using features extracted from pretrained models, but fine-tuning yielded the best performance, with an impressive 0.99 $R^2$ value on a diverse set of held-out test micrographs. Notably, features extracted from MicroNet showed significantly better performance than those from ImageNet, but fine-tuning on ImageNet demonstrated the reverse. As the problem is natively a segmentation task, the segmentation models excelled in determining crystal coverage on image patches. However, when applied to full images rather than patches, the performance of segmentation models degraded considerably, while the regressors did not, suggesting that regression models may be more robust to scale and dimension changes compared to segmentation models. Our results demonstrate the efficacy of computer vision models for automating sample characterization in 2D materials while providing important practical considerations for their use in the development of chalcogenide thin films.

Broken inversion symmetry in van der Waals topological ferromagnetic metal iron germanium telluride. (arXiv:2312.14384v1 [cond-mat.mtrl-sci])
Kai-Xuan Zhang, Hwiin Ju, Hyuncheol Kim, Jingyuan Cui, Jihoon Keum, Je-Geun Park, Jong Seok Lee

Inversion symmetry breaking is critical for many quantum effects and fundamental for spin-orbit torque, which is crucial for next-generation spintronics. Recently, a novel type of gigantic intrinsic spin-orbit torque has been established in the topological van-der-Waals (vdW) magnet iron germanium telluride. However, it remains a puzzle because no clear evidence exists for interlayer inversion symmetry breaking. Here, we report the definitive evidence of broken inversion symmetry in iron germanium telluride directly measured by the second harmonic generation (SHG) technique. Our data show that the crystal symmetry reduces from centrosymmetric P63/mmc to noncentrosymmetric polar P3m1 space group, giving the three-fold SHG pattern with dominant out-of-plane polarization. Additionally, the SHG response evolves from an isotropic pattern to a sharp three-fold symmetry upon increasing Fe deficiency, mainly due to the transition from random defects to ordered Fe vacancies. Such SHG response is robust against temperature, ensuring unaltered crystalline symmetries above and below the ferromagnetic transition temperature. These findings add crucial new information to our understanding of this interesting vdW metal, iron germanium telluride: band topology, intrinsic spin-orbit torque and topological vdW polar metal states.

Electronic structure, magnetic and transport properties of antiferromagnetic Weyl semimetal GdAlSi. (arXiv:2312.14415v1 [cond-mat.str-el])
Antu Laha, Asish K. Kundu, Niraj Aryal, Emil S. Bozin, Juntao Yao, Sarah Paone, Anil Rajapitamahuni, Elio Vescovo, Tonica Valla, Milinda Abeykoon, Ran Jing, Weiguo Yin, Abhay N. Pasupathy, Mengkun Liu, Qiang Li

We report the topological electronic structure, magnetic, and magnetotransport properties of a noncentrosymmetric compound GdAlSi. Magnetic susceptibility shows an antiferromagnetic transition at $T_\mathrm{N}$ = 32 K. In-plane isothermal magnetization exhibits an unusual hysteresis behavior at higher magnetic field, rather than near zero field. Moreover, the hysteresis behavior is asymmetric under positive and negative magnetic fields. First-principles calculations were performed on various magnetic configurations, revealing that the antiferromagnetic state is the ground state, and the spiral antiferromagnetic state is a close competing state. The calculations also reveal that GdAlSi hosts multiple Weyl points near the Fermi energy. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) shows relatively good agreement with the theory, with the possibility of Weyl nodes slightly above the Fermi energy. Within the magnetic ordered state, we observe an exceptionally large anomalous Hall conductivity (AHC) of ~ 1310 $\Omega^{-1}$cm$^{-1}$ at 2 K. Interestingly, the anomalous Hall effect persists up to room temperature with a significant value of AHC (~ 155 $\Omega^{-1}$cm$^{-1}$). Our analysis indicates that the large AHC originates from the Berry curvature associated with the multiple pairs of Weyl points near Fermi energy.

Thermodynamic and Stoichiometric Laws Ruling the Fates of Growing Systems. (arXiv:2312.14435v1 [cond-mat.stat-mech])
Atsushi Kamimura, Yuki Sughiyama, Tetsuya J. Kobayashi

We delve into growing open chemical reaction systems (CRSs) characterized by autocatalytic reactions within a variable volume, which changes in response to these reactions. Understanding the thermodynamics of such systems is crucial for comprehending biological cells and constructing protocells, as it sheds light on the physical conditions necessary for their self-replication. Building on our recent work, where we developed a thermodynamic theory for growing CRSs featuring basic autocatalytic motifs with regular stoichiometric matrices, we now expand this theory to include scenarios where the stoichiometric matrix has a nontrivial left kernel space. This extension introduces conservation laws, which limit the variations in chemical species due to reactions, thereby confining the system's possible states to those compatible with its initial conditions. By considering both thermodynamic and stoichiometric constraints, we clarify the environmental and initial conditions that dictate the CRSs' fate-whether they grow, shrink, or reach equilibrium. We also find that the conserved quantities significantly influence the equilibrium state achieved by a growing CRS. These results are derived independently of specific thermodynamic potentials or reaction kinetics, therefore underscoring the fundamental impact of conservation laws on the growth of the system.

Spontaneous gap opening and potential excitonic states in an ideal Dirac semimetal Ta$_2$Pd$_3$Te$_5$. (arXiv:2312.14456v1 [cond-mat.mtrl-sci])
Peng Zhang, Yuyang Dong, Dayu Yan, Bei Jiang, Tao Yang, Jun Li, Zhaopeng Guo, Yong Huang, Bo Hao, Qing Li, Yupeng Li, Kifu Kurokawa, Rui Wang, Yuefeng Nie, Makoto Hashimoto, Donghui Lu, Wen-He Jiao, Jie Shen, Tian Qian, Zhijun Wang, Youguo Shi, Takeshi Kondo

The opening of an energy gap in the electronic structure generally indicates the presence of interactions. In materials with low carrier density and short screening length, long-range Coulomb interaction favors the spontaneous formation of electron-hole pairs, so-called excitons, opening an excitonic gap at the Fermi level. Excitonic materials host unique phenomenons associated with pair excitations. However, there is still no generally recognized single-crystal material with excitonic order, which is, therefore, awaited in condensed matter physics. Here, we show that excitonic states may exist in the quasi-one-dimensional material Ta$_2$Pd$_3$Te$_5$, which has an almost ideal Dirac-like band structure, with Dirac point located exactly at Fermi level. We find that an energy gap appears at 350 K, and it grows with decreasing temperature. The spontaneous gap opening is absent in a similar material Ta$_2$Ni$_3$Te$_5$. Intriguingly, the gap is destroyed by the potassium deposition on the crystal, likely due to extra-doped carriers. Furthermore, we observe a pair of in-gap flat bands, which is an analog of the impurity states in a superconducting gap. All these observations can be properly explained by an excitonic order, providing Ta$_2$Pd$_3$Te$_5$ as a new and promising candidate realizing excitonic states.

High Magnetoresistance Ratio on hBN Boron-Vacancy/Graphene Magnetic Tunnel Junction. (arXiv:2312.14476v1 [cond-mat.mes-hall])
Halimah Harfah, Yusuf Wicaksono, Gagus Ketut Sunnardianto, Muhammad Aziz Majidi, Koichi Kusakabe

We presents a new strategy to create a van der Waals-based magnetic tunnel junction (MTJ) that consists of a three-atom layer thickness of graphene (Gr) sandwiched with hexagonal boron nitride (hBN) by introducing a monoatomic Boron vacancy in both hBN layers. The magnetic properties and electronic structure of the system were investigated using density functional theory (DFT), while the transmission probability of the MTJ was investigated using the Landauer-B\"uttiker formalism within the non-equilibrium Green function method. The Stoner gap was found to be created between the spin-majority channel and the spin-minority channel on LDOS of the hBN monoatomic boron-vacancy (V$_B$) near the vicinity of Fermi energy, creating a possible control of the spin valve by considering two different magnetic allignment of hBN(V$_B$) layers, anti-parallel and parallel configuration. The results of the transmission probability calculation showed a high electron transmission in the parallel configuration of the hBN(V$_B$) layers and a low transmission when the antiparallel configuration was considered. A high TMR ratio of approximately 400% was observed when comparing the antiparallel and parallel configuration of hBN(V$_B$) layers in the hBN (V$_B$)/Gr/hBN(V$_B$), giving the highest TMR for the thinnest MTJ system.

Quasi-localization and Wannier Obstruction in Partially Flat Bands. (arXiv:2312.14553v1 [cond-mat.str-el])
Jin-Hong Park, Jun-Won Rhim

The localized nature of a flat band is understood by the existence of a compact localized eigenstate. However, the localization properties of a partially flat band, ubiquitous in surface modes of topological semimetals, have been unknown. We show that the partially flat band is characterized by a non-normalizable compact localized state(NCLS). The partially flat band develops only in a momentum range, where normalizable Bloch wave functions can be obtained by the linear combination of the NCLSs. Outside this momentum region, a ghost flat band, unseen from the band structure, is introduced for the consistent counting argument with the full set of NCLSs. Then, we demonstrate that the Wannier function corresponding to the partially flat band exhibits an algebraic($\sim 1/r^{1+\epsilon}$ in 1D and $\sim 1/r^{3/2+\epsilon}$ in 2D) decay behavior, where $\epsilon$ is a positive number. Namely, one can have the Wannier obstruction even in a topologically trivial band if it is partially flat. Finally, we develop a construction scheme of a tight-binding model of the topological semimetal by designing an NCLS.

Optical wood with switchable solar transmittance for all-round thermal management. (arXiv:2312.14560v1 [physics.optics])
He Gao, Ying Li, Yanjun Xie, Daxin Liang, Jian Li, Yonggui Wang, Zefang Xiao, Haigang Wang, Wentao Gan, Lorenzo Pattelli, Hongbo Xu

Technologies enabling passive daytime radiative cooling and daylight harvesting are highly relevant for energy-efficient buildings. Despite recent progress demonstrated with passively cooling polymer coatings, however, it remains challenging to combine also a passive heat gain mechanism into a single substrate for all-round thermal management. Herein, we developed an optical wood (OW) with switchable transmittance of solar irradiation enabled by the hierarchically porous structure, ultralow absorption in solar spectrum and high infrared absorption of cellulose nanofibers. After delignification, the OW shows a high solar reflectance (94.9%) in the visible and high broadband emissivity (0.93) in the infrared region (2.5-25 $\mu$m). Owing to the exceptional mass transport of its aligned cellulose nanofibers, OW can quickly switch to a new highly transparent state following phenylethanol impregnation. The solar transmittance of optical wood (OW-II state) can reach 68.4% from 250 to 2500 nm. The switchable OW exhibits efficient radiative cooling to 4.5 {\deg}C below ambient temperature in summer (81.4 W m$^{-2}$ cooling power), and daylight heating to 5.6 {\deg}C above the temperature of natural wood in winter (heating power 229.5 W m$^{-2}$), suggesting its promising role as a low-cost and sustainable solution to all-season thermal management applications.

Towards a comprehensive understanding of the low energy luminescence peak in 2D materials. (arXiv:2312.14604v1 [cond-mat.mtrl-sci])
Keerthana S Kumar, Ajit Kumar Dash, Hasna Sabreen H, Manvi Verma, Vivek Kumar, Kenji Watanabe, Takashi Taniguchi, Gopalakrishnan Sai Gautam, Akshay Singh

An intense low-energy broad luminescence peak (L-peak) is usually observed in 2D transition metal dichalcogenides (TMDs) at low temperatures. L-peak has earlier been attributed to bound excitons, but its origins are widely debated with direct consequences on optoelectronic properties. To decouple the contributions of physisorbed and chemisorbed oxygen, organic adsorbates, and strain on L-peak, we measured a series of monolayer (ML) MoS2 samples (mechanically exfoliated (ME), synthesized by oxygen-assisted chemical vapour deposition (O-CVD), hexagonal boron nitride (hBN) covered and hBN encapsulated). Emergence of L-peak below 150 K and saturation of photoluminescence (PL) intensity with laser power confirm bound nature of L-peak. Anomalously at room temperature, O-CVD samples show high A-exciton PL (c.f. ME), but reduced PL at low temperatures, which is attributed to strain-induced direct-to-indirect bandgap change in low defect O-CVD MoS2. Further, L-peak redshifts dramatically ~ 130 meV for O-CVD samples (c.f. ME). These observations are fully consistent with our predictions from density functional theory (DFT) calculations, considering effects of both strain and defects, and supported by Raman spectroscopy. In ME samples, charged oxygen adatoms are identified as thermodynamically favourable defects which can create in-gap states, and contribute to the L-peak. The useful effect of hBN is found to originate from reduction of charged oxygen adatoms and hydrocarbon complexes. This combined experimental-theoretical study allows an enriched understanding of L-peak and beneficial impact of hBN, and motivates collective studies of strain and defects with direct impact on optoelectronics and quantum technologies.

Magnetic droplet solitons. (arXiv:2312.14621v1 [cond-mat.mes-hall])
Martina Ahlberg, Sheng Jiang, Roman Khymyn, Sunjae Chung, Johan Åkerman

Magnetic droplets are nanoscale, non-topological, dynamical solitons that can be nucleated in different spintronic devices, such as spin torque nano-oscillators (STNOs) and spin Hall nano-oscillators (SHNOs). This chapter first briefly discusses the theory of spin current driven dissipative magnetic droplets in ferromagnetic thin films with uniaxial anisotropy. We then thoroughly review the research literature on magnetic droplets and their salient features, as measured using electrical, microwave, and synchrotron techniques, and as envisaged by micromagnetic simulations. We also touch upon a closely related soliton, the dynamical skyrmion. Finally, we present an outlook of new routes in droplet science.

Dipole coupling of a bilayer graphene quantum dot to a high-impedance microwave resonator. (arXiv:2312.14629v1 [cond-mat.mes-hall])
Max J. Ruckriegel, Lisa M. Gächter, David Kealhofer, Mohsen Bahrami Panah, Chuyao Tong, Christoph Adam, Michele Masseroni, Hadrien Duprez, Rebekka Garreis, Kenji Watanabe, Takashi Taniguchi, Andreas Wallraff, Thomas Ihn, Klaus Ensslin, Wei Wister Huang

We implement circuit quantum electrodynamics (cQED) with quantum dots in bilayer graphene, a maturing material platform for semiconductor qubits that can host long-lived spin and valley states. The presented device combines a high-impedance ($Z_\mathrm{r} \approx 1 \mathrm{k{\Omega}}$) superconducting microwave resonator with a double quantum dot electrostatically defined in a graphene-based van der Waals heterostructure. Electric dipole coupling between the subsystems allows the resonator to sense the electric susceptibility of the double quantum dot from which we reconstruct its charge stability diagram. We achieve sensitive and fast detection with a signal-to-noise ratio of 3.5 within 1 ${\mu}\mathrm{s}$ integration time. The charge-photon interaction is quantified in the dispersive and resonant regimes by comparing the coupling-induced change in the resonator response to input-output theory, yielding a maximal coupling strength of $g/2{\pi} = 49.7 \mathrm{MHz}$. Our results introduce cQED as a probe for quantum dots in van der Waals materials and indicate a path toward coherent charge-photon coupling with bilayer graphene quantum dots.

Photoinduced topological phase transition in monolayer Ti$_2$SiCO$_2$. (arXiv:2312.14639v1 [physics.optics])
Pu Liu, Chaoxi Cui, Zhi-Ming Yu

The TiSiCO-family monolayer $X_2Y$CO$_2$($X$=Ti, Zr, Hf; $Y$=Si, Ge) is a two-dimensional second-order topological insulator with unique valley-layer coupling in equilibrium condition. In this work, based on the four-band tight-binding (TB) model of monolayer Ti$_2$SiCO$_2$ (ML-TiSiCO) and the Floquet theory, we study the non-equilibrium properties of the ML-TiSiCO under a periodic field of laser and a gate-electric field. We find the interaction between the time-periodic polarized light and the electric field can lead to a variety of intriguing topological phase transitions. By driving the system with only circularly polarized light (CPL), a photoinduced topological phase transition occurs from a second-order topological insulator to a Chern insulator with a Chern number of $C=\pm$2, and the sign of the Chern number $C$ is determined by the chirality of the incident light. Further adding a perpendicular electric field, we find that the ML-TiSiCO exhibits a rich phase diagram, consisting of Chern insulators with different Chern numbers and various topological semimetals. In contrast, since the linearly polarized light (LPL) does not break time-reversal symmetry, the Chern number of the system would not be changed under the irradiation of LPL. However, there still exist many topological phases, including second-order topological insulator, topological semi-Dirac, Dirac and valley-polarized Dirac semimetals under the interaction between the LPL and the electric field. Our results not only enhance the understanding of the fundamental properties of ML-TiSiCO but also broaden the potential applications of such materials in optoelectronic devices.

Thermodynamics and dynamics of coupled complex SYK models. (arXiv:2312.14644v1 [hep-th])
Jan C. Louw, Linda M. van Manen, Rishabh Jha

It has been known that the large-$q$ complex SYK model falls under the same universality class as that of van der Waals (mean-field) which is also shared by a variety of black holes. At the same time, it also saturates the Maldacena-Shenker-Stanford (MSS) bound and is thus maximally chaotic. This work establishes the robustness of shared universality class and quantum chaos for SYK-like models by extending to a system of coupled large-$q$ complex SYK models of different orders. We provide a detailed derivation of thermodynamic (critical exponents) properties observing a phase transition and dynamic (Lyapunov exponent) properties via the out-of-time correlator (OTOC) calculations. Our analysis reveals that, despite the introduction of an additional scaling parameter through interaction strength ratios, the system undergoes a continuous phase transition at low temperatures, similar to that of a single SYK model. The critical exponents align with the Landau-Ginzburg (mean-field) universality class, shared with van der Waals gases and various AdS black holes. Furthermore, we demonstrate that the coupled SYK system remains maximally chaotic in the large-$q$ limit at low temperatures, adhering to the Maldacena-Shenker-Stanford (MSS) bound, a feature consistent with single large-$q$ complex SYK model. These findings open avenues for broader inquiries into the universality and chaos in complex quantum systems by showing that our coupled SYK system belong to the same universality class as that of van der Waals and various AdS black holes while saturating the MSS bound of quantum chaos.

Tunneling in ABC trilayer graphene superlattice. (arXiv:2312.14704v1 [cond-mat.mes-hall])
Mouhamadou Hassane Saley, Jaouad El-hassouny, Abderrahim El Mouhafid, Ahmed Jellal

We investigate the transport properties of Dirac fermions in ABC trilayer graphene {(ABC-TLG)} superlattices. Based on the transfer matrix method and using the continuity conditions of the system, we calculate the transmission probabilities {and the corresponding conductance}. In the context of two-band tunneling, Klein tunneling is observed, but it decreases with an increase in the number of cells. An interlayer bias opens a gap when the number of cells is increased. Furthermore, increasing the barrier/well width and the cell number results in an increase in the number of gaps and oscillations in both two-band and six-band cases. Asymmetry is found in the scattered transmission due to the presence of the interlayer bias. The conductance decreases when the number of cells increases and a gap region is found. Our results indicate that adjusting the number of cells, the width of the barrier/well, and the barrier heights makes it possible to control electron tunneling and the gap number in ABC-TLG. These findings provide valuable insights for the development of electronic devices using graphene materials.

Line defects in nematic liquid crystals as charged superelastic rods with negative twist--stretch coupling. (arXiv:2312.14735v1 [cond-mat.soft])
Shengzhu Yi, Hao Chen, Xinyu Wang, Miao Jiang, Bo Li, Qi-huo Wei, Rui Zhang

Topological defects are a ubiquitous phenomenon in diverse physical systems. In nematic liquid crystals (LCs), they are dynamic, physicochemically distinct, sensitive to stimuli, and are thereby promising for a range of applications. However, our current understanding of the mechanics and dynamics of defects in nematic LCs remain limited and are often overwhelmed by the intricate details of the specific systems. Here, we unify singular and nonsingular line defects as superelastic rods and combine theory, simulation, and experiment to quantitatively measure their effective elastic moduli, including line tension, torsional rigidity, and twist--stretch coefficient. Interestingly, we found that line defects exhibit a negative twist--stretch coupling, meaning that twisted line defects tend to unwind under stretching, which is reminiscent of DNA molecules. A patterned nematic cell experiment further confirmed the above findings. Taken together, we have established an effective elasticity theory for nematic defects, paving the way towards understanding and engineering their deformation and transformation in driven and active nematic materials.

Action formalism for geometric phases from self-closing quantum trajectories. (arXiv:2312.14760v1 [quant-ph])
Dominic Shea, Alessandro Romito

When subject to measurements, quantum systems evolve along stochastic quantum trajectories that can be naturally equipped with a geometric phase observable via a post-selection in a final projective measurement. When post-selecting the trajectories to form a close loop, the geometric phase undergoes a topological transition driven by the measurement strength. Here, we study the geometric phase of a subset of self-closing trajectories induced by a continuous Gaussian measurement of a single qubit system. We utilize a stochastic path integral that enables the analysis of rare self-closing events using action methods and develop the formalism to incorporate the measurement-induced geometric phase therein. We show that the geometric phase of the most likely trajectories undergoes a topological transition for self-closing trajectories as a function of the measurement strength parameter. Moreover, the inclusion of Gaussian corrections in the vicinity of the most probable self-closing trajectory quantitatively changes the transition point in agreement with results from numerical simulations of the full set of quantum trajectories.

Evidence for correlated electron pairs and triplets in quantum Hall interferometers. (arXiv:2312.14767v1 [cond-mat.mes-hall])
Wenmin Yang, David Perconte, Corentin Déprez, Kenji Watanabe, Takashi Taniguchi, Sylvain Dumont, Edouard Wagner, Frédéric Gay, Inès Safi, Hermann Sellier, Benjamin Sacépé

Pairing of electrons is ubiquitous in electronic systems featuring attractive inter-electron interactions, as exemplified in superconductors. Counter-intuitively, it can also be mediated in certain circumstances by the repulsive Coulomb interaction alone. Quantum Hall (QH) Fabry-P\'erot interferometers (FPIs) tailored in two-dimensional electron gas under a perpendicular magnetic field has been argued to exhibit such unusual electron pairing seemingly without attractive interaction. Here, we show evidence in graphene QH FPIs revealing not only a similar electron pairing at bulk filling factor nu=2 but also an unforeseen emergence of electron tripling characterized by a fractional Aharonov-Bohm flux period h/3e (h is the Planck constant and e the electron charge) at nu=3. Leveraging a novel plunger-gate spectroscopy, we demonstrate that electron pairing (tripling) involves correlated charge transport on two (three) entangled QH edge channels. This spectroscopy indicates a quantum interference flux-periodicity determined by the sum of the phases acquired by the distinct QH edge channels having slightly different interfering areas. While recent theory invokes the dynamical exchange of neutral magnetoplasmons -- dubbed neutralons -- as mediator for electron pairing, our discovery of three entangled QH edge channels with apparent electron tripling defies understanding and introduces a new three-body problem for interacting fermions.

Enhancement of superconducting transition temperature and exotic stoichiometries in Lu-S system under high pressure. (arXiv:2312.14780v1 [cond-mat.supr-con])
Juefei Wu, Bangshuai Zhu, Chi Ding, Dexi Shao, Cuiying Pei, Qi Wang, Jian Sun, Yanpeng Qi

Binary metal sulfides are potential material family for exploring high Tc superconductors under high pressure. In this work, we study the crystal structures, electronic structures and superconducting properties of the Lu-S system in the pressure range from 0 GPa to 200 GPa, combining crystal structure predictions with ab-initio calculations. We predict 14 new structures, encompassing 7 unidentified stoichiometries. Within the S-rich structures, the formation of S atom cages is beneficial for superconductivity, with the superconducting transition temperature 25.86 K and 25.30 K for LuS6-C2/m at 70 GPa and LuS6-R-3m at 90 GPa, respectively. With the Lu/(Lu+S) ratio increases, the Lu-d electrons participate more in the electronic properties at the Fermi energy, resulting in the coexistence of superconductivity and topological non-triviality of LuS2-Cmca, as well as the superconductivity of predicted Lu-rich compounds. Our calculation is helpful for understanding the exotic properties in transition metal sulfides system under high pressure, providing possibility in designing novel superconductors for future experimental and theoretical works.

Pressure-induced structure phase transitions and superconductivity in dual topological insulator BiTe. (arXiv:2312.14784v1 [cond-mat.supr-con])
Shihao Zhu, Bangshuai Zhu, Cuiying Pei, Qi Wang, Jing Chen, Qinghua Zhang, Tianping Ying, Lin Gu, Yi Zhao, Changhua Li, Weizheng Cao, Mingxin Zhang, Lili Zhang, Jian Sun, Yulin Chen, Juefei Wu, Yanpeng Qi

The (Bi2)m(Bi2Te3)n homologous series possess natural multilayer heterostructure with intriguing physical properties at ambient pressure. Herein, we report the pressure-dependent evolution of the structure and electrical transport of the dual topological insulator BiTe, a member of the (Bi2)m(Bi2Te3)n series. With applied pressure, BiTe exhibits several different crystal structures and distinct superconducting states, which is remarkably similar to other members of the (Bi2)m(Bi2Te3)n series. Our results provide a systematic phase diagram for the pressure-induced superconductivity in BiTe, contributing to the highly interesting physics in this (Bi2)m(Bi2Te3)n series.

SuperVortexNet: Reconstructing Superfluid Vortex Filaments Using Deep Learning. (arXiv:2312.14815v1 [cond-mat.quant-gas])
Nick Keepfer, Thomas Flynn, Nick Parker, Thomas Billam

We introduce a novel approach to the three-dimensional reconstruction of superfluid vortex filaments using deep convolutional neural networks. Superfluid vortices, quantum mechanical phenomena of immense scientific interest, are challenging to image due to their small dimensions and intricate topology. Here, we propose a deep-learning methodology that serves as a proof-of-principle for fully reconstructing the topology of superfluid vortex filaments. We have trained a convolutional neural network on a large dataset of simulated superfluid density images obtained by solving the Gross--Pitaevskii equation at scale, enabling it to learn the complex patterns and features inherent to superfluid vortex filaments. The network ingests the integrated density along the axial, coronal, and sagittal directions and outputs the reconstructed superfluid vortex filaments in three dimensions. We demonstrate the success of this approach over a range of vortex densities of simulated isotropic quantum turbulence, enabling access to the characteristic scaling law of the decaying vortex line length.

The Impact of Local Strain Fields in Non-Collinear Antiferromagnetic Films. (arXiv:2312.14864v1 [cond-mat.mtrl-sci])
Freya Johnson (1), Frederic Rendell-Bhatti (2), Bryan D. Esser (3), Aisling Hussey (4), David W. McComb (5), Jan Zemen (6), David Boldrin (2), Lesley Cohen (7) ((1) Cavendish Laboratory University of Cambridge, (2) School of Physics and Astronomy University of Glasgow, (3) Monash Centre for Electron Microscopy Monash University, (4) School of Physics Trinity College Dublin, (5) Center for Electron Microscopy and Analysis The Ohio State University, (6) Faculty of Electrical Engineering Czech Technical University in Prague, (7) Blackett Laboratory Imperial College London)

Antiferromagnets hosting structural or magnetic order that breaks time reversal symmetry are of increasing interest for 'beyond von Neumann computing' applications because the topology of their band structure allows for intrinsic physical properties, exploitable in integrated memory and logic function. One such group are the non-collinear antiferromagnets. Essential for domain manipulation is the existence of small net moments found routinely when the material is synthesised in thin film form and attributed to symmetry-breaking caused by spin canting, either from the Dzyaloshinskii-Moriya interaction or from strain. Although the spin arrangement of these materials makes them highly sensitive to strain, there is little understanding about the influence of local strain fields caused by lattice defects on global properties, such as magnetisation and anomalous Hall effect. This premise is investigated by examining non-collinear films that are either highly lattice mismatched or closely matched to their substrate. In either case, edge dislocation networks are generated and for the former case these extend throughout the entire film thickness, creating large local strain fields. These strain fields allow for finite intrinsic magnetisation in seemly structurally relaxed films and influence the antiferromagnetic domain state and the intrinsic anomalous Hall effect.

Untangling the valley structure of states for intravalley exchange anisotropy in lead chalcogenides quantum dots. (arXiv:2312.14918v1 [cond-mat.mes-hall])
I. D. Avdeev, M. O. Nestoklon

We put forward a generalized procedure is which allows to restore the bulk-like electron and hole wave functions from the wave functions of quantum confined electron/hole states obtained in atomistic calculations. The procedure is applied to the lead chalcogenide quantum dots and the effective Hamiltonian of the exchange interaction for the ground state of an exciton localized in PbS and PbSe quantum dots was extracted. The results demonstrate that the matrix elements of intravalley exchange in PbS quantum dots are much more anisotropic than ones in PbSe.

Topological Green's function zeros in an exactly solved model and beyond. (arXiv:2312.14926v1 [cond-mat.str-el])
Steffen Bollmann, Chandan Setty, Urban F. P. Seifert, Elio J. König

The interplay of topological electronic band structures and strong interparticle interactions provides a promising path towards the constructive design of robust, long-range entangled many-body systems. As a prototype for such systems, we here study an exactly integrable, local model for a fractionalized topological insulator. Using a controlled perturbation theory about this limit, we demonstrate the existence of topological bands of zeros in the exact fermionic Green's function and show that {in this model} they do affect the topological invariant of the system, but not the quantized transport response. Close to (but prior to) the Higgs transition signaling the breakdown of fractionalization, the topological bands of zeros acquire a finite ``lifetime''. We also discuss the appearance of edge states and edge zeros at real space domain walls separating different phases of the system. This model provides a fertile ground for controlled studies of the phenomenology of Green's function zeros and the underlying exactly solvable lattice gauge theory illustrates the synergetic cross-pollination between solid-state theory, high-energy physics and quantum information science.

Corrections to the reflectance of graphene by light emission. (arXiv:2208.01311v4 [cond-mat.mtrl-sci] UPDATED)
Ken-ichi Sasaki, Kenichi Hitachi, Masahiro Kamada, Takamoto Yokosawa, Taisuke Ochi, Tomohiro Matsui

Monolayer graphene absorbs 2.3 percent of the incident visible light. This 'small' absorption has been used to emphasize the visual transparency of graphene, but it in fact means that multilayer graphene absorbs a sizable fraction of incident light, which causes non-negligible fluorescence. In this paper, we formulate the light emission properties of multilayer graphene composed of tens to hundreds of layers using a transfer matrix method and confirm the method's validity experimentally. We could quantitatively explain the measured contrasts of multilayer graphene on SiO$_2$/Si substrates and found sizable corrections, which cannot be classified as incoherent light emissions, to the reflectance of visible light. The new component originates from coherent emission caused by absorption at each graphene layer. Multilayer graphene thus functions as a partial coherent light source of various wavelengths, and it may have surface-emitting laser applications.

Signatures of many-body localization of quasiparticles in a flat band superconductor. (arXiv:2302.06250v3 [cond-mat.supr-con] UPDATED)
Koushik Swaminathan, Poula Tadros, Sebastiano Peotta

We construct a class of exact eigenstates of the Hamiltonian obtained by projecting the Hubbard interaction term onto the flat band subspace of a generic lattice model. These exact eigenstates are many body states in which an arbitrary number of localized fermionic particles coexist with a sea of mobile Cooper pairs with zero momentum. By considering the dice lattice as an example, we provide evidence that these exact eigenstates are in fact manifestation of local integrals of motions of the projected Hamiltonian. In particular the spin and particle densities retain memory of the initial state for a very long time, if localized unpaired particles are present at the beginning of the time evolution. This shows that many-body localization of quasiparticles and superfluidity can coexist even in generic two-dimensional lattice models with flat bands, for which it is not known how to construct local conserved quantities. Our results open new perspectives on the old condensed matter problem of the interplay between superconductivity and localization.

Continuous phase transitions between fractional quantum Hall states and symmetry-protected topological states. (arXiv:2302.06501v3 [cond-mat.str-el] UPDATED)
Ying-Hai Wu, Hong-Hao Tu, Meng Cheng

We study quantum phase transitions in Bose-Fermi mixtures driven by interspecies interaction in the quantum Hall regime. In the absence of such an interaction, the bosons and fermions form their respective fractional quantum Hall (FQH) states at certain filling factors. A symmetry-protected topological (SPT) state is identified as the ground state for strong interspecies interaction. The phase transitions between them are proposed to be described by Chern-Simons-Higgs field theories. For a simple microscopic Hamiltonian, we present numerical evidence for the existence of the SPT state and a continuous transition to the FQH state. It is also found that the entanglement entropy between the bosons and fermions exhibits scaling behavior in the vicinity of this transition.

Transition to the Haldane phase driven by electron-electron correlations. (arXiv:2304.11154v2 [cond-mat.str-el] UPDATED)
A. Jażdżewska, M. Mierzejewski, M. Środa, A. Nocera, G. Alvarez, E. Dagotto, J. Herbrych

One of the most famous quantum systems with topological properties, the spin $\mathcal{S}=1$ antiferromagnetic Heisenberg chain, is well-known to display exotic $\mathcal{S}=1/2$ edge states. However, this spin model has not been analyzed from the more general perspective of strongly correlated systems varying the electron-electron interaction strength. Here, we report the investigation of the emergence of the Haldane edge in a system of interacting electrons -- the two-orbital Hubbard model -- with increasing repulsion strength $U$ and Hund interaction $J_\mathrm{H}$. We show that interactions not only form the magnetic moments but also form a topologically nontrivial fermionic many-body ground-state with zero-energy edge states. Specifically, upon increasing the strength of the Hubbard repulsion and Hund exchange, we identify a sharp transition point separating topologically trivial and nontrivial ground-states. Surprisingly, such a behaviour appears already at rather small values of the interaction, in a regime where the magnetic moments are barely developed.

Solving optimization problems with local light shift encoding on Rydberg quantum annealers. (arXiv:2308.07798v2 [quant-ph] UPDATED)
Kapil Goswami, Rick Mukherjee, Herwig Ott, Peter Schmelcher

We provide a non-unit disk framework to solve combinatorial optimization problems such as Maximum Cut (Max-Cut) and Maximum Independent Set (MIS) on a Rydberg quantum annealer. Our setup consists of a many-body interacting Rydberg system where locally controllable light shifts are applied to individual qubits in order to map the graph problem onto the Ising spin model. Exploiting the flexibility that optical tweezers offer in terms of spatial arrangement, our numerical simulations implement the local-detuning protocol while globally driving the Rydberg annealer to the desired many-body ground state, which is also the solution to the optimization problem. Using optimal control methods, these solutions are obtained for prototype graphs with varying sizes at time scales well within the system lifetime and with approximation ratios close to one. The non-blockade approach facilitates the encoding of graph problems with specific topologies that can be realized in two-dimensional Rydberg configurations and is applicable to both unweighted as well as weighted graphs. A comparative analysis with fast simulated annealing is provided which highlights the advantages of our scheme in terms of system size, hardness of the graph, and the number of iterations required to converge to the solution.

Toward a global phase diagram of the fractional quantum anomalous Hall effect. (arXiv:2308.10406v2 [cond-mat.mes-hall] UPDATED)
Aidan P. Reddy, Liang Fu

Recent experiments on the twisted semiconductor bilayer system $t$MoTe$_2$ have observed integer and fractional quantum anomalous Hall effects, which occur in topological moir\'e bands at zero magnetic field. Here, we present a global phase diagram of $t$MoTe$_2$ throughout the filling range $0< n\leq 1$ substantiated by exact diagonalization calculations. At a magic angle, we find that the system resembles the lowest Landau level (LLL) to a remarkable degree, exhibiting an abundance of incompressible fractional quantum anomalous Hall states and compressible anomalous composite Fermi liquid states. Away from the magic angle, particle-hole symmetry is strongly broken. Some LLL-like features remain robust near half-filling, while others are replaced, predominantly by charge density waves near $n=0$ and anomalous Hall Fermi liquids near $n=1$. Among LLL-like phases, we find the anomalous composite Fermi liquid at $n=\frac{1}{2}$ to be most robust against deviations from the magic angle. Within the band-projected model, we show that strong particle-hole asymmetry above the magic angle results from interaction-enhanced quasiparticle dispersion near $n=1$. Our work sets the stage for future exploration of LLL-like and beyond-LLL phases in fractional quantum anomalous Hall systems.

Second-order optical response of superconductors induced by supercurrent injection. (arXiv:2309.14077v3 [cond-mat.supr-con] UPDATED)
Linghao Huang, Jing Wang

We develop a theory of the nonlinear optical responses in superconducting systems in the presence of a dc supercurrent. The optical transitions between particle-hole pair bands across the superconducting gap are allowed in clean superconductors as the inversion symmetry breaking by supercurrent. Vertex correction is included in optical conductivity to maintain the $U(1)$ gauge symmetry in the mean-field formalism, which contains the contributions from collective modes. We show two pronounced current-dependent peaks in the second-harmonic generalization $\sigma^{(2)}(2\omega,\omega,\omega)$ at the gap edge $2\hbar\omega=2\Delta$ and $\hbar\omega=2\Delta$ and one in the photocurrent effect $\sigma^{(2)}(0,\omega,-\omega)$ at $\hbar\omega=2\Delta$, all of which diverge in the clean limit. We demonstrate this in the models of a single-band superconductor with $s$-wave and $d$-wave pairings, and Dirac fermion systems with $s$-wave pairing. Our theory predicts that the current-induced peak in $\text{Im}[\sigma^{(2)}(\omega)]$ is proportional to the square of the supercurrent density in the $s$-wave single-band model, with the same order of magnitude as the recent experimental observation of second-harmonic generation in NbN by Nakamura et al. [Phys. Rev. Lett. 125, 097004 (2020)]. Supercurrent induced nonlinear optical spectroscopy provides a valuable toolbox to explore novel superconductors.

Non-reciprocity permits edge states and strong localization in stochastic topological phases. (arXiv:2310.16720v2 [cond-mat.stat-mech] UPDATED)
Aleksandra Nelson, Evelyn Tang

Topological phases of matter exhibit edge responses with the attractive property of robustness against deformations and defects. Such phases have recently been realized in stochastic systems, which model a large class of biological and chemical phenomena. However, general theoretical principles are lacking for these systems, such as the relation between the bulk topological invariant and observed edge responses, i.e. the celebrated bulk-edge correspondence. We show that contrary to established topological phases, stochastic systems require non-reciprocal (or non-Hermitian) transitions to have edge responses. In both 1D and 2D models with different edge states, we demonstrate that stochastic topological responses grow dramatically with non-reciprocity while the quantum version plateaus. We further present a novel mechanism by which non-reciprocity engenders robust edge currents in stochastic systems. Our work establishes the crucial role of non-reciprocal interactions in permitting robust responses in soft and living matter.

Gate-tunable topological superconductivity in a supramolecular electron spin lattice. (arXiv:2310.18134v2 [cond-mat.supr-con] UPDATED)
Rémy Pawlak, Jung-Ching Liu, Chao Li, Richard Hess, Hongyan Chen, Carl Drechsel, Ping Zhou, Robert Häner, Ulrich Aschauer, Thilo Glatzel, Silvio Decurtins, Daniel Loss, Jelena Klinovaja, Shi-Xia Liu, Wulf Wulfhekel, Ernst Meyer

Topological superconductivity emerges in chains or arrays of magnetic atoms coupled to a superconductor. However, the external controllability of such systems with gate voltages is detrimental for their future implementation in a topological quantum computer. Here we showcase the supramolecular assembly of radical molecules on Pb(111), whose discharge is controlled by the tip of a scanning tunneling microscope. Charged molecules carry a spin-1/2 state, as confirmed by observing Yu-Shiba-Rusinov in-gap states by tunneling spectroscopy at millikelvin temperature. Low energy modes are localized at island boundaries with a long decay towards the interior, whose spectral signature is consistent with Majorana zero modes protected by mirror symmetry. Our results open up a vast playground for the synthesis of gate-tunable organic topological superconductors.

Observation of an electronic microemulsion phase emerging from a quantum crystal-to-liquid transition. (arXiv:2311.18069v2 [cond-mat.str-el] UPDATED)
Jiho Sung, Jue Wang, Ilya Esterlis, Pavel A. Volkov, Giovanni Scuri, You Zhou, Elise Brutschea, Takashi Taniguchi, Kenji Watanabe, Yubo Yang, Miguel A. Morales, Shiwei Zhang, Andrew J. Millis, Mikhail D. Lukin, Philip Kim, Eugene Demler, Hongkun Park

Strongly interacting electronic systems possess rich phase diagrams resulting from the competition between different quantum ground states. A general mechanism that relieves this frustration is the emergence of microemulsion phases, where regions of different phase self-organize across multiple length scales. The experimental characterization of these phases often poses significant challenges, as the long-range Coulomb interaction microscopically mingles the competing states. Here, we use cryogenic reflectance and magneto-optical spectroscopy to observe the signatures of the mixed state between an electronic Wigner crystal and an electron liquid in a MoSe2 monolayer. We find that the transit into this 'microemulsion' state is marked by anomalies in exciton reflectance, spin susceptibility, and Umklapp scattering, establishing it as a distinct phase of electronic matter. Our study of the two-dimensional electronic microemulsion phase elucidates the physics of novel correlated electron states with strong Coulomb interactions.

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Holographic Weyl anomaly in string theory, by Lorenz Eberhardt, Sridip Pal
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Submitted on 2023-12-24, refereeing deadline 2023-12-24.