Found 27 papers in cond-mat
Date of feed: Mon, 18 Dec 2023 01:30:00 GMT

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Comparative study of Kondo effect in Vanadium dichalcogenides VX$_2$ (X=Se & Te). (arXiv:2312.09258v1 [cond-mat.mtrl-sci])
Indrani Kar, Susanta Ghosh, Shuvankar Gupta, Sudip Chakraborty, S. Thirupathaiah

We report on the electrical transport, magnetotransport, and magnetic properties studies on the transition metal dichalcogenides VSe$_2$ and VTe$_2$ and draw a comprehensive comparison between them. We observe Kondo effect in both systems induced by the exchange interaction between localized moments and conduction electrons at low temperature, resulting into resistance upturn at 6 K for VSe$_2$ and 17 K for VTe$_2$. From the field dependent resistance measurements we find that the data is fitted best with modified Hamann equation corrected by the quantum Brillouin function for VSe$_2$, while the data is fitted best with modified Hamann equation corrected by the classical Langevin function for VTe$_2$. Interestingly, we observe a contrasting magnetoresistance (MR) property between these systems across the Kondo temperature. That means, negative MR is found in both systems in the Kondo state. In the normal state MR is positive for VSe$_2$, while it is negligible for VTe$_2$. In addition, both systems show weak ferromagnetism at low temperature due to intercalated V atoms.


Non-invertible symmetry-protected topological order in a group-based cluster state. (arXiv:2312.09272v1 [cond-mat.str-el])
Christopher Fechisin, Nathanan Tantivasadakarn, Victor V. Albert

Despite growing interest in beyond-group symmetries in quantum condensed matter systems, there are relatively few microscopic lattice models explicitly realizing these symmetries, and many phenomena have yet to be studied at the microscopic level. We introduce a one-dimensional stabilizer Hamiltonian composed of group-based Pauli operators whose ground state is a $G\times \text{Rep}(G)$-symmetric state: the $G \textit{ cluster state}$ introduced in $[\href{this http URL}{\text{Brell, New Journal of Physics }\textbf{17}\text{, 023029 (2015)}}]$. We show that this state lies in a symmetry-protected topological (SPT) phase protected by $G\times \text{Rep}(G)$ symmetry, distinct from the symmetric product state by a duality argument. We identify several signatures of SPT order, namely protected edge modes, string order parameters, and topological response. We discuss how $G$ cluster states may be used as a universal resource for measurement-based quantum computation, explicitly working out the case where $G$ is a semidirect product of abelian groups.


Solid-state single-photon sources: recent advances for novel quantum materials. (arXiv:2312.09280v1 [quant-ph])
Martin Esmann, Stephen C. Wein, Carlos Antón-Solanas

In this review, we describe the current landscape of emergent quantum materials for quantum photonic applications. We focus on three specific solid-state platforms: single emitters in monolayers of transition metal dichalcogenides, defects in hexagonal boron nitride, and colloidal quantum dots in perovskites. These platforms share a unique technological accessibility, enabling the rapid implementation of testbed quantum applications, all while being on the verge of becoming technologically mature enough for a first generation of real-world quantum applications.

The review begins with a comprehensive overview of the current state-of-the-art for relevant single-photon sources in the solid-state, introducing the most important performance criteria and experimental characterization techniques along the way. We then benchmark progress for each of the three novel materials against more established (yet complex) platforms, highlighting performance, material-specific advantages, and giving an outlook on quantum applications. This review will thus provide the reader with a snapshot on latest developments in the fast-paced field of emergent single-photon sources in the solid-state, including all the required concepts and experiments relevant to this technology.


Vacancy-induced tunable Kondo effect in twisted bilayer graphene. (arXiv:2312.09286v1 [cond-mat.str-el])
Yueqing Chang, Jinjing Yi, Ang-Kun Wu, Fabian B. Kugler, Eva Andrei, David Vanderbilt, Gabriel Kotliar, J. H. Pixley

In single sheets of graphene, vacancy-induced states have been shown to host an effective spin-1/2 hole that can be Kondo-screened at low temperatures. Here, we show how these vacancy-induced impurity states survive in twisted bilayer graphene (TBG), which thus provides a tunable system to probe the critical destruction of the Kondo effect in pseudogap hosts. Ab-initio calculations and atomic-scale modeling are used to determine the nature of the vacancy states in the vicinity of the magic angle in TBG, demonstrating that the vacancy can be treated as a quantum impurity. Utilizing this insight, we construct an Anderson impurity model with a TBG host that we solve using the numerical renormalization group combined with the kernel polynomial method. We determine the phase diagram of the model and show how there is a strict dichotomy between vacancies in the AA / BB versus AB / BA tunneling regions. In AB / BA vacancies, we find that the Kondo temperature at the magic angle develops a broad distribution with a tail to vanishing temperatures due to multifractal wavefunctions at the magic angle. We argue that the scanning tunneling microscopy response in the vicinity of the vacancy can act as a non-trivial probe of both the critical single-particle states and the underlying many-body ground state in magic-angle TBG.


Self-Diffusion and Structure of a Quasi Two-Dimensional, Classical Coulomb Gas Under Increasing Magnetic Field and Temperature. (arXiv:2312.09318v1 [cond-mat.mes-hall])
J. D. Hernández Velázquez, Z. Nussinov, A. Gama Goicochea

The influence of a magnetic field applied perpendicularly to the plane of a quasi two dimensional, low density classical Coulomb gas, with interparticle potential U of r as 1 over r, is studied using momentum conserving dissipative particle dynamics simulations. The self diffusion and structure of the gas are studied as functions of temperature and strength of the magnetic field. It is found that the gas undergoes a topological phase transition when the temperature is varied, in accord with the Bohr van Leeuwen BvL theorem, the structural properties being unaffected, resembling those of the strictly two dimensional Kosterlitz Thouless transition, with U of r as varying as ln r. Consistent with the BvL theorem, the transition temperature and the melting process of the condensed phase are unchanged by the field. Conversely, the self diffusion coefficient of the gas is strongly reduced by the magnetic field. At the largest values of the cyclotron frequency, the self diffusion coefficient is inversely proportional to the applied magnetic field. The implications of these results are discussed.


Hofstadter Butterfly and Broken-Symmetry Quantum Hall States in \alpha-Type Organic Dirac Fermion Systems. (arXiv:2312.09413v1 [cond-mat.mes-hall])
Toshihito Osada

The electronic state of \alpha-type organic Dirac fermion systems such as \alpha-(ET)_2I_3 or \alpha-(BETS)_2I_3 has been studied under magnetic fields using the four-band tight-binding model with Peierls phase factors. The validity of the Dirac fermion picture in these materials was confirmed by the generated Hofstadter butterfly and its Chern numbers. The four-component envelope function of the N = 0 Landau level with valley degeneracy was studied. It was found that the two degenerate valley states have different weights on A and A' molecules connected by inversion. This feature is also recognized for the N = 0 spin-split Landau levels under the Zeeman effect and the spin-orbit interaction. The spontaneous valley symmetry breaking in the N = 0 Landau levels due to the exchange interaction results in the \nu = 1 and -1 quantum Hall states accompanied by the spatial charge and spin modulations in a unit cell.


Fermionic quartet and vestigial gravity. (arXiv:2312.09435v1 [cond-mat.other])
G.E. Volovik

We discuss the two-step transitions in superconductors, where the intermediate state between the Cooper pair state and the normal metal is the 4-fermion condensate, which is called the intertwined vestigial order. We discuss different types of the vestigial order, which are possible in the spin-triplet superfluid $^3$He, and the topological objects in the vestigial phases. Since in $^3$He the order parameter $A_{\alpha i}$ represents the analog of gravitational tetrads, we suggest that the vestigial states are possible in quantum gravity. As in superconductors, the fermionic vacuum can experience two consequent phase transitions. At first transition the metric appears as the bilinear combination of tetrads $g_{\mu\nu} =\eta_{ab}< \hat E^a_\mu \hat E^b_\nu>$, while the tetrad order parameter is still absent, $e_\mu^a=< \hat E^a_\mu> =0$. This corresponds to the bosonic Einstein general relativity, which emerges in the fermionic vacuum. The nonzero tetrads $e_\mu^a=< \hat E^a_\mu> \neq 0$ appear at the second transition, where a kind of the Einstein-Cartan-Sciama-Kibble tetrad gravity is formed. This suggests that on the levels of particles, gravity acts with different strength on fermions and bosons.


Emergence of 6-particle "hexciton'' states in WS$_2$ and MoSe$_2$ monolayers. (arXiv:2312.09476v1 [cond-mat.mes-hall])
J. Choi, J. Li, D. Van Tuan, H. Dery, S. A. Crooker

When doped with a high density of mobile charge carriers, monolayer transition-metal dichalcogenide (TMD) semiconductors can host new types of composite many-particle exciton states that do not exist in conventional semiconductors. Such multi-particle bound states arise when a photoexcited electron-hole pair couples to not just a single Fermi sea that is quantum-mechanically distinguishable (as for the case of conventional charged excitons or trions), but rather couples simultaneously to \textit{multiple} Fermi seas, each having distinct spin and valley quantum numbers. Composite six-particle ``hexciton'' states were recently identified in electron-doped WSe$_2$ monolayers, but under suitable conditions they should also form in all other members of the monolayer TMD family. Here we present spectroscopic evidence demonstrating the emergence of many-body hexcitons in charge-tunable WS$_2$ monolayers (at the A-exciton) and MoSe$_2$ monolayers (at the B-exciton). The roles of distinguishability and carrier screening on the stability of hexcitons are discussed.


Transport response of topological hinge modes in $\alpha$-Bi$_4$Br$_4$. (arXiv:2312.09487v1 [cond-mat.mes-hall])
Md Shafayat Hossain, Qi Zhang, Zhiwei Wang, Nikhil Dhale, Wenhao Liu, Maksim Litskevich, Brian Casas, Nana Shumiya, Jia-Xin Yin, Tyler A. Cochran, Yongkai Li, Yu-Xiao Jiang, Ying Yang, Guangming Cheng, Zi-Jia Cheng, Xian P. Yang, Nan Yao, Titus Neupert, Luis Balicas, Yugui Yao, Bing Lv, M. Zahid Hasan

Electronic topological phases are renowned for their unique properties, where conducting surface states exist on the boundary of an insulating three-dimensional bulk. While the transport response of the surface states has been extensively studied, the response of the topological hinge modes remains elusive. Here, we investigate a layered topological insulator $\alpha$-Bi$_4$Br$_4$, and provide the first evidence for quantum transport in gapless topological hinge states existing within the insulating bulk and surface energy gaps. Our magnetoresistance measurements reveal pronounced h/e periodic (where h denotes Planck's constant and e represents the electron charge) Aharonov-Bohm oscillation. The observed periodicity, which directly reflects the enclosed area of phase-coherent electron propagation, matches the area enclosed by the sample hinges, providing compelling evidence for the quantum interference of electrons circumnavigating around the hinges. Notably, the h/e oscillations evolve as a function of magnetic field orientation, following the interference paths along the hinge modes that are allowed by topology and symmetry, and in agreement with the locations of the hinge modes according to our scanning tunneling microscopy images. Remarkably, this demonstration of quantum transport in a topological insulator can be achieved using a flake geometry and we show that it remains robust even at elevated temperatures. Our findings collectively reveal the quantum transport response of topological hinge modes with both topological nature and quantum coherence, which can be directly applied to the development of efficient quantum electronic devices.


Uniaxial zero thermal expansion in low-cost Mn2OBO3 from 3.5 to 1250 K. (arXiv:2312.09530v1 [cond-mat.mtrl-sci])
Chi-Hung Lee, Cheng-Yen Lin, Guan-Yu Chen

Unique zero thermal expansion (ZTE) materials are valuable for use in precision instruments, including electronics, aerospace parts, and engines. However, most ZTE materials have a temperature range less than 1000 K under which they do not expand. In this study, we present a uniaxial ZTE in the low-cost Mn2OBO3 with a thermal expansion coefficient of $\alpha$= -1.7$\times$10^(-7) K-1 along the [h00] direction from 3.5 to 1250 K. The monoclinic structure of Mn2OBO3 remains stable over the entire temperature range in ambient conditions. Considerable thermal contraction on the BO3 trigonal planar and thermal expansion on the MnO6 octahedra combine to produce uniaxial ZTE. No charge order-disorder transition, which could cause thermal contraction, was observed up to 1250 K.


Stress correlations in near-crystalline packings. (arXiv:2312.09555v1 [cond-mat.soft])
Roshan Maharana, Kabir Ramola

We derive exact results for stress correlations in near-crystalline systems in two and three dimensions. We study energy minimized configurations of particles interacting through Harmonic as well as Lennard-Jones potentials, for varying degrees of microscopic disorder and quenched forces on grains. Our findings demonstrate that the macroscopic elastic properties of such near-crystalline packings remain unchanged within a certain disorder threshold, yet they can be influenced by various factors, including packing density, pressure, and the strength of inter-particle interactions. We show that the stress correlations in such systems display anisotropic behavior at large lengthscales and are significantly influenced by the pre-stress of the system. The anisotropic nature of these correlations remains unaffected as we increase the strength of the disorder. Additionally, we derive the large lengthscale behavior for the change in the local stress components that shows a $1/r^d$ radial decay for the case of particle size disorder and a $1/r^{d-1}$ behavior for quenched forces introduced into a crystalline network. Finally, we verify our theoretical results numerically using energy-minimised static particle configurations.


Summation of Divergent Series and Quantum Phase Transitions in Kitaev Chains with Long-Range Hopping. (arXiv:2312.09566v1 [cond-mat.stat-mech])
Hao Fu, Peiqing Tong

We study the quantum phase transitions (QPTs) in extended Kitaev chains with long-range ($1/r^{\alpha}$) hopping. Formally, there are two QPT points at $\mu=\mu_0(\alpha)$ and $\mu_\pi(\alpha)$ ($\mu$ is the chemical potential) which correspond to the summations of $\sum_{m=1}^{\infty}m^{-\alpha}$ and $\sum_{m=1}^{\infty}(-1)^{m-1}m^{-\alpha}$, respectively. When $\alpha\leq0$, both the series are divergent and it is usually believed that no QPTs exist. However, we find that there are two QPTs at $\mu=\mu_0(0)$ and $\mu_\pi(0)$ for $\alpha=0$ and one QPT at $\mu=\mu_\pi(\alpha)$ for $\alpha<0$. These QPTs are second order. The $\mu_0(0)$ and $\mu_\pi(\alpha\leq0)$ correspond to the summations of the divergent series obtained by the analytic continuation of the Riemann $\zeta$ function and Dirichlet $\eta$ function. Moreover, it is found that the quasiparticle energy spectra are discontinue functions of the wave vector $k$ and divide into two branches. This is quite different from that in the case of $\alpha>0$ and induces topological phases with the winding number $\omega:=\pm1/2$. At the same time, the von Neumann entropy are power law of the subchain length $L$ no matter in the gapped region or not. In addition, we also study the QPTs, topological properties, and von Neumann entropy of the systems with $\alpha>0$.


Infrared anomalies in ultrathin Ti3C2Tx MXene films. (arXiv:2312.09573v1 [cond-mat.mtrl-sci])
Meng Li, Tao Cheng, Gongze Liu, He Huang, Keqiao Li, Yang Li, Jiayue Yang, Baoling Huang

Visible transparent but infrared reflective materials are ideal candidates for both transparent conductive films and low-emissivity glass, which are highly desired in a broad variety of areas such as touchscreens and displays, photovoltaics, smart windows, and antistatic coatings. Ultrathin Ti3C2Tx MXene films are emerging as promising low-emissivity transparent candidates. However, the fundamental IR properties of Ti3C2Tx has not been revealed experimentally due to daunting challenges in the preparation of continuous, large-area, and ultrathin films of optical quality on flat substrates. Herein, we proposed a tape-free transfer method that can help prepare centimeter-size and ultrathin (down to 8 nm) Ti3C2Tx films on diverse optical substrates. Benefitting from this method, the refractive index and permittivity for Ti3C2Tx were successfully measured. Ti3C2Tx films exhibit large in-plane permittivity in the IR region, yielding maximum IR reflectance of 88% for bulk films. Interestingly, three anomalies were found in ultrathin Ti3C2Tx films: strong dispersion in the permittivity, interlayer space-dependent optical properties, and abnormally high IR absorption for a 15-nm-thick film. These anomalies are important guidelines in the design of Ti3C2Tx-based low-emissivity transparent films and other related devices, and may inspire other intriguing applications such as ultrathin IR absorption coatings and tunable IR optical devices.


Topological atom optics and beyond with knotted quantum wavefunctions. (arXiv:2312.09619v1 [cond-mat.quant-gas])
Maitreyi Jayaseelan, Joseph D. Murphree, Justin T. Schultz, Janne Ruostekoski, Nicholas P. Bigelow

Atom optics demonstrates optical phenomena with coherent matter waves, providing a foundational connection between light and matter. Significant advances in optics have followed the realisation of structured light fields hosting complex singularities and topologically non-trivial characteristics. However, analogous studies are still in their infancy in the field of atom optics. Here, we investigate and experimentally create knotted quantum wavefunctions in spinor Bose--Einstein condensates which display non-trivial topologies. In our work we construct coordinated orbital and spin rotations of the atomic wavefunction, engineering a variety of discrete symmetries in the combined spin and orbital degrees of freedom. The structured wavefunctions that we create map to the surface of a torus to form torus knots, M\"obius strips, and a twice-linked Solomon's knot. In this paper we demonstrate striking connections between the symmetries and underlying topologies of multicomponent atomic systems and of vector optical fields--a realization of topological atom-optics.


Topological Band Inversion and Chiral Majorana Mode in Hcp Thallium. (arXiv:2312.09637v1 [cond-mat.supr-con])
Motoaki Hirayama, Takuya Nomoto, Ryotaro Arita

The chiral Majorana fermion is an exotic particle that is its own antiparticle. It can arise in a one-dimensional edge of topological materials, and especially that in a topological superconductor can be exploited in non-Abelian quantum computation. While the chiral Majorana mode (CMM) remains elusive, a promising situation is realized when superconductivity coexists with a topologically non-trivial surface state. Here, we perform fully non-empirical calculation for the CMM considering superconductivity and surface relaxation, and show that hexagonal close-packed thallium (Tl) has an ideal electronic state that harbors the CMM. The $k_z=0$ plane of Tl is a mirror plane, realizing a full-gap band inversion corresponding to a topological crystalline insulating phase. Its surface and hinge are stable and easy to make various structures. Another notable feature is that the surface Dirac point is very close to the Fermi level, so that a small Zeeman field can induce a topological transition. Our calculation indicates that Tl will provide a new platform of the Majorana fermion.


Non-monotonic temperature dependence of electron viscosity and crossover to high-temperature universal viscous fluid in monolayer and bilayer graphene. (arXiv:2312.09701v1 [cond-mat.mes-hall])
Indra Yudhistira, Ramal Afrose, Shaffique Adam

Electrons in quantum matter behave like a fluid when the quantum-mechanical carrier-carrier scattering dominates over other relaxation mechanisms. By combining a microscopic treatment of electron-electron interactions within the random phase approximation with a phenomenological Navier-Stokes like equation, we predict that in the limit of high temperature and strong Coulomb interactions, both monolayer and bilayer graphene exhibit a universal behavior in dynamic viscosity. We find that the dynamic viscosity to entropy density ratio for bilayer graphene is closer to the holographic bound suggesting that such a bound might be observable in a condensed matter system. We discuss how this could be observed experimentally using a magnetoconductance measurements in a Corbino geometry for a realistic range of temperature and carrier density.


Exposing the odd-parity superconductivity in CeRh$_2$As$_2$ with hydrostatic pressure. (arXiv:2312.09729v1 [cond-mat.supr-con])
Meike Pfeiffer, Konstantin Semeniuk, Javier F. Landaeta, Michael Nicklas, Christoph Geibel, Manuel Brando, Seunghyun Khim, Elena Hassinger

Odd-parity superconductivity is a rare and sought-for state of matter with a potential for applications in topological quantum computing. Crystals with staggered locally non-centrosymmetric structures have been proposed as platforms where a magnetic field can induce a transition between even- and odd-parity superconducting (SC) states. The superconductor CeRh$_2$As$_2$ with a critical temperature $T_{\mathrm{c}}\approx0.4\,\mathrm{K}$ is likely the first example material showing such a phase transition at a magnetic field $\mu_{0}H^{*}=4\,\mathrm{T}$ applied along the crystallographic $c$ axis. CeRh$_2$As$_2$ also undergoes a phase transition of an unknown origin at $T_{0}=0.5\,\mathrm{K}$ and presents signs of an antiferromagnetism below $0.25\,\mathrm{K}$. Under a hydrostatic pressure of $P_0\approx0.5\,\mathrm{GPa}$, the $T_{0}$ order vanishes, resulting in a quantum critical point. Here, using resistivity measurements under pressure, we investigate how the correlations and normal-state orders affect the SC phase switching. We find an enhancement of the in-plane critical field near $P_0$. At the same time, the two SC states persist well past $P_{0}$, until at least $2.7\,\mathrm{GPa}$ and $H^{*}$ is reduced to $0.3\,\mathrm{T}$, making the putative odd-parity state stable almost down to zero field.


Hyperbolic Bloch points in ferrimagnetic exchange spring. (arXiv:2312.09836v1 [cond-mat.mes-hall])
Javier Hermosa-Muñoz, Aurelio Hierro-Rodríguez, Andrea Sorrentino, José I. Martín, Luis M. Alvarez-Prado, Eva Pereiro, Carlos Quirós, María Velez, Salvador Ferrer

Bloch points in magnetic materials are attractive entities in view of magnetic information transport. Here, Bloch point configuration has been investigated and experimentally determined in a magnetic trilayer ($Gd_{12}Co_{88}/Nd_{17}Co_{83}/Gd_{24}Co_{76}$) with carefully adjusted composition within the ferrimagnetic $Gd_{x}Co_{1-x}$ alloys in order to engineer saturation magnetization, exchange length, and interlayer couplings (ferromagnetic vs antiferromagnetic). X-ray vector magnetic tomography has allowed us to determine experimentally Bloch point polarity (related to topological charge) and Bloch point helicity ${\gamma}$ (determined by magnetostatic energy). At the bottom layer (close to the ferromagnetic interface), Bloch points adopt a standard circulating configuration with helicity ${\gamma}$ close to ${\pi}/2$. Within the top layer (with much lower saturation magnetization), Bloch points nucleate within a Neel-like exchange spring domain wall created by the antiferromagnetic coupling and adopt an uncommon hyperbolic configuration, characterized by much larger helicity angles. Our results indicate a path for Bloch point engineering in future applications adjusting material parameters and domain wall characteristics.


Topological magnon gap engineering in van der Waals CrI$_3$ ferromagnets. (arXiv:2312.09903v1 [cond-mat.mtrl-sci])
Verena Brehm, Pawel Sobieszczyk, Jostein Kløgetvedt, Richard F. L. Evans, Elton J. G. Santos, Alireza Qaiumzadeh

The microscopic origin of the topological magnon band gap in CrI$_3$ ferromagnets has been a subject of controversy for years since two main models with distinct characteristics, i.e., Dzyaloshinskii-Moriya (DM) and Kitaev, provided possible explanations with different outcome implications. Here we investigate the angular magnetic field dependence of the magnon gap of CrI$_3$ by elucidating what main contributions play a major role in its generation. We implement stochastic atomistic spin dynamics simulations to compare the impact of these two spin interactions on the magnon spectra. We observe three distinct magnetic field dependencies between these two gap opening mechanisms. First, we demonstrate that the Kitaev-induced magnon gap is influenced by both the direction and amplitude of the applied magnetic field, while the DM-induced gap is solely affected by the magnetic field direction. Second, the position of the Dirac cones within the Kitaev-induced magnon gap shifts in response to changes in the magnetic field direction, whereas they remain unaffected by the magnetic field direction in the DM-induced gap scenario. Third, we find a direct-indirect magnon band-gap transition in the Kitaev model by varying the applied magnetic field direction. These differences may distinguish the origin of topological magnon gaps in CrI$_3$ and other van der Waals magnetic layers. Our findings pave the way for exploration and engineering topological gaps in van der Waals materials.


Kagome and honeycomb flat bands in moir\'e graphene. (arXiv:2303.03352v2 [cond-mat.mes-hall] UPDATED)
Michael G. Scheer, Biao Lian

We propose a class of graphene-based moir\'e systems hosting flat bands on kagome and honeycomb moir\'e superlattices. These systems are formed by stacking a graphene layer on a 2D substrate with lattice constant approximately $\sqrt{3}$ times that of graphene. When the moir\'e potentials are induced by a 2D irreducible corepresentation in the substrate, the model shows a rich phase diagram of low energy bands including eigenvalue fragile phases as well as kagome and honeycomb flat bands. Spin-orbit coupling in the substrate can lift symmetry protected degeneracies and create spin Chern bands, and we observe spin Chern numbers up to three. We additionally propose a moir\'e system formed by stacking two graphene-like layers with similar lattice constants and Fermi energies but with Dirac Fermi velocities of opposite sign. This system exhibits multiple kagome and honeycomb flat bands simultaneously. Both models we propose resemble the hypermagic model of [Scheer $\textit{et al.}$, Phys. Rev. B $\textbf{106}$, 115418 (2022)] and may provide ideal platforms for the realization of strongly correlated topological phases.


Observation of Flat Bands and Dirac Cones in a Pyrochlore Lattice Superconductor. (arXiv:2304.09066v2 [cond-mat.str-el] UPDATED)
Jianwei Huang, Chandan Setty, Liangzi Deng, Jing-Yang You, Hongxiong Liu, Sen Shao, Ji Seop Oh, Yucheng Guo, Yichen Zhang, Ziqin Yue, Jia-Xin Yin, Makoto Hashimoto, Donghui Lu, Sergey Gorovikov, Pengcheng Dai, Jonathan D. Denlinger, M. Zahid Hasan, Yuan-Ping Feng, Robert J. Birgeneau, Youguo Shi, Ching-Wu Chu, Guoqing Chang, Qimiao Si, Ming Yi

Emergent phases often appear when the electronic kinetic energy is comparable to the Coulomb interactions. One approach to seek material systems as hosts of such emergent phases is to realize localization of electronic wavefunctions due to the geometric frustration inherent in the crystal structure, resulting in flat electronic bands. Recently, such efforts have found a wide range of exotic phases in the two-dimensional kagome lattice, including magnetic order, time-reversal symmetry breaking charge order, nematicity, and superconductivity. However, the interlayer coupling of the kagome layers disrupts the destructive interference needed to completely quench the kinetic energy. Here we demonstrate that an interwoven kagome network-a pyrochlore lattice-can host a three dimensional (3D) localization of electron wavefunctions. Meanwhile, the nonsymmorphic symmetry of the pyrochlore lattice guarantees all band crossings at the Brillouin zone X point to be 3D gapless Dirac points, which was predicted theoretically but never yet observed experimentally. Through a combination of angle-resolved photoemission spectroscopy, fundamental lattice model and density functional theory calculations, we investigate the novel electronic structure of a Laves phase superconductor with a pyrochlore sublattice, CeRu$_2$. We observe flat bands originating from both the Ce 4$f$ orbitals as well as from the 3D destructive interference of the Ru 4$d$ orbitals. We further observe the nonsymmorphic symmetry-protected 3D gapless Dirac cones at the X point. Our work establishes the pyrochlore structure as a promising lattice platform to realize and tune novel emergent phases intertwining topology and many-body interactions.


Bending Stiffness Collapse, Buckling, Topological Bands of Freestanding Twisted Bilayer Graphene. (arXiv:2305.07543v2 [cond-mat.mes-hall] UPDATED)
Jin Wang, Ali Khosravi, Andrea Silva, Michele Fabrizio, Andrea Vanossi, Erio Tosatti

The freestanding twisted bilayer graphene (TBG) is unstable, below a critical twist angle {\theta}_c~3.7 degrees, against a moire (2 \times 1) buckling distortion at T=0. Realistic simulations reveal the concurrent unexpected collapse of the bending rigidity, an unrelated macroscopic mechanical parameter. An analytical model connects bending and buckling anomalies at T=0, but as temperature rises the former fades, while buckling persists further. The (2 \times 1) electronic properties are also surprising. The magic twist angle narrow bands, now eight in number, fail to show zone boundary splittings despite the new periodicity. Symmetry shows how this is dictated by an effective single valley physics. These structural, critical, and electronic predictions promise to make the freestanding state of TBG especially interesting.


Kinetic Friction of Structurally Superlubric 2D Material Interfaces. (arXiv:2306.00205v2 [cond-mat.mtrl-sci] UPDATED)
Jin Wang, Ming Ma, Erio Tosatti

The ultra-low kinetic friction F_k of 2D structurally superlubric interfaces, connected with the fast motion of the incommensurate moir\'e pattern, is often invoked for its linear increase with velocity v_0 and area A, but never seriously addressed and calculated so far. Here we do that, exemplifying with a twisted graphene layer sliding on top of bulk graphite -- a demonstration case that could easily be generalized to other systems. Neglecting quantum effects and assuming a classical Langevin dynamics, we derive friction expressions valid in two temperature regimes. At low temperatures the nonzero sliding friction velocity derivative dF_k/dv_0 is shown by Adelman-Doll-Kantorovich type approximations to be equivalent to that of a bilayer whose substrate is affected by an analytically derived effective damping parameter, replacing the semi-infinite substrate. At high temperatures, friction grows proportional to temperature as analytically required by fluctuation-dissipation. The theory is validated by non-equilibrium molecular dynamics simulations with different contact areas, velocities, twist angles and temperatures. Using 6^{\circ}-twisted graphene on Bernal graphite as a prototype we find a shear stress of measurable magnitude, from 25 kPa at low temperature to 260 kPa at room temperature, yet only at high sliding velocities such as 100 m/s. However, it will linearly drop many orders of magnitude below measurable values at common experimental velocities such as 1 {\mu}m/s, a factor 10^{-8} lower. The low but not ultra-low "engineering superlubric" friction measured in existing experiments should therefore be attributed to defects and/or edges, whose contribution surpasses by far the negligible moir\'e contribution.


Charge conservation in spin torque oscillators leads to a self-induced torque. (arXiv:2307.05105v3 [cond-mat.mes-hall] UPDATED)
Pieter M. Gunnink, Tim Ludwig, Rembert A. Duine

Spin torque oscillators are conventionally described by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, at the onset of oscillations, the predictions of the conventional LLGS equation differ qualitatively from experimental results and thus appear to be incomplete. In this work we show that taking charge conservation into account leads to a previously-overlooked self-induced torque, which modifies the LLGS equation. We show that the self-induced torque originates from the pumping current that a precessing magnetization drives through a magnetic tunnel junction. To illustrate the importance of the self-induced torque, we consider an in-plane magnetized nanopillar, where it gives clear qualitative corrections to the conventional LLGS description.


Perturbative RG flows in AdS: an \'etude. (arXiv:2309.10031v2 [hep-th] UPDATED)
Edoardo Lauria, Michael N. Milam, Balt C. van Rees

We discuss general properties of perturbative RG flows in AdS with a focus on the treatment of boundary conditions and infrared divergences. In contrast with flat-space boundary QFT, general covariance in AdS implies the absence of independent boundary flows. We illustrate how boundary correlation functions remain conformally covariant even if the bulk QFT has a scale. We apply our general discussion to the RG flow between consecutive unitary diagonal minimal models which is triggered by the $\phi_{(1,3)}$ operator. For these theories we conjecture a flow diagram whose form is significantly simpler than that in flat-space boundary QFT. In several stand-alone appendices we discuss two-dimensional BCFTs in general and the minimal model BCFTs in particular. These include both an extensive review as well as the computation of several new BCFT correlation functions.


Topological phases of monolayer and bilayer depleted Lieb lattices. (arXiv:2310.10286v2 [cond-mat.mes-hall] UPDATED)
Arghya Sil, Asim Kumar Ghosh

Existence of nontrivial topological phases in a tight binding Haldane-like model on the depleted Lieb lattice is reported. This two-band model is formulated by considering the nearest-neighbor, next-nearest-neighbor and next-next-nearest-neighbor hopping terms along with complex phase which breaks the time reversal symmetry of this semi-metallic system. Topological feature of this model is studied along with the presence of sublattice symmetry breaking staggered onsite energy. Combined effect of these two broken symmetries is found crucial for an additional transition between nontrivial and trivial phases. System exhibits two types of phase transitions, say, between two nontrivial phases and nontrivial to trivial phases. Nonzero Chern numbers, existence of Hall plateau and symmetry protected edge states confirm the presence of the nontrivial phases. This two-band system hosts four different types of phases where two are topological. Additionally topological properties of stacked bilayer of the depleted Lieb lattices is also studied with similar Haldane-like Hamiltonian. This four-band system is found to host Chern insulating phases, with higher values of Chern numbers supported by in-gap edge states.


Evidence for the novel type of orbital Fulde-Ferrell-Larkin-Ovchinnikov state in the bulk limit of 2H-NbSe2. (arXiv:2312.03215v2 [cond-mat.supr-con] UPDATED)
Chang-woo Cho, Kwan To Lo, Cheuk Yin Ng, Timothée T. Lortz, Abdel Rahman Allan, Mahmoud Abdel-Hafiez, Rolf Lortz

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, an unusual superconducting state, exhibits remarkable resilience against high magnetic fields that surpass the Pauli paramagnetic limit for superconductivity. This state, characterized by a spatial modulation of the superconducting order parameter in real space, is extremely rare. Recently, an even more exotic variant, the orbital FFLO state, has been first predicted and later identified in the transition metal dichalcogenide superconductor 2H-NbSe2. This state is observed in thin samples at the boundary between two and three dimensions, specifically for sample thicknesses below approximately 40 nm. The complex interplay between Ising spin orbit coupling and the Pauli paramagnetic effect can lead to a stabilization of the FFLO state in a relatively large range of the phase diagram, even below the Pauli limit. In this study, we present experimental evidence of the formation of this orbital FFLO state in true bulk 2H-NbSe2 samples. This evidence was obtained using high-resolution magnetic torque experiments in magnetic fields applied strictly parallel to the NbSe2 basal plane. The magnetic torque reveals a small step-like reversible anomaly, indicating a magnetic field-induced thermodynamic phase transition within the superconducting state. This anomaly bears many similarities to the FFLO transitions in other FFLO superconductors, suggesting the potential existence of an orbital FFLO state in bulk 2H-NbSe2 samples. Additionally, we observe a pronounced in-plane 6-fold symmetry of the upper critical field in the field range above this phase transition, which has previously been interpreted as a hallmark of the orbital FFLO state in thin 2H-NbSe2.