We report on the specific heat, magnetic properties, and ultrasonic measurements of a single-crystalline DyCuGe. We reveal an antiferromagnetic ordering in the (001) plane at $T_{\mathrm{N}}=7.4\mathrm{K}$, which is higher than the reported transition temperature of polycrystalline samples. The transverse elastic moduli show a pronounced softening down to $T_{\mathrm{N}}$ due to interlevel quadrupole-mediated interactions between the ground and excited Kramers doublets. Below $T_{\mathrm{N}}$, we further detected a huge softening accompanied by ultrasound attenuation in the transverse $C_{44}$ mode, suggesting a lattice instability. Our experimental and calculated results manifest that ferroquadrupolar interactions give rise to the lattice instability even in an antiferromagnetically ordered state.

Quenched disorders can strongly influence the physical properties of quantum many-body systems. The real-space strong-disorder renormalization group (SDRG) analysis has shown that the spin-1/2 random Heisenberg chain is controlled by the infinite-randomness fixed point (IRFP) and forms a random singlet (RS) ground state. Motivated by recent thermal transport experiments on the quasi-one-dimensional antiferromagnet copper benzoate [B. Y. Pan et al., Phys. Rev. Lett. 129, 167201 (2022)], we adapt the SDRG to study the low-temperature properties of the random Heisenberg chain by assuming that its low-energy excited states are captured by the parent Hamiltonian of the RS ground state as well. We find that while the specific heat coefficient and the uniform magnetic susceptibility scale as $C/T\sim T^{-{\alpha }_c}$ and $\chi \sim T^{-{\alpha }_s}$ with $0<{\alpha }_{c,s}<1$, indicating a divergent low-energy density of states, the thermal and the spin conductivities scale as $\kappa /T\sim T$ and ${\sigma }_s\sim T$, which implies a vanishing density of extended states in the low-energy limit. We believe that such a disparity in the thermodynamic and transport properties is a common feature of random systems controlled by the IRFP.

We report the magnetic and magnetotransport properties and electronic band structure of the kagome Dirac semimetal ${\mathrm{HoMn}}_6{\mathrm{Ge}}_6$. Temperature-dependent electrical resistivity demonstrates various magnetic-transition-driven anomalies. Notably, a crossover from negative to positive magnetoresistance (MR) is observed at around 150 K. While the linear nonsaturating positive MR in the low-temperature region is mainly driven by the linear Dirac-like band dispersions as predicted by the first-principles calculations, the negative MR observed in the high-temperature region is due to the spin-flop-type magnetic transition. Consistent with anisotropic Fermi surface topology, we observe anisotropic magnetoresistance at low temperatures. A significant anomalous Hall effect has been noticed at high temperatures in addition to a switching of the dominant charge carrier from electron to hole at around 215 K.

The only kinematically allowed phonon-scattering events for bands of subsonic fermions ($v_F<v_p$) are interband transitions, leading to different low-$T$ transport physics in nearly flat-band systems (NFBs) than in the typical supersonic case. We apply a kinetic theory of phonon-limited transport to a generic two-band system of subsonic fermions, deriving formulas for relaxation times and resistivity that are accurate in the “ultrasubsonic” limit defined by small $v_F/v_p$ and small band separation. We predict regimes of $\rho \propto T, \rho \propto T^2$, and perfect conductivity. Our theory predicts linear-in-$T$ resistivity down to a crossover temperature that is suppressed from its supersonic analog by a factor of $v_F/v_p$, offering a different explanation for low-$T$ “strange metal” behavior observed in NFBs. Understanding NFBs thus requires updated expectations for “normal” transport physics.

The exotic correlated phases that emerge in the low-dispersing moiré bands of twisted van der Waals structures have opened new exciting opportunities in condensed-matter physics. These systems exhibit a rich phase diagram of novel physical phenomena and exotic correlated phases that emerge in the low-dispersing bands. Despite its popularity, the resulting mini-bands in the conduction band of ${\mathrm{MoS}}_2$ moiré superlattices remained elusive so far. In this work, we resolve these spin-valley mini-bands via transport spectroscopy in micron-scale devices. The theoretical energy scales exhibit an astounding agreement with our experimental observations, which together with the behavior under thermal activation, suggest an electronic phase transition. These intriguing observations highlight the potential of twisted ${\mathrm{MoS}}_2$ structures as a playground to explore correlated electron states and associated phenomena.

Topological insulators are described by topological invariants that can be computed by integrals over momentum space but also as traces over local, real-space topological markers. These markers are useful to detect topological insulating phases in disordered crystals, quasicrystals, and amorphous systems. Among these markers, only the spectral localizer operator can be used to distinguish topological metals that show zero-modes of the localizer spectrum. However, it remains unclear whether trivial metals also display zero-modes and if their localizer spectrum is distinguishable from topological ones. Here we show that trivial metals generically display zero-modes of the localizer spectrum. The localizer zero-modes are determined by the zero-mode solutions of a Dirac equation with a varying mass parameter. We use this observation, valid in any dimension, to determine the difference between the localizer spectrum of trivial and topological metals and study the spectrum of the localizer for fractional quantum Hall edges. Because the localizer is a local, real-space operator, it may be used as a tool to differentiate between noncrystalline topological and trivial metals, and characterize strongly correlated systems, for which local topological markers are scarce.

Motivated by the exotic phenomenology of certain quantum materials and recent advances in programmable quantum emulators, we here study fermions and bosons in ${\mathbb{Z}}_N$ lattice gauge theories. We introduce a family of exactly soluble models, and characterize their orthogonal (semi)metallic ground states, the excitation spectrum, and the correlation functions. We further study integrability-breaking perturbations using an appropriately derived set of Feynman diagrammatic rules and borrowing physics associated to Anderson's orthogonality catastrophe. In the context of the ground states, we revisit Luttinger's theorem following Oshikawa's flux insertion argument and furthermore demonstrate the existence of a Luttinger surface of zeros in the fermionic Green's function. Upon inclusion of perturbations, we address the transition from the orthogonal metal to the normal state by condensation of certain excitations in the gauge sectors, so-called “$e$ particles.” We furthermore discuss the effect of dynamics in the dual “$m$-particle” excitations, which ultimately leads to the formation of charge-neutral hadronic $N$-particle bound states. We present analytical arguments for the most important phases and estimates for phase boundaries of the model. Specifically, and in sharp distinction to quasi-one-dimensional ${\mathbb{Z}}_N$ lattice gauge theories, renormalization group arguments imply that the phase diagram does not include an emergent deconfining U(1) phase in the limit of large number of fermion flavors. Therefore, in regards to lattice QED problems, ${\mathbb{Z}}_N$ quantum emulators with $N<\infty$ can at best be used for approximate solutions at intermediate length scales.

In this study, we investigate $\mathrm{Te}M\mathrm{Si}{X}_2$ ($M=\mathrm{Mo}$ and W; $X=\mathrm{P}$ and As) Janus monolayers, identifying them as direct band-gap semiconductors exhibiting two distinctive valley within both the conduction and valence bands. $\mathrm{Te}M\mathrm{Si}{X}_2$ exhibits significant valley spin splitting (VSS) in the valence band, especially in ${\mathrm{TeWSiP}}_2$ and ${\mathrm{TeWSiAs}}_2$ (up to 0.48 and 0.53 eV). Furthermore, the construction of $\mathrm{Te}M\mathrm{Si}{X}_2/{\mathrm{CrI}}_3$ heterostructure introduces valley polarization, with magnitudes ranging from 2.9 meV for ${\mathrm{TeMoSiP}}_2/{\mathrm{CrI}}_3$ to up to 26.1 meV for ${\mathrm{TeWSiAs}}_2/{\mathrm{CrI}}_3$. Manipulating the magnetization direction of ${\mathrm{CrI}}_3$ or the interlayer spacing results in alterations of valley polarization in $\mathrm{Te}M\mathrm{Si}{X}_2/{\mathrm{CrI}}_3$. These findings provide valuable guidance for the further development of valleytronic devices based on $\mathrm{Te}M\mathrm{Si}{X}_2$ Janus monolayers.

In the magic angle twisted bilayer graphene (MATBG), non-Fermi liquidlike transport phenomena are universally observed. To understand their origin, we perform the self-consistent analysis of the self-energy due to SU(4) valley $+$ spin fluctuations induced by the electron-electron correlation. In the SU(4) fluctuation mechanism, the 15 channels of fluctuations contribute additively to the self-energy. Therefore, the SU(4) fluctuation mechanism gives much higher electrical resistance than the spin fluctuation mechanism. By the same reason, SU(4) fluctuations of intermediate strength provide $T$-linear resistivity down to $\sim 1\mathrm{K}$. Interestingly, the $T$-linear resistivity is robustly realized for a wide range of electron filling, even away from the van Hove filling. This study provides a strong evidence for the importance of electron-electron correlation in MATBG.

The discovery of the ideal topological semimetallic state in experimentally accessible materials is of both fundamental and technological interest. Here, based on first-principles calculations using the virtual crystal approximation method, we study the evolution of band structures in the ${\mathrm{Ni}}_3\mathrm{In}$ cubic structure by intercalating the doped C or N atoms, termed as ${\mathrm{Ni}}_3{\mathrm{InC}}_x{\mathrm{N}}_{\left(1\text{–}x\right)}$. Through controlling the ratio of intercalated atoms, we calculate electronic structures of a series of doped samples. A distinctive feature of a nodal line occurring at the Fermi level is present at $x=0.26$, and the charge analysis indicates that the bond interaction between intercalated atoms and nickel atoms induces charge redistribution, giving rise to the rigidlike shift of bands. Within the spin-orbital coupling, the nodal line is gapped and thereby ${\mathrm{Ni}}_3{\mathrm{InC}}_x{\mathrm{N}}_{\left(1\text{–}x\right)}$ transforms into topological insulators due to the spin-rotation symmetry breaking. Moreover, the nontrivial state is insensitive to the external strain and pressure, so one can expect exotic correlation physics of massless nodal-line fermions in the doped systems of ${\mathrm{Ni}}_3{\mathrm{InC}}_x{\mathrm{N}}_{\left(1\text{–}x\right)}$. Our work provides a promising avenue to explore ideal topological massless quasiparticles in doped systems.

When a mobile hole is doped into an antiferromagnet, its movement will distort the surrounding magnetic order and yield a magnetic polaron. The resulting complex interplay of spin and charge degrees of freedom gives rise to very rich physics and is widely believed to be at the heart of high-temperature superconductivity in cuprates. In this paper, we develop a quantitative theoretical formalism, based on the phenomenological parton description, to describe magnetic polarons in the strong-coupling regime. We construct an effective Hamiltonian with weak coupling to the spin-wave excitations in the background, making the use of standard polaronic methods possible. Our starting point is a single hole doped into an antiferromagnet described by a “geometric string” capturing the strongly correlated hopping processes of charge and spin degrees of freedom, beyond linear spin-wave approximation. Subsequently, we introduce magnon excitations through a generalized $1/S$ expansion and derive an effective coupling of these spin waves to the hole plus the string (the meson) to arrive at an effective polaron Hamiltonian with density-density type interactions. After making a Born-Oppenheimer–type approximation, this system is solved using the self-consistent Born approximation to extract the renormalized polaron properties. We apply our formalism (i) to calculate beyond linear spin-wave angle-resolved photoemission spectroscopy spectra, (ii) to reveal the interplay of rovibrational meson excitations, and (iii) to analyze the pseudogap expected at low doping. Moreover, our work paves the way for exploring magnetic polarons out of equilibrium or in frustrated systems, where weak-coupling approaches are desirable and going beyond linear spin-wave theory becomes necessary.

In this study, we present first-principles calculations that introduce a nonvolatile spin field effect transistor (spin-FET) utilizing a van der Waals multiferroic heterostructure, specifically ${\mathrm{VSi}}_2{\mathrm{N}}_4/{\mathrm{Sc}}_2{\mathrm{CO}}_2$. We demonstrate that inverting the ferroelectric polarization in a ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ monolayer can effectively modulate the electronic states of a ${\mathrm{VSi}}_2{\mathrm{N}}_4$ monolayer, enabling a transition from half-metal to half-semiconductor. This transition significantly alters the electronic transport properties. Furthermore, we construct a spin-FET device based on this multiferroic heterostructure and observe that the ${\mathrm{VSi}}_2{\mathrm{N}}_4/{\mathrm{Sc}}_2{\mathrm{CO}}_2$-based spin-FET exhibits exceptional all-electric-controlled performance. Notably, the inversion of the ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ ferroelectric polarization yields a substantial on-off current ratio, approximately 650%, under a minimal bias voltage of 0.02 V. Additionally, we identify a unique spatially separated spin-polarized transport phenomenon, wherein pure spin-up electrons transport exclusively through ${\mathrm{VSi}}_2{\mathrm{N}}_4$, and spin-down electrons through ${\mathrm{Sc}}_2{\mathrm{CO}}_2$. Our findings suggest a promising pathway for developing low-energy dissipation and nonvolatile FET devices.

We reconsider the phase diagram of a three-dimensional ${\mathbb{Z}}_2$ topological insulator in the presence of short-ranged potential disorder, with the insight that nonperturbative rare states destabilize the noninteracting Dirac semimetal critical point separating different topological phases. Based on our numerical data on the density of states, conductivity, and wave functions, we argue that the putative Dirac semimetal line is destabilized into a diffusive metal phase of finite extent due to nonperturbative effects of rare regions. We discuss the implications of these results for past and current experiments on doped topological insulators.

Quantum computers promise a highly efficient approach to investigate quantum phase transitions, which describe abrupt changes between different ground states of many-body systems. At quantum critical points, the divergent correlation length and entanglement entropy render the ground state preparation difficult. In this work, we explore the imaginary-time evolution for probing the universal critical behavior as the universal information of the ground state can be extracted in the early time relaxation process. We propose a systematic and scalable scheme to probe the universal behaviors via imaginary-time critical dynamics on quantum computers and demonstrate the validness of our approach by both numerical simulation and quantum hardware experiments. With the full form of the universal scaling function in terms of imaginary time, system size, and circuit depth, we successfully probe the universality by scaling analysis of the critical dynamics at an early time and with shallower quantum circuit depth. Equipped with quantum error mitigation, we also confirm the expected scaling behavior from experimental results on a superconducting quantum processor which stands as the first experimental demonstration of universal imaginary-time quantum critical dynamics.

We investigate the properties of the two-dimensional model with Rashba-type spin-orbit coupling cubic in electron momentum. In the normal phase, edge states emerge on open boundaries. In the superconducting phase, edge states could evolve into gapped fermionic edge states. Such edge states can be differentiated by the modulation of critical dc Josephson current of a tunnel junction under an out-of-plane magnetic field. Our results may apply to oxide interface superconductors.

In this work, we have investigated the elastic and mechanical properties, electronic band structure, lattice dynamics, and electron-phonon interaction in four ternary transition metal borides ${\mathrm{Mo}}_5{\mathrm{PB}}_2$, ${\mathrm{Mo}}_5{\mathrm{SiB}}_2$, ${\mathrm{W}}_5{\mathrm{SiB}}_2$, and ${\mathrm{Mo}}_5{\mathrm{GeB}}_2$ by executing systematic ab initio calculations based on density functional theory within the generalized gradient approximation. The calculated elastic constants and elastic moduli for ${\mathrm{Mo}}_5{\mathrm{SiB}}_2$ agree well with available experimental data. The calculated elastic constants and phonon dispersion relations show that all the considered borides are mechanically and dynamically stable. The electronic density of states near the Fermi level of these compounds is dominated by the transition metal $d$ orbitals, which leads to low-frequency phonon modes arising from the vibrations of transition metal atoms being more strongly involved in the process of scattering of electrons rather than high-frequency phonon modes arising from the vibrations of lighter other two atoms. Our electron-phonon interaction calculations reveal that the electron-phonon coupling strength of ${\mathrm{Mo}}_5{\mathrm{PB}}_2$ is the strongest with a value of 0.959 among all the studied compounds, which in turn yields a superconducting transition temperature ($T_c$) value of 9.527 K, being higher than the $T_c$ values of ${\mathrm{Mo}}_5{\mathrm{SiB}}_2$ (5.845 K), ${\mathrm{W}}_5{\mathrm{SiB}}_2$ (5.931 K), and ${\mathrm{Mo}}_5{\mathrm{GeB}}_2$ (6.193 K). All these $T_c$ values harmonize with their respective experimental values of 9.2, 5.8, 5.8, and 5.8 K, indicating the reliability of our ab initio calculations.

Optical responses of atomically thin 2D materials are greatly influenced by electron-hole interactions. It is by far established that exciton signatures can be well identified in the optical absorption spectrum of quasi-2D materials. However, the same level of understanding of excitonic effects on nonlinear optical responses and the ability to compute them accurately is still much desired. Based on the functional integral formalisms and working in the velocity gauge, we introduce a convenient Feynman diagram approach for calculating nonlinear responses including excitonic effects. By dressing electron-photon interactions with electron-hole ladder diagrams, we derive an expression for second-order optical responses and provide a comprehensive description of excitonic effects. We apply our approach to a monolayer h-BN model and show qualitative changes in the second harmonic generation spectrum when comparing with results assuming independent particles. Our approach can be readily extended to higher-order optical responses and is feasible for first-principles calculations.

The topological semimetals have been the research focus of condensed-matter physics, materials science, and spintronics in recent decades. Here, we report the magnetoelectric transport properties of the topological nodal-line semimetal ${\mathrm{ZrP}}_2$. A clear magnetic-field induced metal-to-insulator transition is observed, and a resistivity plateau appears as the temperature drops to 15 K. An extremely large magnetoresistance of $15000\%$ obtained at low temperatures confirms Kohler's rule, which is ascribed to the electron-hole compensation. The beating patterns of Shubnikov–de Haas quantum oscillations are also observed below 10 K, which mainly stems from the superposition of two oscillations from electron and hole pockets, which is also consistent with the theoretical calculations. These findings provide a candidate material for the study of topological nodal-line semimetal in exploring physical properties and topological electronics.