Twisted two-dimensional (2D) van der Waals (vdW) quantum materials are renowned for their uncanny features like unconventional superconductivity, metal insulator transition (Mott transition), spin liquid phase etc., offering a rich landscape for strong electron correlations. Such electronic correlations also account for unusual magnetism in twisted crystals. However, advancement in the field of 2D twisted magnetism has been constrained, owing to lack of ideal materials as well as proper ways to design moiré magnets correlating their emergent magnetic and electronic properties. Here, we design a vdW moiré magnet and demonstrate that the simple act of rotating two monolayers i.e., $\mathrm{1T}\text{−}{\mathrm{NbSe}}_2$ and $\mathrm{1T}\text{−}{\mathrm{VSe}}_2$ at various twist angles, produces an inhomogeneous mixture of augmented and quenched localized magnetic moments per transition metal vanadium (V) and niobium (Nb) atoms. Precisely, twist angle affects the induced local magnetic moments of each constituent layers. Notable flat bands and itinerant ferromagnetism emerge in vdW moiré superlattice, the latter satisfying Stoner criterion. These features result from orbital rehybridization at atomic lattice sites instead of interlayer coupling between layers. Moreover, orbital magnetism is identified in untwisted heterobilayer system. The results present an effective strategy for designing moiré magnets with insights into new quantum-mechanical phenomenon of twist regulated on-site magnetism.

We studied the high-field phase diagram of a chiral-lattice antiferromagnet $\mathrm{Sr}\left(\mathrm{TiO}\right){\mathrm{Cu}}_4{\left({\mathrm{PO}}_4\right)}_4$ by means of ultrasound, dielectric, and magnetocaloric-effect measurements. These experimental techniques reveal two new phase transitions at high fields, which have not been resolved by previous magnetization experiments. Specifically, the $c_{66}$ acoustic mode shows drastic changes with hysteresis for magnetic fields applied along the $c$ axis, indicating a strong magnetoelastic coupling. Combined with cluster mean-field theory, we discuss the origin of these phase transitions. By considering the chiral-twist effect of ${\mathrm{Cu}}_4{\mathrm{O}}_{12}$ cupola units, which is inherent to the chiral crystal structure, the phase diagram is reasonably reproduced. The agreement between experiment and theory suggests that this material is a unique quasi-two-dimensional spin system with competing exchange interactions and chirality, leading to a rich phase diagram.

The Josephson diode effect (JDE) is the asymmetry of the critical supercurrent flowing along opposite current directions. We study a mechanism in this work to generate a possible JDE in the two-dimensional line-centered honeycomb (LCH) lattice based Josephson junction in which the supercurrent flows via the topological edge states. Since the two helical edge states of the LCH topological insulator have different Fermi velocities, the field-free Josephson junction is shown to exhibit a spin-resolved JDE in which the critical supercurrent is asymmetric over the two opposite current directions for each spin species. By introducing magnetization in the circuit, a charge version of JDE occurs in the system.

Superconducting systems that simultaneously lack space-inversion and time-reversal symmetries have recently been the subject of a flurry of experimental and theoretical research activities. Their ability to carry supercurrents with magnitudes depending on the polarity (current direction)—termed the supercurrent diode effect—might be practically exploited to design dissipationless counterparts of contemporary semiconductor-based diodes. Magnetic Josephson junctions realized in the two-dimensional electron gas (2DEG) within a narrow quantum well through proximity to conventional superconductors perhaps belong to the most striking and versatile platforms for such supercurrent rectifiers. Starting from the Bogoliubov–de Gennes approach, we provide a minimal theoretical model to explore the impact of the spin-orbit coupling and magnetic exchange inside the 2DEG on the Andreev bound states and Josephson current-phase relations. Assuming realistic junction parameters, we evaluate the polarity-dependent critical currents to quantify the efficiency of these Josephson junctions as supercurrent diodes, and discuss the tunability of the Josephson supercurrent diode effect in terms of spin-orbit coupling, magnetic exchange, and transparency of the nonsuperconducting weak link. Furthermore, we demonstrate that the junctions might undergo current-reversing 0–$\pi$-like phase transitions at large-enough magnetic exchange, which appear as sharp peaks followed by a sudden suppression in the supercurrent-diode-effect efficiency. The characteristics of the Josephson supercurrent diode effect obtained from our model convincingly reproduce many unique features observed in recent experiments, validating its robustness and suitability for further studies.

High pressure technology is an effective method for studying whether there is Mott or Peierls phase transition (PT) from insulator to metal phase. It is unclear whether there is a Mott or Peierls PT in the ambient $\mathrm{M}1/\mathrm{R}\to$ the high-pressure X phase for bulk $\mathrm{V}{\mathrm{O}}_2$. Based on first-principles calculation, insulator $\mathrm{M}1\to$ semimetallic X and metallic $\mathrm{R}$ $\to$ semimetallic X PT path were studied. During the pressurization of the ambient M1 phase, it will transform into two nearly degenerate intermediate phases of M1′ and M1′′. The M1′′ phase has a significantly lower band gap (no zero band gap) owing to its relatively longer length of V-V V-V shorter bond and V-O bond. The $\mathrm{M}{1}^{″}$ $\to$ $\mathrm{X}$ path is the more probable route owing to its lower potential barrier and shorter moving route of V atoms relative to those of $\mathrm{M}{1}^{\prime }$ $\to$ $\mathrm{X}$ path. Those V atoms at the (002) crystal plane of crystal cell will shuffle nearly along the [100] crystal direction as volume pressurization in the $\mathrm{M}{1}^{″}$ $\to$ $\mathrm{X}$ path, while the averaging of V-V spacing occur and the band gap gradually decreases to zero or even a negative value. Uniaxial stressing of the metal $\mathrm{R}$ phase along [100] or [010] crystal direction leads to the $\mathrm{Ca}{\mathrm{Cl}}_2$-type orthogonal metal phase (O phase, space group No. 58, Pnnm). Upon cooling the O phase under high pressure, it will transform to $\mathrm{M}{1}^{″}$ and then the X phase. Those V atoms at the (002) crystal plane of crystal cell continuously shuffle along the [10-1] crystal direction in the O $\to$ $\mathrm{M}{1}^{″}$ path, moreover the difference in longer and shorter V-V bond lengths tends to increase and a new dimerization of the V-V dimers appears. The PT paths from the semiconductor $\mathrm{M}1\to$ semiconductor $\mathrm{M}{1}^{″}$ $\to$ semimetal X phase and from metal $\mathrm{R}$ $\to$ metal O phase only involve geometrically structural PT. Especially, it is a typical reconstruction-type and martensiticlike PT, and no PT of electronic structure from semiconductor to metal phase is involved in the path from semiconductor $\mathrm{M}{1}^{″}$ $\to$ semimetal X phase. The structural PT path from metal O $\to$ semiconductor $\mathrm{M}{1}^{″}$ phase is a typical Peierls PT or pseudo Mott PT within our DFT+U calculations even though the Coulomb repulsion between electrons is always present and works. The Coulomb repulsion between electrons cannot cause the electronic structural PT to happen properly.

Bounded domains have discrete eigenfrequencies/spectra, and cavities with different boundaries and areas have different spectra. A general methodology for isospectral twinning, whereby the spectra of different cavities are made to coincide, is created by combining ideas from across physics including transformation optics, inverse problems, and metamaterial cloaking. We twin a hexagonal drum with a deformed hexagonal drum using a nonsingular coordinate transform that adjusts the deformed shape by mapping a near boundary domain to a zone of heterogeneous anisotropic medium. Splines define the mapping zone for twinning these two drums and we verify isospectrality by a finite-element analysis.

The tight-binding model for a chain, where the hopping constants follow a Fibonacci sequence, predicts multifractality in the spectrum and wave functions. Experimentally, we realize this model by chains of small dielectric resonators with a high refractive index (${\epsilon }_r\approx 45$) of cylindrical form that exhibit evanescent coupling. We show that the fractality of the measured local density of state (LDOS) is best understood when the sites are rearranged according to the similarities in their local surrounding, i.e., their conumbers. This allows us to deduce simple recursive construction schemes for the LDOS for the two cases of dominant strong and weak coupling, despite our limited resolution due to nonzero resonance width and size constraints. We measure the singularity spectrum and the fractal dimensions of the wave functions, and we find good agreement with theoretical predictions for the multifractality based on a perturbative description in the quasiperiodic limit.

We investigate disorder-induced localization in metals that break time-reversal and inversion symmetries through their energy dispersion, ${\epsilon }_k\ne {\epsilon }_{-k}$, but lack Berry phases. In the perturbative regime of disorder, we show that weak localization is suppressed due to a mismatch of the Fermi velocities of left and right movers. To substantiate this analytical result, we perform quench numerics on chains shorter than the Anderson localization length $\xi$—the latter computed and verified to be finite using the recursive Green's function method—and find a sharp rise in the saturation value of the participation ratio due to band asymmetry, indicating a tendency to delocalize. Interestingly, for weak disorder strength $\eta$, we see a better fit to the scaling behavior $\xi \propto 1/{\eta }^2$ for asymmetric bands than conventional symmetric ones.

This paper aims to address the structural, magnetic, electronic, and thermal properties of the rare-earth-based Heusler compound ${\mathrm{GdPd}}_2\mathrm{Bi}$. Our experimental findings indicate that our sample orders antiferromagnetically below the Néel temperature $T_N=9$ K. Resistivity versus temperature data show a sharp jump below about 175 K upon cooling, which is also associated with thermal hysteresis. This is further supported by specific heat and thermopower measurements, although no such signature is observed in the magnetization study. Temperature-dependent powder x-ray diffraction measurements confirm a martensitic transition around the region of thermal hysteresis ($\sim 175$ K) from the high-temperature (HT) cubic Heusler $L{2}_1$ structure to the low-temperature (LT) orthorhombic $Pmma$ structure similar to many previously reported shape memory alloys. We observe robust Bain distortion between cubic and orthorhombic lattice parameters, related by $a_{\mathrm{orth}}=\sqrt{2}{a}_{\mathrm{cub}}$, $b_{\mathrm{orth}}=a_{\mathrm{cub}}$, and $c_{\mathrm{orth}}=a_{\mathrm{cub}}/\sqrt{2}$, that occurs by contraction along the $c$ axis and elongation along the $a$ axis, respectively. The sample shows an unusual “nonsaturating” $H^2$-dependent negative magnetoresistance for magnetic field as high as 150 kOe, and such behavior can be accounted for by the Gd $4f\text{−}5d$ interaction leading to the polarization of the $5d$ band. In addition, nonlinear field dependence of the Hall resistivity is observed below about 30 K, which coincides with the sign change of the Seebeck coefficient. The electronic structure calculations confirm metallic states in both the LT and HT phases. This indicates the complex nature of the Fermi surface along with the existence of both electron and hole charge carriers. The anomalous transport behaviors can be related to the presence of both electrons and holes at the Fermi surface.

We theoretically investigate the nonlinear response current of a two-dimensional system under an in-plane magnetic field. Based on the extended semiclassical theory, we develop a unified theory including both longitudinal and transverse currents and classify contributions according to their scaling with the relaxation time. Besides time-reversal-even contributions, we reveal a previously unknown time-reversal-odd contribution to the Hall current, which occurs in magnetic systems, exhibits band geometric origin, and is linear in relaxation time. We show that the different contributions exhibit different symmetry characters, especially in their angular dependence on the field orientation, which can be used to distinguish them in experiment. The theory is explicitly demonstrated in the study of the Rashba model. Our work presents a deepened understanding of nonlinear planar transport, proposes approaches to distinguish different contributions, and sheds light on possible routes to enhance the effect in practice.

We use a newly developed quantum Monte Carlo method to investigate the impact of finite dispersion of the optical phonon mode on the properties of the bond Su-Schrieffer-Heeger polaron in two dimensions. We compare the properties of the bond polaron, such as effective mass, ground state energy, and $Z$ factor, with and without positive phonon bandwidth. Our findings reveal that when we exclude phonon dispersion, at the same electron-phonon coupling strength, the effective mass increases as the phonon frequency decreases, indicating a heavier polaron in the deep adiabatic regime. However, in the dispersive case, we observe that the effective mass increases as the phonon bandwidth increases. Moreover, we notice a crossover from a light polaron state to a heavy polaron state in the deep adiabatic regime for both the dispersionless and dispersive cases.

Orbitronics has recently emerged as a very active research topic after several proposals aiming to exploit the orbital degree of freedom for charge-free electronics. In this communication, we investigate orbital transport in selected two-dimensional systems to better understand which parameters govern the intra-atomic and interatomic contributions to the orbital Hall effect. We study the impact of the gap, the role of the materials' topology and the influence of the disorder on spin and orbital Hall transport. Starting from the Kane-Mele model, we describe how the orbital moment behaves depending on the material's topology and clarify the influence of the gap on the orbital Hall conductivity. We then extend the study to realistic topologically trivial and nontrivial materials, and find that the topology has little qualitative influence on the orbital Hall conductivity. In contrast, we observe that the energy dispersion has a more dramatic impact, especially in the presence of disorder. Remarkably, our results suggest that the intra-atomic orbital Hall current is more robust against scattering than the interatomic one, without further impact of the topological properties of the system under consideration.

We study a $1+1$ (space-time)-dimensional extension of the $0+1$-dimensional Sachdev-Ye-Kitaev model for $N$ Majorana fermions, with random all-to-all quartic interactions, averaged over disorder. At large interaction couplings and large $N$, the conformal symmetry of the effective action emerges, which is not broken spontaneously as in the original $0+1$ SYK model. Two-point correlators are obtained from a coupling expansion of the Schwinger-Dyson equations. For $N=4$, the model can be mapped onto complex fermions and solved exactly via the bosonization technique, featuring two branches of excitations: a gapped “pseudocharge” mode and a gapless “pseudospin” mode. We give an approximate analytic form of the two-point correlators at large distances and zero temperature for $N=4$, which is adopted heuristically to evaluate an approximation to the large-$N$ free energy in the zero temperature limit, numerically. The fact that this energy displays an absolute minimum at a finite value of the gap, though in a restricted range of parameters, suggests that a gapped branch of excitations is also present in our extension of the model at least in that range of parameters.

The hidden-order phase of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ has eluded identification for over 35 years. A compelling proposal that explains the Ising heavy-fermion nature of the material is hastatic order: a symmetry-breaking heavy Fermi liquid arising from a spinorial microscopic hybridization. The original hastatic proposal cannot microscopically model the pressure-induced antiferromagnetic phase; and while it predicts a spinorial order parameter, it does not provide any detectable signatures of the spinorial nature. Here, we present a more realistic microscopic model of hastatic order in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ based on two conducting electron bands and explore its phase diagram in detail. Our model nontrivially preserves the Ising heavy-fermion physics of the original, while allowing us to tune between the antiferromagnet and hidden order using pressure analogs and magnetic field. Our model is also consistent with recent phenomenological predictions of not one, but two, vector order parameters associated with the spinorial order that provide the first microscopic predictions for detecting the spinorial nature.

In the spirit of multiscale modeling, we develop a theoretical framework for spin-lattice coupling that connects, on the one hand, to ab initio calculations of spin-lattice coupling parameters and, on the other hand, to the magnetoelastic continuum theory. The derived Hamiltonian describes a closed system of spin and lattice degrees of freedom and explicitly conserves the total momentum, angular momentum, and energy. Using a numerical implementation that corrects earlier Suzuki-Trotter decompositions we perform simulations on the basis of the resulting equations of motion to investigate the combined magnetic and mechanical motion of a ferromagnetic nanoparticle, thereby validating our developed method. In addition to the ferromagnetic resonance mode of the spin system, we find another low-frequency mechanical response and a rotation of the particle according to the Einstein–de Haas effect. The framework developed herein will enable the use of multiscale modeling for investigating and understanding a broad range of magnetomechanical phenomena from slow to ultrafast timescales.

The influence of Rashba spin-orbit coupling (SOC) on superconducting correlations in thin-film superconductor/antiferromagnet (S/AF) structures with compensated interfaces is studied. A unique effect of anisotropic enhancement of proximity-induced triplet correlations by the SOC is predicted. It manifests itself in the anisotropy of the superconducting critical temperature $T_c$ with respect to orientation of the Néel vector relative to the S/AF interface, which is opposite to the behavior of $T_c$ in superconductor/ferromagnet structures. We show that the anisotropy is controlled by the chemical potential of the superconductor and, therefore, can be adjusted in (quasi-)2D structures.

Graphene quantum dots (GQDs) can exhibit a range of spectacular phenomena such as the Klein tunneling induced quasibound states and Berry phase tuned energy spectra. According to previous studies, all these interesting quantum phenomena seem to be well understood in the free electron picture. However, electronic motion in the GQDs is locally reduced to quantized orbits by quantum confinement, which implies that the kinetic energy in the GQDs may be comparable to or even smaller than the Coulomb energy of the quasiparticles, possibly resulting in exotic correlated phases. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable GQDs in $\text{graphene}\text{/}{\mathrm{WSe}}_2$ heterostructure devices and report a correlation-induced exotic phase in the GQDs. Gating allows us to precisely characterize effects of the electron-electron interaction on the energy spectra of the GQDs. By measuring density of states as a function of energy and position, we explicitly uncover two density waves with different velocities in the GQDs, attributing to spin-charge separation in real space.

It is now widely believed that $p$-wave superfluidity is the key to generate a novel critical phase in the non-Abelian Aubry-André-Harper model. However, we here establish that this belief is incorrect. In this work, we systemically investigate the phase transition of a non-Abelian quasiperiodic mosaic lattice model with $p$-wave superfluidity. The results show that the critical phase exists only in the quasiperiodic model, whereas in the mosaic model, despite the presence of $p$-wave superfluidity, the critical phase is absent, and mobility edge (ME) phases are generated instead. Furthermore, if the period of the mosaic modulation $\kappa \ge 3$, the results even show that regardless of the strength of the $p$-wave superfluidity, there are only extended and ME phases, but neither critical nor localized phases. This work clearly reveals the phase transition of a non-Abelian quasiperiodic mosaic lattice model with $p$-wave superfluidity, and it may be testified in near-term state-of-the-art experimental settings.

Dynamic nucleation of dislocations caused by a stress front (“shock”) of amplitude ${\sigma }_a$ moving with speed $V$ is investigated by solving numerically the dynamic Peierls equation with an efficient method. Speed $V$ and amplitude ${\sigma }_a$ are considered as independent variables, with $V$ possibly exceeding the longitudinal wave speed $c_L$. Various reactions between dislocations take place such as scattering, dislocation-pair nucleation, annihilation, and crossing. Pairs of edge dislocation are always nucleated with speed $v\gtrsim c_L$ (and likewise for screws with $c_L$ replaced by $c_S$, the shear wave speed). The plastic wave exhibits self-organization, forming distinct “bulk” and “front” zones. Nucleations occur either within the bulk or at the zone interface, depending on the value of $V$. The front zone accumulates dislocations that are expelled from the bulk or from the interface. In each zone, dislocation speeds and densities are measured as functions of simulation parameters. The densities exhibit a scaling behavior with stress, given by ${[{({\sigma }_a/{\sigma }_{\text{th}})}^2-1]}^{\beta }$, where ${\sigma }_{\text{th}}$ represents the nucleation threshold and $0<\beta <1$.