Coulomb drag experiments have been an essential tool to study strongly interacting low-dimensional systems. Historically, this effect has been explained in terms of momentum transfer between electrons in the active and the passive layer. Here, we report Coulomb drag measurements between laterally coupled GaAs/AlGaAs quantum wires in the multiple one-dimensional (1D) sub-band regime that break Onsager's reciprocity upon both layer and current direction reversal, in contrast to prior 1D Coulomb drag results. The drag signal shows nonlinear current-voltage (I-V) characteristics, which are well characterized by a third-order polynomial fit. These findings are qualitatively consistent with a rectified drag signal induced by charge fluctuations. However, the nonmonotonic temperature dependence of this drag signal suggests that strong electron-electron interactions, expected within the Tomonaga-Luttinger liquid framework, remain important and standard interaction models are insufficient to capture the qualitative nature of rectified 1D Coulomb drag.

We design a kagome compound ${\mathrm{Pt}}_3{\mathrm{P}}_2{\mathrm{Te}}_8$ and find it is a topological semimetal with a symmetry-protected three-dimensional (3D) bulk Dirac point along the $\mathrm{\Gamma }$-A path. This Dirac point is not intrinsic to kagome lattice and brings a parity inversion, leading to topological surface states and fragile Fermi arcs on the (100) surface. These features are very close to the Fermi level and distinct from the bulk states, which suggests they can be observed in experiments. We further characterize the 3D bulk Dirac point and compute a high-order topological invariant, i.e., the change of the filling anomaly, whose value of 4 indicates the high-order bulk-hinge correspondence and the existence of high-order Fermi arcs. Moreover, ${\mathrm{Pt}}_3{\mathrm{P}}_2{\mathrm{Te}}_8$ exhibits rich topological phase transitions under hydrostatic pressure. Our results provide a potential platform to study topological properties related to 3D bulk Dirac points and pressure-induced topological phase transitions in the kagome lattice.

Recent technological advances allow fabricating ultraclean two-dimensional electronic systems where the electron mean free path due to static disorder and phonons is much larger compared to the conducting channel width. It makes possible to realize novel, ballistic and hydrodynamic, regimes of electron transport resulting in drastic modifications of the normal Hall effect. Here we develop a theory of anomalous Hall effect—generation of the electric field transverse to the flowing current unrelated to the Lorentz force action—in ultraclean channels with two-dimensional electron gas and demonstrate that both in ballistic and hydrodynamic regimes the anomalous Hall effect, similarly to the normal one, strongly differs from that in the standard diffusive case. We take into account all relevant contributions to the anomalous Hall electric field and Hall voltage: the skew scattering of electrons, side-jump, and anomalous velocity effects that appear as a result of the spin-orbit coupling. We study both ballistic and hydrodynamic transport regimes which are realized depending on the relation between the electron-electron mean free path and the channel width. The role of electron-electron interactions is analyzed. Compact analytical expressions for the anomalous Hall field and voltage are derived. Possible experimental scenarios for observation of the anomalous Hall effect in ultraclean channels are briefly discussed.

In this work the extreme nonlinear optical response of the giant fullerene molecule ${\mathrm{C}}_{240}$ in strong laser field is studied. The investigation of high-order harmonic generation in such quantum nanostructure is presented modeling the ${\mathrm{C}}_{240}$ molecule and its interaction with the laser field in the scope of the tight-binding mean-field approach. The electron-electron interaction is modeled by the parametrized Ohno potential, which takes into account long-range Coulomb interaction. The essential role of the many-body Coulomb interaction in determining harmonic intensities is demonstrated. We also consider vacancy-defected molecule ${\mathrm{C}}_{240}$. The presence of a single vacancy breaks the icosahedral symmetry leading to the emergence of intense even-order harmonics. We examine the dependence of moderate harmonics on laser frequency that shows the multiphoton resonant nature of high harmonics generation. The dependence of cutoff harmonics on both laser intensity and frequency is examined too.

Frustrated magnets with degenerate ground states exhibit exotic ground states and rich phase structures when perturbations and/or thermal fluctuations lift the degeneracy. In two-dimensional models with short-range interactions, continuous symmetries cannot spontaneously break at finite temperatures, leading to the suppression of conventional magnetic long-range ordering. In this paper, we numerically study the classical $J_1\text{−}{J}_2$ $XY$ antiferromagnet on the kagome lattice as a prototype model of such frustrated magnets, where $J_2$ denotes the next-nearest-neighbor exchange interaction. We map out the $J_2\text{−}T$ phase diagram of this model employing extensive classical Monte Carlo simulations. The obtained phase diagram features Berezinskii-Kosterlitz-Thouless transitions of $q=0$, $\sqrt{3}\times \sqrt{3}$ magnetic orders, and octupole orders, in addition to finite-temperature phase transitions of both ferrochiral and antiferrochiral long-range orders. Additionally, we find a nontrivial first-order transition for antiferromagnetic $J_2/J_1<0$. The origin of this transition is discussed in the context of nonlocal loop structures present in local ${120}^{\circ }$ spin structures.

The present studies show that long-range magnetic order takes place in ${\mathrm{RuBr}}_3$ at $\approx 34$ K. The observations of clear oscillations in the muon time spectra demonstrate the presence of well-defined internal fields at the muon sites. The magnetic ordering appears to be very robust and static, suggesting a more conventional nature of magnetic ordering in the ${\mathrm{RuBr}}_3$ system at zero field. Present investigations prove that in ${\mathrm{RuBr}}_3$ the Kitaev interactions are likely to be weakened at zero field in comparison to the $\alpha \text{−}{\mathrm{RuCl}}_3$ system. This proves that it is possible to tune the Kitaev interactions by replacing Cl with heavier halogen elements such as Br.

Motivated by the recent discovery of anomalously strong nearest-neighbor attraction in the one-dimensional hole-doped cuprate ${\text{Ba}}_{2+x}{\text{Sr}}_x{\text{CuO}}_{3+\delta }$, we investigate the $d$-wave superconducting property of an extended checkerboard Hubbard model, which includes an intraplaquette nearest-neighbor attraction $V$. Our quantum Monte Carlo simulations reveal that, for small interplaquette hopping integral $t^{\prime }$, $V$ induces a drastic enhancement of long-range $d$-wave pairing correlations in the parameter range of $0.0\le U\le 6.0t$ ($t$ is the intraplaquette hopping integral) when the electron density lies between half-filling and quarter-filling. At $V=-0.4t$, one order of magnitude larger $d$-wave pairing correlations are observed compared to the ones at $V=0.0t$ and, more importantly, they exceed the values of the free electron system, signifying the existence of $d$-wave superconductivity. The exact diagonalization calculations on the $2\times 2$ plaquette indicate that the enhancement of $d$-wave pairing correlations can be attributed to the $V$-induced enhancement of pair-binding energy. Our findings provide a new perspective on the realization of high-temperature $d$-wave superconductivity via the combination of nearest-neighbor attraction and electronic inhomogeneity.

The magnetoelectric (ME) effect, with the cross coupling between magnetism and electric polarization, provides a new controlling dimension for creating novel information storage devices. Here, we report crystal structure, magnetic field-induced spin flop transition, and ME behaviors of Fe-doped ${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ by using comprehensive characterization methods of x-ray diffraction, high-angle annular dark-field images, magnetization, specific-heat capacity, dielectric and electric polarization measurements. The Fe/Ni honeycomb lattice is endowed with strong magnetic frustration. Through Fe doping, a metamagnetic spin flop (SF) transition and the related linear magnetoelectric effect are achieved under a lower magnetic field, which is due to the delicate modification of magnetic interactions. Consequently, the maximum direct ME coefficient ${\alpha }_{\mathrm{H}}$ ($=238\mathrm{ps}{\mathrm{m}}^{-1}$) and converse ME coefficient ${\alpha }_{\mathrm{E}}$ ($=110\mathrm{ps}{\mathrm{m}}^{-1}$) were achieved in ${\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_{1.5}{\mathrm{Mo}}_3{\mathrm{O}}_8$ at the phase boundary of antiferromagnetic-SF and SF phase, respectively.

We study the Josephson effect in the spin-singlet superconductor/altermagnet/spin-triplet superconductor junctions using the Green's function method. The current-phase difference relationships in the junctions strongly depend on the orientation of altermagnet and the types of the Cooper pairs. For the orientation angle equal to odd multiples of $\pi /4$, the current-phase difference relationships are of the $sin2\varphi$ type, which are irrespective of the pairing wave functions in superconductors. For the other orientation angles, the emergence of the lowest order current becomes possible and its form, $sin\varphi$ or $cos\varphi$, depends on the pairing wave functions in superconductors. The ${\varphi }_0$ phase and the $0\text{−}\pi$ transition can be realized in our junctions due to the appearance of the lowest order current. The selection rules for the lowest order current are presented. The symmetric relations satisfied by the current-phase difference relationships are analyzed through considering the transformations of the junctions under the mirror reflection, the time-reversal and the spin-rotation operations. Our results not only provide a method to detect the intrinsic spin-triplet superconductivity but also possess application values in the design of the field-free quantum devices.

Interlayer coupling is strongly implicated in the complex electronic properties of $1T-{\mathrm{TaS}}_2$. Uniaxial strain engineering offers a route to modify this coupling in order to elucidate its interplay with the electronic structure and electronic correlations. Here, we employ angle-resolved photoemission spectroscopy (ARPES) to reveal the effect of uniaxial strain on the electronic structure in $1T-{\mathrm{TaS}}_2$. The gap of the normally insulating ground state is significantly reduced, with a correlated flat band appearing close to the Fermi level. Temperature-dependent ARPES measurements reveal that the flat band only develops below the commensurate charge density wave (CCDW) transition, where interlayer dimerization produces a band insulator in unstrained samples. Electronic structure calculations suggest that the correlated flat band is stabilized by a modified interlayer coupling of the Ta $d_{z^2}$ electrons. Further hints of a strain-induced structural modification of the interlayer order are obtained from x-ray diffraction. Our combined approach provides critical input for understanding the complex phase diagram of this platform material for correlated physics.

While free fermion topological crystalline insulators have been largely classified, the analogous problem in the strongly interacting case has been only partially solved, and the relationship between the free and interacting classifications is not well understood. In this paper, we develop a characterization and classification of interacting, invertible fermionic topological phases in ($2+1$) dimensions with charge conservation, discrete magnetic translation, and $M$-fold point group rotation symmetries, which form the group $G_f=\text{U}{\left(1\right)}^f{\times }_{\varphi }[{\mathbb{Z}}^2⋊{\mathbb{Z}}_M]$ for $M=1,2,3,4,\text{and}6$. $\varphi$ is the magnetic flux per unit cell. We derive a topological response theory in terms of background crystalline gauge fields, which gives a complete classification of different phases and a physical characterization in terms of quantized response to symmetry defects. We then derive the same classification in terms of a set of real-space invariants $\left\{{\mathrm{\Theta }}_{\text{o}}^{\pm }\right\}$ that can be obtained from ground state expectation values of suitable partial rotation operators. We explicitly relate these real-space invariants to the quantized coefficients in the topological response theory, and find the dependence of the invariants on the chiral central charge $c_{-}$ of the invertible phase. Finally, when $\varphi =0,$ we derive an explicit map between the free and interacting classifications.

Self-consistent Hartree-Fock approximation combined with solutions of the Bethe-Salpeter equation offers a powerful tool for studies of strong correlation effects arising in condensed matter models, nuclear physics, quantum field theories, and real materials. The standard finite-temperature approach would be to first solve the problem in the Matsubara representation and then apply numerical analytic continuation to the real-frequency axis to link theoretical results with experimental probes, but this ill-conditioned procedure often distorts important spectral features even for very accurate imaginary-frequency data. We demonstrate that the ladder-type finite-temperature Bethe-Salpeter equation in the Hartree-Fock basis for the three-point vertex function and, ultimately, system's polarization can be accurately solved directly on the real frequency axis using the diagrammatic Monte Carlo technique and series resummation. We illustrate the method by applying it to the homogeneous electron gas model and demonstrate how multiple scattering events renormalize Landau damping.

Machine learning is used to generate empirical pseudopotentials that characterize the local screened interactions in the Kohn-Sham Hamiltonian. Our approach incorporates momentum-range-separated rotation-covariant descriptors to capture crystal symmetries as well as crucial directional information of bonds, thus realizing accurate descriptions of anisotropic solids. Trained empirical potentials are shown to be versatile and transferable such that the calculated energy bands and wave functions without cumbersome self-consistency reproduce conventional ab initio results even for semiconductors with defects, thus fostering faster and faithful data-driven material researches.

Motivated by high-temperature superconductivity in pressurized ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, we investigate the pair correlations in the two-orbital Hubbard ladder, which consists of the nearly half-filled and nearly quarter-filled orbitals. By employing the density matrix renormalization group method, we demonstrate that the pair correlation exhibits a power-law decay against the distance while the spin correlation decays exponentially. The decay exponent of the pair correlation of the nearly half-filled orbital is comparable to the exponent of the quasi-long-range superconducting correlation in the doped single-orbital Hubbard ladder, which suggests the importance of the $d_{3z^2-r^2}$ orbital in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$.

We consider electronic systems with a spontaneously broken continuous symmetry. The scattering vertex between electrons and Goldstone modes is calculated over the entire Brillouin zone using the random phase approximation. This calculation reveals two things: (1) electrons always couple to both $\varphi$ and ${\partial }_t\varphi$, where $\varphi$ is the Goldstone field, and (2) quasiparticles in a state with continuous symmetry breaking have to be defined in a rotating frame, which locally follows the fluctuations of the order parameter. The implications of these findings for electron spectral functions in both symmetry-broken and thermally disordered systems are discussed, and the examples of antiferromagnetism in the Hubbard model and spin spiral order in the three-band model are worked out in detail.

We investigate the morphologies of the Ge(001) surface that are produced by bombardment with a normally incident, broad argon ion beam at sample temperatures above the recrystallization temperature. Two previously observed kinds of topographies are seen, i.e., patterns consisting of upright and inverted rectangular pyramids, as well as patterns composed of shallow, isotropic basins. In addition, we observe the formation of an unexpected third type of pattern for intermediate values of the temperature, ion energy, and ion flux. In this type of intermediate morphology, isolated peaks with rectangular cross-sections stand above a landscape of shallow, rounded basins. We also extend past theoretical work to include a second-order correction term that comes from the curvature dependence of the sputter yield. For a range of parameter values, the resulting continuum model of the surface dynamics produces patterns that are remarkably similar to the intermediate morphologies we observe in our experiments. The formation of the isolated peaks is the result of a term that is not ordinarily included in the equation of motion, a second-order correction to the curvature dependence of the sputter yield.

Nascent quantum computers motivate the exploration of quantum many-body systems in nontraditional scenarios. For example, it has become natural to explore the dynamics of systems evolving under both unitary evolution and measurement. Such systems can undergo dynamical phase transitions in the entanglement properties of quantum trajectories conditional on the measurement outcomes. Here, we explore dynamics in which one attempts to (locally) use those measurement outcomes to steer the system toward a target state, and we study the resulting phase diagram as a function of the measurement and feedback rates. Steering succeeds when the measurement and feedback rates exceed a threshold, yielding an absorbing-state transition in the trajectory-averaged density matrix. We argue that the absorbing-state transition generally occurs at different critical parameters from the entanglement transition in individual trajectories and has distinct critical properties. The efficacy of steering depends on the nature of the target state: in particular, for local dynamics targeting long-range correlated states, steering is necessarily slow and the entanglement and steering transitions are well separated in parameter space.

We show that the ferroelectric system $(1-x){\mathrm{SrTiO}}_3\text{−}\left(x\right){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ exhibits an interesting interplay of local polarization, ionic size effect, and octahedral tilt. This interplay results in (i) a nonmonotonic composition dependence of the lattice volume at room temperature and (ii) existence of a unique temperature ($300{}^{\circ }\mathrm{C}$) at which the competing effects of electrostriction caused by the formation of polar nanoregions and geometrical ionic size make the system exhibit composition independent lattice parameters in a large composition interval $x<0.40$.

Picosecond time-resolved and cw magneto-optical methods have been used for studying the spin-related properties of excitons and photocarriers in methyl-ammonium lead bromide (${\mathrm{MAPbBr}}_3$) thin film, single crystal, and light-emitting diodes (LED), focusing on the Landé $g$ values of these species. Using the transient circularly polarized photoinduced quantum beatings (QB) under an applied magnetic field, B in ${\mathrm{MAPbBr}}_3$ single crystal, we obtained the anisotropic $g$ values of electrons for B field along [010] and [001]: $|g_{\left[001\right]}^{\mathrm{e}}|=2.15$ and $|g_{\left[010\right]}^{\mathrm{e}}|=1.75$, and for holes $|g_{\left[001\right]}^{\mathrm{h}}|=0.42$ and $|g_{\left[010\right]}^{\mathrm{h}}|=0.60$. We also used the magnetic circular dichroism method for measuring the bright excitons’ $g$ value, $g_{\mathrm{ex}}=g_{\mathrm{e}}+g_{\mathrm{h}}=2.5$. From these two types of measurements we conclude that $g_{\mathrm{h}}>0$ in ${\mathrm{MAPbBr}}_3$. This conclusion was corroborated by measuring the magnetoelectroluminescence response of LED based on ${\mathrm{MAPbBr}}_3$ active layer. The $g$ values in single crystal and their average in films are in excellent agreement with a k·p model that shows similarity and difference to those of ${\mathrm{MAPbI}}_3$. We also observed the influence of the Overhauser field on the QB frequencies that is induced by the dynamic nuclear polarization generated by the spin-aligned electrons using circularly polarized pump or probe beams.