The in-plane photoelectric (IPPE) effect is a recently discovered [Sci. Adv. 8, eabi8398 (2022)] quantum phenomenon, which enables efficient detection of terahertz (THz) radiation in semiconductor structures with a two-dimensional (2D) electron gas. Here, we develop a theory of the IPPE effect in quasi-one-dimensional electron systems in which the width of the 2D conducting channel is so small that the transverse quantization energy is larger than the thermal energy. We calculate the THz photoresponse of such a system as a function of the THz frequency, control gate voltages, and geometrical parameters of the detector. We show that the transverse quantization of the electron motion manifests itself in oscillating gate-voltage dependences of the photocurrent, if the THz photon energy is less than the one-dimensional quantization energy. Results of the theory are applicable to any semiconductor systems with 2D electron gases, including III–V structures, silicon-based field effect transistors, and the novel 2D layered graphene-related materials.

Single crystals of ${\mathrm{TmNiC}}_2$ were grown by the optical floating-zone technique and were investigated by x-ray diffraction (XRD), thermal expansion, electrical resistivity, specific heat, and magnetic susceptibility measurements. Single-crystal XRD reveals the formation of a commensurate charge density wave (CDW) characterized by a CDW modulation vector ${\mathbf{q}}_{\mathbf{2}\mathbf{c}}=(0.5,0.5,0.5)$, which is accompanied by a symmetry change from the orthorhombic space group $Amm2$ to the monoclinic space group $Cm$, i.e., to a CDW superstructure which is isostructural with that of ${\mathrm{LuNiC}}_2$. For all transport and thermodynamic properties, anomalies related to a second order-type thermodynamic CDW phase transition are observed at around $T_{\mathrm{CDW}}\simeq 375\mathrm{K}$. The large specific heat anomaly at $T_{\mathrm{CDW}}, \mathrm{\Delta }C\simeq 6.2\mathrm{J}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-1}$, together with noticeable changes in entropy and enthalpy related to the CDW transition, suggests that this point group symmetry breaking CDW phase transition affects more significant parts of the Fermi surface as compared to the incommensurate CDW transition of, e.g., ${\mathrm{SmNiC}}_2$ with no change in point group symmetry. The results on the antiferromagnetic and paramagnetic state of ${\mathrm{TmNiC}}_2$ obtained by the above macroscopic techniques were complemented by microscopic studies via inelastic neutron scattering. A crystalline electric field modeling of macroscopic susceptibility and magnetic specific heat and entropy contributions as well as microscopic neutron scattering data, reveal crystal field eigenstates and eigenvalues with a ground-state doublet of the Tm-$4f$ electrons, which is well separated by about 25 meV from exited states of the $J=6$ ground-state multiplet.

We use tensor network simulations to calculate the time evolution of the lower part of the entanglement spectrum and return rate functions after global quantum quenches in the Ising model. We consider ground state quenches toward mesonic parameter ranges with confined fermion pairs as nonperturbative bound states in a semiclassical regime and the relativistic ${\mathrm{E}}_8$ theory. We find that in both cases only the dominant eigenvalue of the modular Hamiltonian fully encodes the meson content of the quantum many-body system or quantum field theory, giving rise to nearly identical entanglement oscillations in the entanglement entropy. When the initial state is prepared in the paramagnetic phase, the return rate density exhibits regular cusps at unequally spaced positions, signaling the appearance of dynamical quantum phase transitions, at which the entanglement spectrum remains gapped. Our analyses provide a deeper understanding on the role of quantum information quantities for the dynamics of emergent phenomena reminiscent of systems in high-energy physics.

We study numerically the impact of many-body interactions on the quantum boomerang effect. We consider various cases: weakly interacting bosons, the Tonks-Girardeau gas, and strongly interacting bosons (which may be mapped onto weakly interacting fermions). Numerical simulations are performed using the time-evolving block decimation algorithm, a quasiexact method based on matrix product states. In the case of weakly interacting bosons, we find a partial destruction of the quantum boomerang effect, in agreement with the earlier mean-field study [J. Janarek et al., Phys. Rev. A 102, 013303 (2020)]. For the Tonks-Girardeau gas, we show the presence of the full quantum boomerang effect. For strongly interacting bosons, we observe a partial boomerang effect. We show that the destruction of the quantum boomerang effect is universal and does not depend on the details of the interaction between particles.

Quantum Griffiths singularity (QGS), which is closely correlated with the quenched disorder, is characterized by the divergence of the dynamical critical exponent when approaching zero temperature and the applicability of activated scaling. Typically such a phenomenon is rather rare and only observed in extremely low temperatures. Here we report the experimental observation of a robust QGS in NbN thin films, which survives in a rather high temperature range. The electrical transport properties were measured under the magnetic field up to 12 T. The field induced superconductor-metal transitions were observed with the continuously changed transition points with the decrease of temperature. The dynamical critical exponent based on the conventional power-law scaling reveals a divergent trend when approaching the low-temperature limit. Moreover, the temperature and field dependence of sheet resistance can be described by the activated scaling analysis in the temperature region up to 1.6 K $\le T\le$ 4.0 K. We argue that the robustness of QGS in the present system originates from the pronounced Pauli paramagnetic effect due to the strong coupling between magnetic field and the spin degree of freedom of the superconducting electrons.

We have performed path-integral Monte Carlo calculations to investigate various quantum phases of ${}^4\mathrm{He}$ layers adsorbed on a biphenylene sheet, a recently synthesized two-dimensional network of carbon atoms. Three different commensurate solid phases are identified in the first ${}^4\mathrm{He}$ layer, which is found to be completed to a ${\mathrm{C}}_{2/2}$ commensurate solid at an areal density of 0.119 ${\AA }^{-2}$. We have also found that the commensurate structure of the completed first layer allows second-layer ${}^4\mathrm{He}$ atoms to constitute a stable commensurate ${\mathrm{C}}_{2/3}$ structure at a total helium coverage of 0.199 ${\AA }^{-2}$. Furthermore, this second-layer commensurate structure is observed to be stable even with the formation of vacancies that are mobile to instigate the superfluid response at low temperatures. This unequivocally shows that vacancy-induced supersolidity can be realized in the second ${}^4\mathrm{He}$ layer on biphenylene.

We present an inelastic neutron scattering study of liquid and solid hydrogen carried out using the wide Angular Range Chopper Spectrometer at Oak Ridge National Laboratory. From the observed dynamic structure factor, we obtained empirical estimates of the molecular mean-squared displacement and average translational kinetic energy. We find that the former quantity increases with temperature, indicating that a combination of thermal and quantum effects is important near the liquid-solid phase transition, contrary to previous measurements. We also find that the kinetic energy drops dramatically on melting of the crystals, a consequence of the large increase in molar volume together with the Heisenberg indeterminacy principle. Our results are compared with quantum Monte Carlo simulations based on different model potentials. In general, there is good agreement between our findings and theoretical predictions based on the Silvera-Goldman and Buck potentials.

Recently, a Dirac exceptional point (EP) was reported in a non-Hermitian system. Unlike a Dirac point in Hermitian systems, this Dirac EP has coalesced eigenstates in addition to the degenerate energy. Also different from a typical EP, the two energy levels connected at this Dirac EP remain real in its vicinity and display a linear instead of square root dispersion, forming a tilted Dirac cone in the hybrid space consisting of a momentum dimension and a synthetic dimension for the strength of non-Hermiticity. In this paper, we first present simple three-band and two-band matrix models with a Dirac EP, where the linear dispersion of the tilted Dirac cone can be expressed analytically. Importantly, our analysis also reveals that there exist Hermitian and non-Hermitian systems that have the same (real-valued) energy spectrum in their entire parameter space, with the exception that one or more degeneracies in the former are replaced by Dirac EPs in the latter. Finally, we show the existence of an imaginary Dirac cone with an EP at its center.

In spin-density functional theory (SDFT) for noncollinear magnetic materials, the Kohn-Sham system features exchange-correlation (xc) scalar potentials and magnetic fields. The significance of the xc magnetic fields is not very well explored; in particular, they can give rise to local torques on the magnetization, which are absent in standard local and semilocal approximations. Exact benchmark solutions for a five-site extended Hubbard lattice at half filling and in the presence of spin-orbit coupling are compared with SDFT results obtained using orbital-dependent exchange-only approximations. The magnetization dynamics following short-pulse excitations is found to be reasonably well described in the exchange-only approximation for weak to moderate interactions. For stronger interactions and near transitions between magnetically ordered and frustrated phases, exchange and correlation torques tend to compensate each other and must both be accounted for.

We predict the existence of a pair-liquid phase in lattice fermion systems with finite-range attractive interactions. This exotic state competes on one side with a normal Fermi liquid of unpaired fermions and on the other side with a phase-separated state where all fermions are coupled into macroscopic clusters. We show that such a phase is absent in bosonic systems and therefore is protected by the exclusion principle. In contrast with zero-range attractive systems where clustering of more than two fermions is directly prohibited, here quantum statistics acts dynamically and in a more subtle way. Since a many-fermion wave function must have nodes, the cluster formation threshold is larger than the pair formation threshold. By directly solving a four-body Schrödinger equation on one- and two-dimensional lattices, we map the boundaries of pair stability. The pair liquid phase should be observable in cold-atom systems. We also discuss implications for the preformed-pair mechanism of high-temperature superconductivity.

Time- and angle-resolved photoelectron spectroscopy is employed to study the thermalization of a nonequilibrium carrier distribution in the Dirac cone of graphite generated upon absorption of linearly polarized near-infrared laser pulses. The data reveal a characteristic time constant of $(20\pm 3)\mathrm{fs}$ for the decay of a photoinduced momentum anisotropy in the nascent carrier population. Additionally, the nascent carrier spectral peak shows a downshift in energy by $\sim 100\mathrm{m}\mathrm{eV}$ within the first $30\mathrm{f}\mathrm{s}$ after excitation, which is associated with the emission of strongly coupled $A_1^{\prime }$ optical phonons at $K$. In agreement with previous findings for graphene, a pronounced momentum anisotropy is also observed for an intermediate quasithermalized state of the carrier population. A fully thermalized distribution (with respect to energy and momentum) is finally established on a characteristic timescale of $(40\pm 10)\mathrm{fs}$. The results underscore the complexity of carrier thermalization on ultrafast timescales resulting from the interplay of carrier-carrier and carrier-phonon interaction.

An approach to calculating current-induced forces in charge transport through nanosystems is introduced. Starting from the fully quantum mechanical hierarchical equations of motion formalism, a timescale separation between electronic and vibrational degrees of freedom is used to derive a classical Langevin equation of motion for the vibrational dynamics as influenced by current-induced forces, such as the electronic friction. The resulting form of the friction is shown to be equivalent to previously derived expressions. The numerical exactness of the hierarchical equations of motion approach, however, allows the investigation of transport scenarios with strong intrasystem and system-environment interactions. As a demonstration, the electronic friction of three example systems is calculated and analyzed: a single electronic level coupled to one classical vibrational mode, an Anderson impurity model coupled to one classical vibrational mode, and a single electronic level coupled to both a classical and quantum vibrational mode.

We present an efficient preconditioning technique for accelerating the fixed-point iteration in real-space Kohn-Sham density functional theory (DFT) calculations. The preconditioner uses a low-rank approximation of the dielectric matrix (LRDM) based on Gâteaux derivatives of the residual of fixed-point iteration along appropriately chosen direction functions. We develop a computationally efficient method to evaluate these Gâteaux derivatives in conjunction with the Chebyshev filtered subspace iteration procedure, an approach widely used in large-scale Kohn-Sham DFT calculations. Further, we propose a variant of LRDM preconditioner based on adaptive accumulation of low-rank approximations from previous self-consistent field iterations, and also extend the LRDM preconditioner to spin-polarized Kohn-Sham DFT calculations. We demonstrate the robustness and efficiency of the LRDM preconditioner against other widely used preconditioners on a range of benchmark systems with sizes ranging from $\sim 100$ to 1100 atoms ($\sim 500\text{–}\mathrm{20 000}$ electrons). The benchmark systems include various combinations of metal-insulating-semiconducting heterogeneous material systems, nanoparticles with localized $d$ orbitals near the Fermi energy, nanofilm with metal dopants, and magnetic systems. In all benchmark systems, the LRDM preconditioner converges robustly within 20–30 iterations. In contrast, other widely used preconditioners show slow convergence in many cases, as well as divergence of the fixed-point iteration in some cases. Finally, we demonstrate the computational efficiency afforded by the LRDM method, with up to 3.4-fold reduction in computational cost for the total ground-state calculation compared to other preconditioners.

The quantum dimer and loop models attract great attention, partially because the fundamental importance in the phases and phase transitions emerging in these prototypical constrained systems, and partially due to their intimate relevance toward the on-going experiments on Rydberg atom arrays in which the blockade mechanism naturally enforces the local constraint. Here we show, by means of the sweeping cluster quantum Monte Carlo method, the complete ground state phase diagram of the fully packed quantum loop model on the square lattice. We find between the lattice nematic (LN) phase with strong dimer attraction and the staggered phase (SP) with strong dimer repulsion, there emerges a resonating plaquette (RP) phase with off-diagonal translational symmetry breaking. Such a quantum phase is separated from the LN via a first order transition and from the SP by the famous Rokhsar-Kivelson point. Our renormalization group analysis reveals the different flow directions, fully consistent with the order parameter histogram in Monte Carlo simulations. The realization and implication of our phase diagram in Rydberg experiments are proposed.

In the real-space formalism of orbital magnetization (OM) for a Chern insulator without an external electric field, it is correct to average the local OM either over the bulk region or over the whole sample. However, for a layered Chern insulator in an external electric field, which is directly related to the nontrivial nature of orbital magnetoelectric coupling, the role of boundaries remains ambiguous in this formalism. Based on a bilayer model with an adjustable Chern number at half-filling, we numerically investigate the OM with the above two different average types under a nonzero perpendicular electric field. The result shows that in this case, the nonzero Chern number gives rise to a gauge shift of the OM with the bulk region average, while this gauge shift is absent for the OM with the whole sample average. This indicates that only the whole sample average is reliable to calculate the OM under a nonzero electric field for Chern insulators. On this basis, the orbital magnetoelectric polarizablity (OMP) and the Chern-Simons orbital magnetoelectric polarizablity (CSOMP) with the whole sample average are studied. We also present the relationship between the OMP (CSOMP) and the response of Berry curvature to the electric field. The stronger the response of Berry curvature to electric field, the stronger is the OMP (CSOMP). Aside from clarifying the calculation methods, our result also provides an effective method to enhance OMP and CSOMP of materials.

The $\alpha \text{−}{T}_3$ lattice is an interpolate between the graphene $(\alpha =0)$ and the Dice lattices $(\alpha =1)$ and has a nondispersive flat band across the Dirac bands at the band center $(E=0)$. In this paper, we study the delocalization effect of the additional chiral zero-energy modes (CZEM) from the vacancy disorder in the $\alpha \text{−}{T}_3$ lattice and address the influence of the flat band on the CZEM. It is shown that the mere broadening of the flat band without the CZEM could produce a supermetallic phase around the band center $E\sim 0$ as well as an adjoined narrow localization regime. When the CZEM from the vacancy disorder is turned on, the transport property resembles the graphene case $(\alpha =0)$ that electrons in the low-energy regime are fully localized except the Dirac point of $E=0$, which is a critical delocalization point because the zero-energy conductivity (ZEC) is independent of the inelastic-scattering strength $\eta$. But the ZEC itself is shown to weakly rely on both the model parameter $\alpha$ and the vacancy density $n_c$ due to the broadening effect of the flat band. The extreme vacancy imbalance among each ABC site of a primitive cell is also studied, and the electron transport around $E\sim 0$ is changed to be either the pure inelastic disorder case without vacancy or the fully localized phase without the critical delocalization point of $E=0$, which depends on the central site $B$ atom replaced by vacancy or not.