The search for novel two-dimensional (2D) giant Rashba semiconductors is a crucial step in the development of spintronic technology. The electronic properties and growth mode of antimonene grown by molecular beam epitaxy were investigated using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy. Results reveal the semiconducting nature of antimonene and the presence of a highly mobile 2D hole gas (2DHG) near the Fermi level. The anisotropy factor and Fermi velocity of the 2DHG are sensitively dependent on the crystal structure and in-plane lattice parameters of antimonene. Antimonene with small lattice parameters exhibits a small anisotropy factor and electron effective mass. Furthermore, the perpendicular (out of plane) electric field breaks the inversion symmetry and, when combined with strong spin-orbit coupling, induces the spin splitting of the valence band of antimonene. ARPES and work function measurements shed light on the band alignment between antimonene and the substrates and confirmed the presence of a nonignorable interface dipole of approximately 0.2 eV. This dipole could be the cause of approximately 80 meV spin splitting at the valence-band maximum of antimonene, a value that is significantly greater than the thermal energy at room temperature. In addition, the spin-splitting degree depends on the interface dipole strength. The gate voltage can generate the out of plane electric field and regulate the spin state similar to an internal electric field. Our results suggest that antimonene and its van der Waals heterostructure system grown on substrates could be used to develop highly efficient spin field-effect transistors and nanospintronic devices.

The evolution of the low-temperature electronic structure of the cuprate metals from the overdoped to the underdoped side has recently been addressed through angle-dependant magnetoresistance (ADMR) experiments in ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. The results show a striking difference between hole dopings $p=0.24$ and $p=0.21$, which lie on either side of a putative quantum critical point at intermediate $p$. Motivated by this, we here study the theory of ADMR in correlated layered materials, paying special attention to the role of angle-dependent quasiparticle weights $Z_{\mathbf{k}}$. Such a $Z_{\mathbf{k}}$ is expected to characterize a number of popular models of the cuprate materials, particularly when underdoped. Further, in the limit of weak interlayer hopping the quasiparticle weight will affect the $c$-axis transport measured in ADMR experiments. We show that proper inclusion of the quasiparticle weight does not support an interpretation of the data in terms of a $(\pi ,\pi )$ spin density wave ordered state, in agreement with the lack of direct evidence for such order. We show that a simple model of Fermi surface reconfiguring across a van Hove point captures many of the striking differences seen between $p=0.21$ and $p=0.24$. We comment on why such a model may be appropriate for interpreting the ADMR data, despite having a large Fermi surface at $p=0.21$, seemingly in contradiction with other evidence for a small Fermi surface at that doping level.

The search for departures from standard hydrodynamics in many-body systems has yielded a number of promising leads, especially in low dimensions. Here, we study one of the simplest classical interacting lattice models, the nearest-neighbor Heisenberg chain, with temperature as the tuning parameter. Our numerics expose strikingly different spin dynamics between the antiferromagnet, where it is largely diffusive, and the ferromagnet, where we observe strong evidence either of spin superdiffusion or an extremely slow crossover to diffusion. This difference also governs the equilibration after a quench, and, remarkably, is apparent even at very high temperatures.

The Su-Schrieffer-Heeger . (SSH) chain is a one-dimensional lattice that comprises two dimerized sublattices. Recently, Zhu, Prodan, and Ahn (ZPA) [Phys. Rev. B 99, 041117(R) (2019)] proposed that one-dimensional flat bands can occur at the topological domain walls of a two-dimensional array of SSH chains. Here, we suggest a two-dimensional topological insulator that is protected by inversion and time-reversal symmetries without spin-orbit coupling. It is shown that two-dimensional SSH chains realize the proposed topological insulator. Utilizing the first Stiefel-Whitney numbers, a weak type of ${\mathbb{Z}}_2$ topological indices are developed, which identify the proposed topological insulator, dubbed a two-dimensional Stiefel-Whitney insulator (2DSWI). The ZPA model is employed to study the topological phase diagrams and topological phase transitions. It is found that the phase transition occurs via the formation of massless Dirac points that wind the entire Brillouin zone. We argue that this unconventional topological phase transition is a characteristic feature of a 2DSWI, manifesting as one-dimensional domain wall states. Using first-principles calculations, we find the suggested 2DSWI should be realized in 11 known materials, such as ${\mathrm{Zn}}_2{\left(\mathrm{P}{\mathrm{S}}_3\right)}_3$. This insight from our work could help efforts to realize topological flat bands in solid-state systems.

We study the mechanism of the exceptionally large anomalous Hall effect (AHE) in the noncentrosymmetric antiferromagnet $\mathrm{Co}{\mathrm{Nb}}_3{\mathrm{S}}_6$ by angle-resolved photoemission spectroscopy (ARPES) and magnetotransport measurements. From ARPES measurements of $\mathrm{Co}{\mathrm{Nb}}_3{\mathrm{S}}_6$ and its family compounds ($\mathrm{Fe}{\mathrm{Nb}}_3{\mathrm{S}}_6$ and $\mathrm{Ni}{\mathrm{Nb}}_3{\mathrm{S}}_6$), we find a band dispersion unique to the Co intercalation existing near the Fermi level. We further demonstrate that a slight deficiency of sulfur in $\mathrm{Co}{\mathrm{Nb}}_3{\mathrm{S}}_6$ eliminates the ferromagnetism and the AHE simultaneously while hardly changing the band structure, indicating that the weak ferromagnetism is responsible for the emergence of the large AHE. Based on our results, we propose Weyl points near the Fermi level to cause the large AHE.

Recent experiments have demonstrated two-qubit gate fidelities above 99% for semiconductor spin qubits. However, theoretically, the fidelity of controlled-not (cnot) operations is limited by off-resonant driving described by off-diagonal terms in the system Hamiltonian. Here, we investigate these off-diagonal contributions and we propose a fidelity improvement of several orders of magnitude by using asymmetric driving. Therefore, we provide a description of ac virtual gates based on a simple capacitance model which not only enables a high-fidelity cnot but also allows for crosstalk reduction when scaling up spin qubit devices to larger arrays.

We numerically study random-hopping fermions (the Cruetz ladder) with repulsion and investigate how the interactions deform localized eigenstates by means of the one-particle-density matrix (OPDM). The ground state exhibits resurgence of localization from the compact localized state to strong-repulsion-induced localization. On the other hand, excited states in the middle of the spectrum tend to extend by the repulsion. The transition property obtained by numerical calculations of the OPDM is deeply understood by studying a solvable model in which local integrals of motion (LIOMs) are obtained explicitly. The present paper clarifies the utility of the OPDM and also how compact-support LIOMs in the noninteracting limit are deformed by the repulsion.

We calculated the phase diagram of the second layer of para-${\mathrm{H}}_2$ adsorbed on graphite using quantum Monte Carlo methods. The second layer shows an incommensurate triangular crystal structure. By using a symmetric wave function, which makes possible molecule exchanges, we observed that this nearly two-dimensional crystal shows a finite superfluid density around a total density of 0.1650 Å${}^{-2}$. The superfluid fraction of this supersolid phase was found to be small, $(0.41\pm 0.05)\%$, but still experimentally accessible.

Perovskites like ${\mathrm{LaAlO}}_3$ (or ${\mathrm{SrTiO}}_3$) undergo displacive structural phase transitions from a cubic crystal to a trigonal (or tetragonal) structure. For many years, the critical exponents in both these types of transitions have been fitted to those of the isotropic three-components Heisenberg model. However, field theoretical calculations showed that the isotropic fixed point (FP) of the renormalization group is unstable, and renormalization group iterations flow either to a cubic FP or to a fluctuation-driven first-order transition. Here we show that these two scenarios correspond to the cubic-to-trigonal and to the cubic-to-tetragonal transitions, respectively. In both cases, the critical behavior is described by slowly varying effective critical exponents, which exhibit universal features. For the trigonal case, we predict a crossover of the effective exponents from their Ising values to their cubic values (which are close to the isotropic ones). For the tetragonal case, the effective exponents can have the isotropic values over a wide temperature range before exhibiting large changes en route to the first-order transition. New renormalization group calculations near the isotropic FP in three dimensions are presented and used to estimate the effective exponents, and dedicated experiments to test these predictions are proposed. Similar predictions apply to cubic magnetic and ferroelectric systems.

We consider the isotropic spin-$\frac{1}{2}$ Heisenberg spin chain weakly perturbed by a local translationally and SU(2)-invariant perturbation. Starting from the local integrals of motion of the unperturbed model, we modify them in order to obtain quasiconserved integrals of motion (charges) for the perturbed model. Such quasiconserved quantities are believed to be responsible for the existence of the prethermalization phase at intermediate timescales. We find that for a sufficiently local perturbation the quasiconserved quantities indeed exist, and we construct an explicit form for the first few of them.

We show that the correlation functions of a class of periodically driven integrable closed quantum systems approach their steady-state value as $n^{-(\alpha +1)/\beta }$, where $n$ is the number of drive cycles and $\alpha$ and $\beta$ denote positive integers. We find that, generically, $\beta =2$ within a dynamical phase characterized by a fixed $\alpha$; however, its value can change to $\beta =3$ or $\beta =4$ either at critical drive frequencies separating two dynamical phases or at special points within a phase. We show that such decays are realized in both driven Su-Schrieffer-Heeger (SSH) and one-dimensional (1D) transverse field Ising models, discuss the role of symmetries of the Floquet spectrum in determining $\beta$, and chart out the values of $\alpha$ and $\beta$ realized in these models. We analyze the SSH model for a continuous drive protocol using a Floquet perturbation theory which provides analytical insight into the behavior of the correlation functions in terms of its Floquet Hamiltonian. This is supplemented by an exact numerical study of a similar behavior for the 1D Ising model driven by a square pulse protocol. For both models, we find a crossover timescale $n_c$ which diverges at the transition. We also unravel a long-time oscillatory behavior of the correlators when the critical drive frequency, ${\omega }_c$, is approached from below $(\omega <{\omega }_c)$. We tie such behavior to the presence of multiple stationary points in the Floquet spectrum of these models and provide an analytic expression for the time period of these oscillations.

We discuss a model scenario for multiferroic systems of type II (collinear spins) where the electric dipolar order competes with a frustrated magnetic order in determining the elastic distortions of the lattice ion positions. High magnetic frustration due to second-neighbors exchange and small spin easy-axis anisotropy lead to the appearance of the so-called quantum magnetic plateau states. Increasing the magnetic field above the plateau border produces composite excitations, where fractionalized spin tertions arise together with spontaneous dipolar flips (in the form of domain walls) and enhanced localized elastic distortions. This peculiar magnetoelectric effect may be described by magneto-electric-elastic quasiparticles that could be detected by x-ray and neutron diffraction techniques. Our results are supported by extensive DMRG computations on the spin sector and self-consistent equations for the lattice distortions.

Material-specific super-exchange Hamiltonians are the key to studying spin and orbital physics in strongly correlated materials. Recently, via an irreducible-tensor operator representation, we derived the orbital superexchange Hamiltonian for $t_{2g}^1$ perovskites and successfully used it, in combination with many-body approaches, to explain orbital physics in these systems. Here, we generalize our method to $e_g^n$ and $t_{2g}^n$ systems at arbitrary integer filling $n$, including both spin and orbital interactions. The approach is suitable for numerical implementations based on ab initio hopping parameters and realistic screened Coulomb interactions and allows for a systematic exploration of superexchange energy surfaces in a realistic context.

Semiconductor-based emitters of pairwise photonic entanglement are a promising constituent of photonic quantum technologies. They are known for the ability to generate discrete photonic states on-demand with low multiphoton emission, near-unity entanglement fidelity, and high single photon indistinguishability. However, quantum dots typically suffer from luminescence blinking, lowering the efficiency of the source and hampering their scalable application in quantum networks. In this paper, we investigate and adjust the intermittence of the neutral exciton emission in a GaAs/AlGaAs quantum dot under two-photon resonant excitation of the neutral biexciton. We investigate the spectral and quantum optical response of the quantum dot emission to an additional wavelength tunable gate laser, revealing blinking caused by the intrinsic Coulomb blockade due to charge capture processes. Our finding demonstrates that the emission quenching can be actively suppressed by controlling the balance of free electrons and holes in the vicinity of the quantum dot and thereby significantly increasing the quantum efficiency by $30\%$.

The valley degree of freedom in two-dimensional materials provides an opportunity to extend the functionalities of valleytronic devices. Very short valley lifetimes demand the ultrafast control of valley pseudospin. Here we theoretically demonstrate the control of valley pseudospin in ${\mathrm{WSe}}_2$ monolayer by a single-cycle linearly polarized laser pulse. We use the asymmetric electric field controlled by the carrier-envelope phase (CEP) to make the valley polarization between $K$ and $K^{\prime }$ point in the Brillouin zone (BZ). Time-dependent density functional theory with spin-orbit interaction reveals that no valley asymmetry and its CEP dependence is observed within the linear-optical limit. In the nonlinear-optical regime, a linearly polarized pulse induces a high degree of valley polarization and this polarization is robust against the field strength. Valley polarization strongly depends and oscillates as a function of CEP. The carrier density distribution forms nodes as the laser intensity increases, our results indicate that the position of the carrier density in the BZ can be controlled by the laser intensity. From the analysis by the massive Dirac Hamiltonian model, the nodes of the carrier density can be attributed to the Landau-Zener-Stückelberg interference of wave packets of the electron wave function.

We study the influence of an in-plane magnetic field and Coulomb interactions on the physics of quantum spin Hall insulators, like those in InAs/GaSb and HgTe/CdTe quantum wells. Using a Hartree-Fock mean-field theory approximation, we calculate phase diagrams as functions of the band gap, band hybridization, and magnetic field strength. We show that when the band hybridization is weak, the system is unstable against the formation of density-wave states. As the strength of the in-plane magnetic field increases, the density-wave region of the phase diagram expands and distinct density-wave states appear. We discuss possible experimental implications of our results.

Eigenenergies of a non-Hermitian system without parity-time symmetry are complex in general. Here, we show that the chiral boundary states of higher-dimensional non-Hermitian topological insulators without parity-time symmetry can be Hermitian with real eigenenergies under certain conditions. Our approach allows one to construct Hermitian chiral edge and hinge states from non-Hermitian two-dimensional Chern insulators and three-dimensional second-order topological insulators, respectively. Such Hermitian chiral boundary channels have perfect transmission coefficients (quantized values) and are robust against disorders. Furthermore, a non-Hermitian topological insulator can undergo the topological Anderson insulator transition from a topologically trivial non-Hermitian metal or insulator to a topological Anderson insulator with quantized transmission coefficients at finite disorders.