We study interference patterns and Friedel oscillations (FO) due to scattering from two or more localized impurities and scattering from extended inhomogeneities in the two-dimensional lattice systems of interacting fermions. Correlations between particles are accounted for by using an approximate method based on the real-space dynamical mean-field theory and a homogeneous self-energy approximation (HSEA), where the site-dependent part of the self-energy is neglected. We find that the interference maxima and minima change systematically as we vary the relative distance between the two impurities. At the same time, the increase of the interaction does not shift the position of interference fringes but only reduces their intensities. A comparison with the single impurity cases clearly shows complex patterns in FO fringes induced by additional multiple scattering processes. In the case of an extended steplike potential the system becomes more homogeneous when the interaction increases. FO and interference patterns are not present in the Mott insulating phase in both single and many impurity models.

The kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) have recently received enormous attention as they exhibit nontrivial topological electronic structure, flat band, unconventional intertwined charge density wave (CDW), anomalous Hall effect, and superconductivity. However, it is still controversial whether or not electronic correlations play an important role in these novel phenomena. Here we perform an extensive investigation on $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) with density functional theory plus dynamical mean field theory calculations. Our results indicate the electronic correlations effects are in the weak regime in both the normal state and the CDW state. The value of effective mass-enhancement $m^{*}/m_{\mathrm{DFT}}$ is about 1.3, and it is barely changed at temperatures ranging 50–900 K in the normal state. Our static susceptibility calculations with the normal structure show that static susceptibility has a weak $q_z$ dependence and the Fermi-surface nesting mechanism does not work in this system. In the CDW state with the inverse Star of David structure, the spectral function opens a CDW gap at the Fermi level below $T_{\mathrm{CDW}}$ (50 K) compared with the normal state above $T_{\mathrm{CDW}}$ (300 K). We find that there exists orbital multiplicity at the Fermi level owing to two nonequivalent V atoms in CDW states, which may have an important role in forming the CDW order.

The magnetic proximity effect (MPE), ferromagnetic (FM) coupling at the interface of magnetically dissimilar layers, has attracted much attention as a promising pathway for introducing ferromagnetism into a high-mobility nonmagnetic (NM) conducting channel. Recently, our group found giant proximity magnetoresistance (PMR), which is caused by MPE at an interface between a NM semiconductor InAs quantum well (QW) layer and a FM semiconductor (Ga,Fe)Sb layer. The MPE in the NM semiconductor can be modulated by applying a gate voltage and controlling the penetration of the electron wave function in the InAs QW into the neighboring insulating FM (Ga,Fe)Sb layer. However, optimal conditions to obtain strong MPE at the InAs/(Ga,Fe)Sb interface have not been clarified. In this paper, we systematically investigate the PMR properties of ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}$ ($x=0$, 5, 7.5, and 10%)/(Ga,Fe)Sb bilayer semiconductor heterostructures under a wide range of gate voltage. The inclusion of Ga alters the electronic structures of the InAs thin film, changing the effective mass and the QW potential of electron carriers. Our experimental results and theoretical analysis of the PMR in these ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}\text{/}$(Ga,Fe)Sb heterostructures show that the MPE depends not only on the degree of penetration of the electron wave function into (Ga,Fe)Sb but also on the electron density. These findings help us to unveil the microscopic mechanism of MPE in semiconductor-based NM/FM heterojunctions.

The momentum-dependent spin splitting in the conduction band couples orbital motion to spin and enables electrical control of spin. Currently, this control relies on the relativistic spin-orbit interaction (SOI), which limits useful materials to those containing heavy elements. Recently, Naka et al. [Nat. Commun. 10, 4305 (2019)] have found a momentum-dependent spin splitting originating from the exchange interaction, which is expected to extend spintronic materials to those without heavy elements. In this paper, we propose a mechanism of the exchange-induced orbital-spin coupling by extending the $\mathit{k}·\mathit{p}$ theory. As an example, we consider an $n$-type ferromagnetic semiconductor (nFMS) of $T_d$ point group symmetry with the $p\text{−}d$ exchange interaction between an electron in the valence band and the spin of a magnetic ion and evaluate the spin splitting in the conduction band of ${\mathrm{\Gamma }}_6$ irreducible representation from the eight-band $\mathit{k}·\mathit{p}$ Hamiltonian. We find that the lowest-order spin splitting in bulk is of the second order of momentum, which results in a nonzero splitting at $k_x=k_y=0$ in a quantum well with a nonzero quantized momentum $k_z$. An estimation shows that the $p\text{−}d$ exchange interaction is the dominant origin of the conduction-band spin splitting in InFeAs nFMS. We also calculate the intrinsic anomalous Hall conductivity of bulk InFeAs generated by the $p\text{−}d$ exchange, which provides both the coupling of orbital motion to spin and that of spin to nFMS magnetization. We find that the $p\text{−}d$ exchange-induced Hall conductivity exhibits an accelerated increase with Fe density, in contrast to that produced by the $s\text{−}d$ exchange and the Dresselhaus SOI. This finding suggests that the extended $\mathit{k}·\mathit{p}$ mechanism of orbital-spin coupling is expected to help find remarkable phenomena and useful applications in a wide variety of materials and structures.

Heat transport in multiple quantum Hall edge channels at Landau-level filling factors $\nu$ = 2, 4, and 8 is investigated with a quantum point contact as a heat generator and a quantum dot as a local thermometer. Heat distribution among the channels remains highly nonuniform, which can be understood with the plasmon eigenmodes of the multiple channels. The heat transport can be controlled with another quantum point contact as a quantized heat valve, as manifested by stepwise increases of heat current at the thermometer. This encourages developing integrated heat circuits with quantum Hall edge channels.

We have numerically investigated the occurrence of long slips when a point mass representing a tip of a solid surface is elastically driven on a hexagonal surface lattice along an arbitrary direction. Tip pathways and slip length histograms are estimated for different values of the damping coefficient $\gamma$ defining the duration of the slip phases. The results are compared with a map of all possible “slip channels” as obtained analytically with the Prandtl-Tomlinson model in two dimensions. The histograms of the corresponding force drops obtained for different values of $\gamma$ could be directly compared with experimental data obtained by friction force microscopy, which would allow us to quantify the energy dissipation in the slip phase in different loading conditions and shed light on quantum effects (phonon and/or electron-hole generation) accompanying sliding friction on the atomic scale.

Utilizing the time-dependent density-matrix renormalization-group technique, we numerically prove photoinduced pairing states in the extended Falicov-Kimball model (EFKM) at half-filling, both with and without internal SU(2) symmetry. In the time-dependent photoemission spectra, an extra band appears above the Fermi energy after pulse irradiation, indicating an insulator-to-metal transition. Even in the absence of the SU(2) structure, the pair correlations are enhanced during the pump, and afterwards they decrease over time. This implies the possible metallization of ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$, a strong candidate for an excitonic insulator material, for which the EFKM is considered to be the minimal theoretical model. Simulating the photoemission with optimized pulse parameters, we demonstrate a photoinduced quantum phase transition, in accord with recent findings in time- and angle-resolved photoemission spectroscopy experiments on ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$.

We study the twisted Hubbard model of a cuprate bilayer at a fixed twist angle $\theta =53.{13}^{\circ }$ and as a function of doping, using the variational cluster approximation, a method that treats short-range dynamical correlations exactly. At intermediate interlayer tunneling, we observe a sudden change of the relative sign of the $d$-wave order parameters of two layers between the underdoped and overdoped regimes. At strong interlayer tunneling, we observe a clear time-reversal symmetry breaking phase near optimal doping, in which the relative phase of the two layers changes continuously from 0 to $\pi$. However, this phase has trivial topology. We also apply a cluster extension of dynamical mean field theory to the same problem, but fail to detect a time-reversal breaking phase with that method, probably owing to the very small energy difference between the different states involved.

We show that the fractionalization of the constrained lattice electrons into charge/spin degrees of freedom driven by strong electron correlation can recover the anomalous quantum Hall effect that is similar to an integer quantum Hall effect in the absence of an external magnetic field.

We have performed inelastic neutron scattering measurements on optimally doped ${\mathrm{Fe}}_{0.98}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ and 10% Cu-doped ${\mathrm{Fe}}_{0.88}{\mathrm{Cu}}_{0.1}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ to investigate the substitution effects on the spin excitations in the whole energy range up to 300 meV. It is found that substitution of Cu for Fe enhances the low-energy spin excitations ($\le 100\mathrm{meV}$), especially around the (0.5, 0.5) point, and leaves the high-energy magnetic excitations intact. In contrast to the expectation that Cu with spin 1/2 will dilute the magnetic moments contributed by Fe with a larger spin, we find that the 10% Cu doping enlarges the effective fluctuating moment from 2.85 to 3.13 ${\mu }_{\mathrm{B}}/\mathrm{Fe}$, although there is no long- or short-range magnetic order around (0.5, 0.5) and (0.5, 0). The presence of enhanced magnetic excitations in the 10% Cu doped sample which is in the insulating state indicates that the magnetic excitations must have some contributions from the local moments, reflecting the dual nature of the magnetism in iron-based superconductors. We attribute the substitution effects to the localization of the itinerant electrons induced by Cu dopants. These results also indicate that the Cu doping does not act as electron donor as in a rigid-band shift model, but more as scattering centers that localize the system.

The fidelity of two-qubit gates using silicon spin qubits is limited by charge noise. When attempting to dynamically compensate for charge noise using single-qubit echo pulses, crosstalk can cause complications. We present a method of using a deep neural network to optimize the components of an analytically designed composite pulse sequence, resulting in a two-qubit gate robust against charge noise errors while also taking crosstalk into account. We analyze two experimentally motivated scenarios. For a scenario with strong electron dipole spin resonance driving and negligible crosstalk, the composite pulse sequence yields up to an order of magnitude improvement over a simple cosine pulse. In a scenario with moderate electron spin resonance driving and appreciable crosstalk such that simple analytical control fields are not effective, optimization using the neural network approach allows one to maintain order-of-magnitude improvement despite the crosstalk.

We determine the interqubit crosstalk- and charge-noise-induced decoherence time $T_2^{*}$ for a system of $L$ exchange-coupled electronic spin qubits in arrays of size $L=3$–14 for a number of different multiqubit geometries by directly calculating the return probability. We compare the behavior of the return probability to other quantities, namely, the average spin, the Hamming distance, and the entanglement entropy. In all cases, we use a starting state with alternating spins, $|{\mathrm{\Psi }}_0\rangle =|\downarrow \uparrow \downarrow \cdots \rangle$. We show that a power law behavior, $T_2^{*}\propto L^{-\gamma }$, is a good fit to the results for the chain and ring geometries as a function of the number of qubits and provide numerical results for the exponent $\gamma$. We find that $T_2^{*}$ depends crucially on the multiqubit geometry of the system. We also calculate the expectation value of one of the spins, the Hamming distance, and the entanglement entropy and show that they are good proxies for the return probability for measuring $T_2^{*}$. A key finding is that $T_2^{*}$ decreases with increasing $L$. We also demonstrate that these results may be understood in terms of perturbation theory and its breakdown.

We have conducted a thorough theoretical and numerical investigation of the electronic susceptibility, the polarizability, the plasmon modes and their damping rates, and the static screening in pseudospin-1 Dirac-cone materials with a flat band, or in a general $\alpha -{\mathcal{T}}_3$ model, at finite temperatures. This study includes calculating the polarization function and the plasmon dispersions and their damping rates at arbitrary temperatures and obtaining analytical approximations in the long-wavelength limit at low and high temperatures. We further demonstrate that the integral transformation of the polarization function cannot be used directly for a dice lattice, revealing some fundamental properties and important applicability limits of the flat-band dispersion model. As $k_{B}T/E_F\ll 1$, the dominant temperature-dependent change of the polarization function and plasmon modes comes from the mismatch between finite-temperature chemical potential and zero-temperature Fermi energy. In particular, we have obtained several closed-form semianalytical expressions for the static-limit polarization function of an arbitrary $\alpha -{\mathcal{T}}_3$ material and acquired exact analytical formulas in all limits of high, low, and zero temperature. This result is of tremendous importance for all types of transport and screening studies of the flat-band Dirac materials.

We present a theory for the differential shot noise, $dS/dV$, as measured via shot-noise scanning tunneling spectroscopy, and the differential conductance, $dI/dV$, for tunneling into Majorana zero modes (MZMs) in the putative topological superconductor ${\mathrm{FeSe}}_{0.45}{\mathrm{Te}}_{0.55}$. We demonstrate that for tunneling into chiral Majorana edge modes near domain walls, as well as MZMs localized in vortex cores and at the end of defect lines, $dS/dV$ vanishes whenever $dI/dV$ reaches a quantized value being equal to the quantum of conductance. These results are independent of the particular orbital tunneling path, thus establishing a vanishing $dS/dV$ concomitant with a quantized $dI/dV$, as universal signatures for Majorana modes in two-dimensional topological superconductors, irrespective of the material's specific complex electronic band structure.

${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ has attracted much recent attention as the first quasi-two-dimensional (2D) kagome superconductor. While the kagome layers are 2D in nature, increasing evidence has pointed to the importance of out-of-plane correlation in this material. However, it remains unclear whether such correlation can change the fundamental electronic structure of the quasi-2D system. Here, we reveal this missing piece of information, using angle-resolved photoemission spectroscopy, complemented by scanning tunneling microscope measurements. The three-dimensional electronic structures in the high-temperature state are revealed, which agree well with density-functional theory calculations. Electron energy bands are observed in the low-temperature state that exhibit additional periodicities along the out-of-plane momentum. These results reveal a direct response to the out-of-plane electronic supermodulation in the single-particle spectral function of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, thus establishing an electronic platform to examine emergent phenomena beyond 2D limit in kagome superconductors.

The titanium oxide superconductors exhibit many intriguing phenomena analogous to cuprates and iron pnictides/chalcogenides and thus provide an ideal platform for studying the pairing mechanism of high-temperature superconductors. Here we report the fabrication of superconducting Mg-Ti-O films on ${\mathrm{MgAl}}_2{\mathrm{O}}_4$ substrates with three principal orientations by ablating a ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ target. Strikingly, a single-crystalline film with an unexpected structure has been found on the (011)-oriented substrate, with a zero resistance transition temperature ($T_{\mathrm{c}0}$) up to 5.0 K which is the highest among the three principal orientations. Compositional and structural characterizations confirm that the film has a highly reduced Mg/Ti ratio and an orthorhombic ${\mathrm{Ti}}_9{\mathrm{O}}_{10}$-like structure (denoted as Mg: ${\mathrm{Ti}}_9{\mathrm{O}}_{10}$). Such a structure is unstable in bulk but is favorable on the (011)-surface of ${\mathrm{MgAl}}_2{\mathrm{O}}_4$ due to the interfacial strain. Similar to other superconducting titanium oxides, the Mg: ${\mathrm{Ti}}_9{\mathrm{O}}_{10}$ film exhibits a relatively large quasi-isotropic upper critical field ($B_{\mathrm{c}2}\sim 13.7$ T). The similarity points to a common origin for the superconductivity in the family, providing valuable insights for the mechanism of unconventional superconductivity in transition metal compounds.

We derive a boundary condition for the Nambu Keldysh Green's function in diffusive normal metal-unconventional superconductor junctions applicable for mixed parity pairing. Applying this theory to a 1d model of $s+p$-wave superconductor, we calculate LDOS in DN and charge conductance of DN-$s+p$-wave superconductor junctions. When the $s$-wave component of the pair potential is dominant, LDOS has a gap like structure at zero energy and the dominant pairing in DN is even-frequency spin-singlet $s$-wave. On the other hand, when the $p$-wave component is dominant, the resulting LDOS has a zero energy peak and the dominant pairing in DN is odd-frequency spin-triplet $s$-wave. We show the robustness of the quantization of the conductance when the magnitude of $p$-wave component of the pair potential is larger than that of $s$-wave one. These results show the robustness of the anomalous proximity effect specific to spin-triplet superconductor junctions.

In order to develop a high impedance standard for shock physics applications, we investigated the equation of state (EOS) and shock compression of bulk platinum using canonical ab initio molecular dynamics (AIMD) simulations, coupled with experimental data on Sandia's Z machine. In simulations, we sampled the thermodynamic state space along isotherms ranging from 300 to 100 000 K for densities of 18 to 40 $\mathrm{g}/{\mathrm{cm}}^3$. While this rectilinear grid of EOS points was useful for preliminary experimental design, material going outside of the density/temperature space of the AIMD simulations required a SESAME-style, broad-range EOS, which we developed independently and subsequently compared to the AIMD simulations. In the final step we validated the SESAME EOS with experimental shock data to 2177 GPa on Sandia's Z machine. The theoretical and AIMD results are in excellent agreement with experiments.

We address the out-of-equilibrium dynamics of many-body systems subject to time-dependent round-trip protocols across quantum and classical (thermal) phase transitions. They are realized by slowly changing one relevant parameter $w$ across its critical point $w_c=0$, linearly in time with a large timescale $t_s$, from $w_i<0$ to $w_f>0$ and then back to $w_i<0$, thus entailing multiple passages through the critical point. Analogously to the one-way Kibble-Zurek protocols across a critical point, round-trip protocols develop dynamic scaling behaviors at both classical and quantum transitions put forward within renormalization-group frameworks. The scaling scenario is analyzed within some paradigmatic models undergoing quantum and classical transitions belonging to the two-dimensional Ising universality class, such as one-dimensional quantum Ising models and fermionic wires, and two-dimensional classical Ising models (supplemented with a purely relaxational dynamics). While the dynamic scaling frameworks are similar for classical and quantum systems, substantial differences emerge due to the different nature of their dynamics, which is purely relaxational for classical systems (implying thermalization in the large-time limit at fixed model parameters), and unitary in the case of quantum systems. In particular, when the critical point separates two gapped (short-ranged) phases and the extreme value $w_f>0$ is kept fixed in the large-$t_s$ limit of the round-trip protocol, we observe hysteresislike scenarios in classical systems, while quantum systems do not apparently develop a sufficiently robust scaling limit along the return way, due to the presence of rapidly oscillating relative phases among the relevant quantum states.