The emission from a nonlinear photonic mode coupled weakly to a gain medium operating below threshold is predicted to exhibit antibunching. In the steady state regime, analytical solutions for the relevant observable quantities are found in accurate agreement with exact numerical results. Under pulsed excitation, the unequal time second-order correlation function demonstrates the triggered probabilistic generation of single photons well separated in time.

By combined top- and backgating, we explore the correlation of superconductivity with band filling and electron confinement at the ${\mathrm{LaAlO}}_3\text{/}{\mathrm{SrTiO}}_3$ interface. We find that the top- and backgate voltages have distinctly different effects on the superconducting critical temperature, implying that the confining potential well has a profound effect on superconductivity. We investigate the origin of this behavior by comparing the gate dependence of $T_c$ to the corresponding evolution of the band filling with gate voltage. For several backgate voltages, we observe maximum $T_c$ to consistently coincide with a kink in tuning the band filling for high topgate voltage. Self-consistent Schrödinger-Poisson calculations relate this kink to a Lifshitz transition of the second $d_{xy}$ subband. These results establish a major role for confinement-induced subbands in the phase diagram of ${\mathrm{SrTiO}}_3$ surface states, and establish gating as a means to control the relative energy of these states.

SnSe has emerged as an efficient and fascinating thermoelectric material. A fundamental understanding of the effects and nature of intrinsic defects and dopants in SnSe is crucial to optimize its thermoelectric performance. In this paper, we perform first-principles calculations to examine the native and extrinsic point-defect properties in SnSe. We show that the easy formation of acceptorlike Sn vacancy (${\mathrm{V}}_{\mathrm{Sn}}$) is responsible for the $p$-type conductivity in intrinsic SnSe. We also propose a mechanism and explain the anomalous temperature dependence of the carrier concentration in intrinsic SnSe crystals. Concerning the extrinsic defects, we focus on the dopants used in experiments. We find that Na (Ag) substitution on Sn site, ${\mathrm{Na}}_{\mathrm{Sn}}$ (${\mathrm{Ag}}_{\mathrm{Sn}}$), acts as acceptor, whereas, substitutional ${\mathrm{Br}}_{\text{Se}}, {\mathrm{I}}_{\text{Se}}$, and ${\mathrm{Bi}}_{\text{Sn}}$ dopants act as donor. It is shown that for Ag doping, its carrier concentration will be saturated with increasing doping concentration due to the coexistence of compensated defects (${\mathrm{Ag}}_{\mathrm{i}}$ and ${\mathrm{Ag}}_{\text{Sn}}$). Furthermore, we analyze how this doping introduced carrier impact on their thermoelectric characteristics. It is found that the more efficient doping of Na, Br, and I can realize higher $ZT$.

We find magnetic-field-dependent dielectric dispersions specific to successive field-induced magnetic phases of a geometrically frustrated magnet ${\mathrm{CuFeO}}_2$ up to 28 T. The dielectric dispersions in the three field-induced collinear-commensurate magnetic phases are well described by the superposition of Debye-type relaxations, and the number of contributions to the Debye-type dispersions differs in these phases. In contrast, the dielectric dispersions in the noncollinear-incommensurate phase, known as a spin-driven ferroelectric phase, cannot be simply described by the Debye-type relaxations. In addition, we find that the temperature dependence of the Debye relaxation frequencies follows the Arrhenius law, and that the activation energies derived from the Arrhenius equation also depend on the magnetic field. Considering the magnetostriction effect in combination with elongation/contraction of spins resulting from the application of a magnetic field, we show that the number of Debye relaxation components is equivalent to the number of states of local ${\mathrm{Fe}}_3\mathrm{O}$ clusters determined by oxygen displacement within a triangular Fe lattice. Based on this correspondence, we propose a possible explanation that excess charges resulting from a lack of stoichiometry hop over the double-well potentials within each local ${\mathrm{Fe}}_3\mathrm{O}$ cluster, like small polarons.

We develop an analytical approach for studying the ferromagnetic resonance (FMR) frequency shift due to dipolar interactions and surface effects in two-dimensional arrays of nanomagnets with (effective) uniaxial anisotropy along the magnetic field. For this we build a general formalism on the basis of perturbation theory that applies to dilute assemblies but which goes beyond the point-dipole approximation as it takes account of the size and shape of the nanoelements, in addition to their separation and spatial arrangement. The contribution to the frequency shift due to the shape and size of the nanoelements has been obtained in terms of their aspect ratio, their separation, and the lattice geometry. We have also varied the size of the array itself and compared the results with a semianalytical model and reached an agreement that improves as the size of the array increases. We find that the red-shift of the ferromagnetic resonance due to dipolar interactions decreases for smaller arrays. Surface effects may induce either a blue-shift or a red-shift of the FMR frequency, depending on the crystal and magnetic properties of the nanoelements themselves. In particular, some configurations of the nanoelements' assemblies may lead to a full compensation between surface effects and dipole interactions.

Interest in the superconducting proximity effect has recently been reignited by theoretical predictions that it could be used to achieve topological superconductivity. Low-$T_c$ superconductors have predominantly been used in this effort, but small energy scales of $\sim 1$ meV have hindered the characterization of the emergent electronic phase, limiting it to extremely low temperatures. In this work, we use molecular beam epitaxy to grow topological insulator $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ in a range of thicknesses on top of a high-$T_c$ superconductor Fe(Te,Se). Using scanning tunneling microscopy and spectroscopy, we detect ${\mathrm{\Delta }}_{\mathrm{ind}}$ as high as $\sim 3.5$ meV, which is the largest reported gap induced by proximity to an $s$-wave superconductor to date. We find that ${\mathrm{\Delta }}_{\mathrm{ind}}$ decays with $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ thickness, but remains finite even after the topological surface states have been formed. Finally, by imaging the scattering and interference of surface state electrons, we provide a microscopic visualization of the fully gapped $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ surface state due to Cooper pairing. Our results establish Fe-based high-$T_c$ superconductors as a promising new platform for realizing high-$T_c$ topological superconductivity.

The effects of Co substitution on $\mathrm{Ba}{\left({\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ ($0\le x\le 0.251$) single crystals grown out of Pb flux are investigated via transport, magnetic, and thermodynamic measurements. ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ exhibits a first-order tetragonal to triclinic structural phase transition at $T_s=137$ K upon cooling, and enters a superconducting phase below $T_c=0.7$ K. The structural phase transition is sensitive to cobalt content and is suppressed completely by $x\ge 0.133$. The superconducting critical temperature, $T_c$, increases continuously with $x$, reaching a maximum of $T_c=2.3$ K at $x=0.083$ and then decreases monotonically until superconductivity is no longer observable well into the tetragonal phase. In contrast to similar ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ substitutional studies, which show an abrupt change in $T_c$ at the triclinic-tetragonal boundary that extends far into the tetragonal phase, $\mathrm{Ba}{\left({\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ exhibits a domelike phase diagram centered around the zero-temperature tetragonal-triclinic boundary. Together with an anomalously large heat capacity jump $\mathrm{\Delta }{C}_e/\gamma T\sim 2.2$ near optimal doping, the smooth evolution of $T_c$ in the $\mathrm{Ba}{\left({\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ system suggests a mechanism for pairing enhancement other than phonon softening.

The noncentrosymmetric superconductor ${\mathrm{Re}}_6\mathrm{Zr}$ has attracted much interest due to the observation of broken time-reversal symmetry in the superconducting state. Here we report an investigation of the superconducting gap structure of ${\mathrm{Re}}_6\mathrm{Zr}$ single crystals by measuring the magnetic penetration depth shift $\mathrm{\Delta }\lambda \left(T\right)$ and electronic specific heat $C_e\left(T\right)$. $\mathrm{\Delta }\lambda \left(T\right)$ exhibits an exponential temperature dependence behavior for $T\ll T_c$, which indicates a fully open superconducting gap. Our analysis shows that a single gap $s$-wave model is sufficient to describe both the superfluid density ${\rho }_s\left(T\right)$ and $C_e\left(T\right)$ results, with a fitted gap magnitude larger than the weak coupling BCS value, providing evidence for fully gapped superconductivity in ${\mathrm{Re}}_6\mathrm{Zr}$ with moderate coupling.

A Weyl semimetal (WSM) is a topological material that hosts Weyl fermions as quasiparticles in the bulk. We study the combined effect of intra- and interorbital disorders on WSMs by adopting a tight-binding model that supports the WSM, three-dimensional quantum anomalous Hall insulator (3D QAHI), and normal insulator (NI) phases in the clean limit. Based on the calculation of the localization length and the Hall conductivity, we present rich phase diagrams due to the interplay of intra- and interorbital disorders. We find that the WSM with well-separated Weyl nodes is stable to both weak intra- and interorbital disorders. However, weak intraorbital disorder can gap out a WSM close to the 3D QAHI phase in the clean phase diagram, forming a 3D QAHI, and it can also drive a NI near the WSM phase to a WSM. By contrast, weak interorbital disorder can cause a 3D QAHI-WSM transition for a 3D QAHI in proximity to the WSM phase in the clean limit, and it can annihilate a WSM near the NI phase, bringing about a WSM-NI transition. We observe a diffusive anomalous Hall metal (DAHM) phase at moderate disorder strength. The DAHM appears in a wide range of the phase diagram when the intraorbital disorder dominates over the interorbital disorder, while the DAHM is found to exist in a narrow region or be missing in the phase diagram when the interorbital disorder dominates.

We compute the spin susceptibility of the two-dimensional Hubbard model away from half-filling, and analyze the impact of frequency-dependent vertex corrections as obtained from the dynamical mean field theory (DMFT). We find that the local dynamics captured by the DMFT vertex strongly affects nonlocal spin correlations, and thus the momentum dependence of the spin susceptibility. While the widely used random-phase approximation yields commensurate Néel-type antiferromagnetism as the dominant instability over a wide doping range, the vertex corrections favor incommensurate ordering wave vectors away from $(\pi ,\pi )$. Our results indicate that the connection between the magnetic ordering wave vector and the Fermi surface geometry, familiar for weakly interacting systems, can hold in a strongly correlated metal, too.

Triclinic rhenium disulphide ($\mathrm{Re}{\mathrm{S}}_2$) is a promising candidate for postsilicon electronics because of its unique optic-electronic properties. The electrical and optical properties of $\mathrm{Re}{\mathrm{S}}_2$ under high pressure, however, remain unclear. Here we present a joint experimental and theoretical study on the structure, electronic, and vibrational properties, and visible-light responses of $\mathrm{Re}{\mathrm{S}}_2$ up to 50 GPa. There is a direct-to-indirect band-gap transition in $1T\text{−}\mathrm{Re}{\mathrm{S}}_2$ under low-pressure regime up to 5 GPa. Upon further compression, $1T\text{−}\mathrm{Re}{\mathrm{S}}_2$ undergoes a structural transition to distorted-$1T^{\prime }$ phase at 7.7 GPa, followed by the isostructural metallization at 38.5 GPa. Both in situ Raman spectrum and electronic structure analysis reveal that interlayer sulfur–sulfur interaction is greatly enhanced during compression, leading to the remarkable modifications on the electronic properties observed in our subsequent experimental measurements, such as band-gap closure and enhanced photoresponsiveness. This study demonstrates the critical role of pressure in tuning materials properties and the potential usage of layered $\mathrm{Re}{\mathrm{S}}_2$ for pressure-responsive optoelectronic applications.

In this work, we investigate the influence of spin-orbit coupling on the magnetic-field dependence of carrier states in a self-assembled quantum dot. The electron and hole energy levels are calculated using the 6-, 8- and 14-band $\mathbf{k}·\mathbf{p}$ models. Through a detailed study within these models, we extract the information about the impact of various spin-orbital coupling channels on the hole $p$ shell. We demonstrate that the most important contribution comes from the influence of shear strain. We show that the complicated magnetic-field dependence of the hole $p$ shell resulting from numerical simulations can be very well fitted using a phenomenological model. We compare the electron and hole $g$-factors calculated within 8- and 14-band $\mathbf{k}·\mathbf{p}$ models and show that these methods give reasonably good agreement.

Fast, high-fidelity single and two-qubit gates are essential to building a viable quantum information processor, but achieving both in the same system has proved challenging for spin qubits. We propose and analyze an approach to perform a long-distance two-qubit controlled phase (CPHASE) gate between two singlet-triplet qubits using an electromagnetic resonator to mediate their interaction. The qubits couple longitudinally to the resonator, and by driving the qubits near the resonator's frequency, they can be made to acquire a state-dependent geometric phase that leads to a CPHASE gate independent of the initial state of the resonator. Using high impedance resonators enables gate times of order 10 ns while maintaining long coherence times. Simulations show average gate fidelities of over 96% using currently achievable experimental parameters and over 99% using state-of-the-art resonator technology. After optimizing the gate fidelity in terms of parameters tuneable in situ, we find it takes a simple power-law form in terms of the resonator's impedance and quality and the qubits' noise bath.

High-energy electronic excitations of graphene and ${\mathrm{MoS}}_2$ heterostructures are investigated by momentum-resolved electron energy-loss spectroscopy in the range of 1 to 35 eV. The interplay of excitations on different sheets is understood in terms of long-range Coulomb interactions and is simulated using a combination of ab initio and dielectric model calculations. In particular, the layered electron-gas model is extended to thick layers by including the spatial dependence of the dielectric response in the direction perpendicular to the sheets. We apply this model to the case of graphene/${\mathrm{MoS}}_2$/graphene heterostructures and discuss the possibility of extracting the dielectric properties of an encapsulated monolayer from measurements of the entire stack.

Two classes of topological superconductors and Majorana modes in condensed matter systems are known to date: one in which disorder induced by impurities strongly suppresses topological superconducting gap and is detrimental to Majorana modes, and another where Majorana fermions are protected by a disorder-robust topological superconductor gap. Observation and control of Majorana fermions and other non-Abelions often requires a symmetry of an underlying system leading to a gap in the single-particle or quasiparticle spectra. In semiconductor structures, impurities that provide charge carriers introduce states into the gap and enable conductance and proximity-induced superconductivity via the in-gap states. Thus a third class of topological superconductivity and Majorana modes emerges, in which topological superconductivity and Majorana fermions appear exclusively when impurities generate in-gap states. We show that impurity-enabled topological superconductivity is realized in a quantum Hall ferromagnet, when a helical domain wall is coupled to an $s$-wave superconductor. As an example of emergence of topological superconductivity in quantum Hall ferromagnets, we consider the integer quantum Hall effect in Mn-doped CdTe quantum wells. Recent experiments on transport through the quantum Hall ferromagnet domain wall in this system indicated a vital role of impurities in the conductance, but left unresolved the question whether impurities preclude generation of Majorana fermions and other non-Abelions in such systems in general. Here, solving a general quantum-mechanical problem of impurity bound states in a system of spin-orbit coupled Landau levels, we demonstrate that impurity-induced Majorana modes emerge at boundaries between topological and conventional superconducting states generated in a domain wall due to proximity to an $s$ superconductor. We consider both short-range disorder and a smooth random potential. The phase diagram of the system is defined by characteristic disorder, gate voltage induced angular momentum splitting of impurity levels, and by a proximity superconducting gap. The phase diagram exhibits two ranges of gate voltage with conventional superconducting order separated by a gate voltage range with topological superconductivity. We show that electrostatic control of domain walls in an integer quantum Hall ferromagnet allows manipulation of Majorana fermions. Ferromagnetic transitions in the fractional quantum Hall regime may lead to the formation and electrostatic control of higher order non-Abelian excitations.

We derive an effective electronic Hamiltonian for the square lattice Hubbard-Holstein model (HHM) in the strong electron-electron $\left(e\text{−}e\right)$ and electron-phonon $\left(e\text{−}ph\right)$ coupling regime and under nonadiabatic conditions $\left(t/{\omega }_0\le 1\right)$, $t$ and ${\omega }_0$ being the electron hopping and phonon frequency respectively. Using the density matrix renormalization-group method, we simulate this effective electronic model on a four-leg cylinder system at quarter filling and present a phase diagram in the $g\text{−}U$ plane where $g$ and $U$ are the $e\text{−}ph$ coupling constant and Hubbard on-site interaction respectively. For larger $g$, we find that a cluster of spins, i.e., phase separation (PS), gives way to a charge density wave (CDW) phase made of nearest-neighbor singlets which abruptly goes to another CDW phase as we increase $U$. But for smaller $g$, we find a metallic phase sandwiched between PS and the singlet CDW phase. This phase is characterized by a vanishing charge gap but a finite spin gap, suggesting a singlet superconducting phase.

Type-II Dirac semimetal (DSM) phase, in which Lorentz invariance is broken, has been experimentally realized in ${\mathrm{PtSe}}_2$ class. We study the transport properties and Fermi surface (FS) morphology of ${\mathrm{PtTe}}_2$ by de Haas–van Alphen (dHvA) quantum oscillations. Quantum oscillations analyses for out-of-plane and in-plane magnetizations revealed multiple frequencies with low effective mass and high quantum mobility. First-principles calculations are performed to confirm the existence of type-II Dirac fermions in ${\mathrm{PtTe}}_2$ and distinguish the contributing pockets for each oscillation mode. Among all the observed frequencies, one around 4510 T is identified to be derived from the electron pocket that participates in the formation of the type-II Dirac cone.

Motivated by recent experimental studies of hydrogen-bonded molecular conductors $\kappa \text{−}X{}_3$(Cat-EDT-${\mathrm{TTF})}_2[\mathit{X}=\mathrm{H}$, D], interplays of protons and correlated electrons, and their effects on magnetic, dielectric, and optical properties, are studied theoretically. We introduce a model Hamiltonian for $\kappa \text{−}X{}_3$(Cat-EDT-${\mathrm{TTF})}_2$, in which molecular dimers are connected by hydrogen bonds. Ground-state phase diagram and optical conductivity spectra are examined by using the mean-field approximation and the exact diagonalization method in finite-size cluster. Three types of the competing electronic and protonic phases, charge density wave phase, polar charge-ordered phase, and antiferromagnetic dimer-Mott insulating phase are found. Observed softening of the interdimer excitation due to the electron-proton coupling implies reduction of the effective electron-electron repulsion, i.e., “Hubbard $U$,” due to the quantum proton motion. Contrastingly, the intradimer charge excitation is hardened due to the proton-electron coupling. Implications of the theoretical calculations to the recent experimental results in $\kappa \text{−}X{}_3$(Cat-EDT-${\mathrm{TTF})}_2$ are discussed.