We propose a generalization of the supersymmetric representation of spins with symplectic symmetry, generalizing the rotation group of the spin from SU(2) to Sp(N). As a test application of this representation, we consider two toy models involving a competition of the Kondo effect and antiferromagnetism: a two-impurity model and a frustrated three-impurity model. Exploring an ensemble of L-shaped representations with a fixed number of boxes in their respective Young tableaux, we allow the system to choose which representation is energetically more favorable in each region in parameter space. We discuss how the features of these preliminary applications can generalize to Kondo lattice models.

An accurate understanding of the phase diagram of dense hydrogen and helium mixtures is a crucial component in the construction of accurate models of Jupiter, Saturn, and Jovian extrasolar planets. Though density-functional-theory-based first-principles methods have the potential to provide the accuracy and computational efficiency required for this task, recent benchmarking in hydrogen has shown that achieving this accuracy requires a judicious choice of functional, and a quantification of the errors introduced. In this work, we present a quantum Monte Carlo (QMC) -based benchmarking study of a wide range of density functionals for use in hydrogen-helium mixtures at thermodynamic conditions relevant for Jovian planets. Not only do we continue our program of benchmarking energetics and pressures, but we deploy QMC-based force estimators and use them to gain insight into how well the local liquid structure is captured by different density functionals. We find that TPSS, BLYP, and vdW-DF are the most accurate functionals by most metrics, and that the enthalpy, energy, and pressure errors are very well behaved as a function of helium concentration. Beyond this, we highlight and analyze the major error trends and relative differences exhibited by the major classes of functionals, and we estimate the magnitudes of these effects when possible.

The Kondo effect, which originates from the screening of a localized magnetic moment by a spin-spin interaction, is widely observed in nonartificial magnetic materials, artificial quantum dots, and carbon nanotubes. In devices based on quantum dots or carbon nanotubes that target quantum information applications, the Kondo effect can be tuned by a gate voltage, a magnetic field, or light. However, the manipulation of the Kondo effect in nonartificial materials has not been thoroughly studied; in particular, the artificial creation of the Kondo effect remains unexplored. Per this subject study, however, a route for the optical creation of the Kondo effect in the nonartificial material $p$-type semiconductor ${\mathrm{CeZn}}_3{\mathrm{P}}_3$ is presented. The Kondo effect emerges under visible-light illumination of the material by a continuous-wave laser diode and is ultimately revealed by photoinduced electrical resistivity, which clearly exhibits a logarithmic temperature dependency. By contrast, a La-based compound (${\mathrm{LaZn}}_3{\mathrm{P}}_3$) displays only normal metallic behavior under similar illumination. The photoinduced Kondo effect, which occurs at higher temperatures when compared with the Kondo effect in artificial systems, provides a potential range of operation for not only quantum information/computation devices but also for operation of magneto-optic devices, thereby expanding the range of device applications based on the Kondo effect.

In order to investigate the mechanism of ferromagnetic ordering in the new $n$-type magnetic semiconductor (In,Fe)As codoped with Be, we have performed x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) studies of ferromagnetic and paramagnetic samples. The spectral line shapes suggest that the ferromagnetism is intrinsic, originating from Fe atoms incorporated into the zinc-blende-type InAs lattice. The magnetization curves of Fe measured by XMCD were well reproduced by the superposition of a Langevin function representing superparamagnetic (SPM) behavior of nanoscale ferromagnetic domains and a $T$-linear function representing Curie-Weiss paramagnetism even much above the Curie temperatures. The data at 20 K showed a deviation from the Langevin behavior, suggesting a gradual establishment of macroscopic ferromagnetism on lowering temperature. The existence of nanoscale ferromagnetic domains indicated by the SPM behavior suggests spatial fluctuations of Fe concentration on the nanoscale.

Topological insulators (TIs) hold great promise for topological quantum computation in solid-state systems. Recently, several groups reported experimental data suggesting that signatures of Majorana modes have been observed in topological insulator Josephson junctions (TIJJs). A prerequisite for the exploration of Majorana physics is to obtain a good understanding of the properties of low-energy Andreev bound states (ABSs) in a material with a topologically nontrivial band structure. Here, we present experimental data and a theoretical analysis demonstrating that the band-structure inversion close to the surface of a TI has observable consequences for supercurrent transport in TIJJs prepared on surface-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin films. Electrostatic carrier depletion of the film surface leads to an abrupt drop in the critical current of such devices. The effect can be understood as a relocation of low-energy ABSs from a region deeper in the bulk of the material to the more strongly disordered surface, which is driven by the topology of the effective band structure in the presence of surface dopants.

Often, the calculation of Coulomb coupling elements for quantum dynamical treatments, e.g., in cluster or correlation expansion schemes, requires the evaluation of a six dimensional spatial integral. Therefore, it represents a significant limiting factor in quantum mechanical calculations. If the size or the complexity of the investigated system increases, many coupling elements need to be determined. The resulting computational constraints require an efficient method for a fast numerical calculation of the Coulomb coupling. We present a computational method to reduce the numerical complexity by decreasing the number of spatial integrals for arbitrary geometries. We use a Green's function formulation of the Coulomb coupling and introduce a generalized scalar potential as solution of a generalized Poisson equation with a generalized charge density as the inhomogeneity. That enables a fast calculation of Coulomb coupling elements and, additionally, a straightforward inclusion of boundary conditions and arbitrarily spatially dependent dielectrics through the Coulomb Green's function. Particularly, if many coupling elements are included, the presented method, which is not restricted to specific symmetries of the model, presents a promising approach for increasing the efficiency of numerical calculations of the Coulomb interaction. To demonstrate the wide range of applications, we calculate internanostructure couplings, such as the Förster coupling, and illustrate the inclusion of symmetry considerations in the method for the Coulomb coupling between bound quantum dot states and unbound continuum states.

We demonstrate that the energy transport of surface phonon polaritons can efficiently be observed in a crystal made up of a three-dimensional assembly of spheroidal nanoparticles of silicon carbide. The ultralow phonon thermal conductivity of this nanostructure, along with its high surface area-to-volume ratio, allows the predominance of the polariton energy over that generated by phonons. The polariton dispersion relation, propagation length, and thermal conductance are numerically determined as functions of the size, shape, and temperature of the nanoparticles. It is shown that the thermal conductance of a crystal with prolate nanoparticles at 500 K and a minor (major) axis of 50 nm ($5\mu \mathrm{m}$) is $0.5\mathrm{nW}{\mathrm{K}}^{-1}$, which is comparable to the quantum of thermal conductance of polar nanowires. We also show that a nanoparticle size dispersion of up to 200 nm does not change significantly the polariton energy, which supports the technological feasibility of the proposed crystal.

Two-dimensional (2D) topological insulators (TIs), which can be integrated into the modern silicon industry, are highly desirable for spintronics applications. Here, using first-principles electronic structure calculations, we show that the Au/Si(111)-$\sqrt{3}$ substrate can provide a platform for hosting 2D TIs obtained through the formation of surface alloys with a honeycomb pattern of adsorbed atoms. We systematically examined elements from groups III to VI of the periodic table at 2/3 monolayer coverage on Au/Si(111)-$\sqrt{3}$, and found that In, Tl, Ge, and Sn adsorbates result in topologically nontrivial phases with band gaps varying from 0 to 50 meV. Our scanning tunneling microscopy and low-energy electron diffraction experiments confirm the presence of the honeycomb pattern when Bi atoms are deposited on Au/Si(111)-$\sqrt{3}$, in accord with our theoretical predictions. Our findings pave the way for using surface alloys as a potential route for obtaining viable 2D TI platforms.

Decoherence of a central spin coupled to an interacting spin bath via inhomogeneous Heisenberg coupling is studied by two different approaches, namely an exact equations of motion (EOMs) method and a Chebyshev expansion technique (CET). By assuming a wheel topology of the bath spins with uniform nearest-neighbor $XX$-type intrabath coupling, we examine the central spin dynamics with the bath prepared in two different types of bath initial conditions. For fully polarized baths in strong magnetic fields, the polarization dynamics of the central spin exhibits a collapse-revival behavior in the intermediate-time regime. Under an antiferromagnetic bath initial condition, the two methods give excellently consistent central spin decoherence dynamics for finite-size baths of $N\le 14$ bath spins. The decoherence factor is found to drop off abruptly on a short time scale and approach a finite plateau value which depends on the intrabath coupling strength nonmonotonically. In the ultrastrong intrabath coupling regime, the plateau values show an oscillatory behavior depending on whether $N/2$ is even or odd. The observed results are interpreted qualitatively within the framework of the EOM and perturbation analysis. The effects of anisotropic spin-bath coupling and inhomogeneous intrabath bath couplings are briefly discussed. Possible experimental realization of the model in a modified quantum corral setup is suggested.

We study graphene, which has both spin-orbit coupling (SOC), taken to be of the Kane-Mele form, and a Zeeman field induced due to proximity to a ferromagnetic material. We show that a zigzag interface of graphene having SOC with its pristine counterpart hosts robust chiral edge modes in spite of the gapless nature of the pristine graphene; such modes do not occur for armchair interfaces. Next we study the change in the local density of states (LDOS) due to the presence of an impurity in graphene with SOC and Zeeman field, and demonstrate that the Fourier transform of the LDOS close to the Dirac points can act as a measure of the strength of the spin-orbit coupling; in addition, for a specific distribution of impurity atoms, the LDOS is controlled by a destructive interference effect of graphene electrons which is a direct consequence of their Dirac nature. Finally, we study transport across junctions, which separates spin-orbit coupled graphene with Kane-Mele and Rashba terms from pristine graphene both in the presence and absence of a Zeeman field. We demonstrate that such junctions are generally spin active, namely, they can rotate the spin so that an incident electron that is spin polarized along some direction has a finite probability of being transmitted with the opposite spin. This leads to a finite, electrically controllable, spin current in such graphene junctions. We discuss possible experiments that can probe our theoretical predictions.

We investigate the intraband carrier dynamics in Landau-quantized graphene after an optical excitation with low-energetic terahertz pulses. Based on a microscopic theory, we calculate time-dependent differential transmission spectra reflecting the Landau-level dynamics. Our calculations reveal a strong dependence on the Fermi energy $E_F$ of the graphene sample as well as on the applied magnetic field $B$. We find that the pump pulse can lead to both absorption bleaching and absorption enhancement depending on $B$ and the position of $E_F$ with respect to the resonant Landau-level transition. As a result, positive and negative contributions in differential transmission spectra appear, in good agreement with recent pump-probe measurements.

In the quest for quantum spin liquids in three spatial dimensions (3D), we study the half-filled Hubbard model on the simple cubic lattice with hopping processes up to third neighbors. Employing the variational cluster approach (VCA), we determine the zero-temperature phase diagram: In addition to a paramagnetic metal at small interaction strength $U$ and various antiferromagnetic insulators at large $U$, we find an intermediate-$U$ antiferromagnetic metal. Most interestingly, we also identify a nonmagnetic insulating region, extending from intermediate to strong $U$. Using VCA results in the large-$U$ limit, we establish the phase diagram of the corresponding $J_1\text{−}{J}_2\text{−}{J}_3$ Heisenberg model. This is qualitatively confirmed—including the nonmagnetic region—using spin-wave theory. Further analysis reveals a striking similarity to the behavior of the $J_1\text{−}{J}_2$ square-lattice Heisenberg model, suggesting that the nonmagnetic region may host a 3D spin-liquid phase.

We investigate the development of the local bonding and chemical state of boron atoms during the growth of B-doped graphene on $6H\text{−}\mathrm{SiC}\left(0001\right)$. Photoemission experiments reveal the presence of two chemical states, namely, boron in the uppermost SiC bilayers and boron substituted in both the graphene and buffer layer lattices. We demonstrate the participation of the dopant in the $\pi$ electron system of graphene by the presence of the ${\pi }^{*}$ resonance in the near edge x-ray adsorption fine structure (NEXAFS) recorded at the B $K$-edge. The experimental findings are supported by NEXAFS simulations.

We study the electromagnetic response and surface electromagnetic modes in a generic gapped Dirac material under pumping with circularly polarized light. The valley imbalance due to pumping leads to a net Berry curvature, giving rise to a finite transverse conductivity. We discuss the appearance of nonreciprocal chiral edge modes, their hybridization and waveguiding in a nanoribbon geometry, and giant polarization rotation in nanoribbon arrays.

A physical mechanism that may enable electrical control of carrier spin-valley polarization is theoretically examined in a monolayer transition-metal dichalcogenide (TMD) structure. The idea is based on the interplay between the strongly spin-orbit coupled nature of the TMD band structure and the exchange interaction with a proximate magnet that can spontaneously lift the valley degeneracy when the carrier density exceeds a certain threshold. The analysis based on the free energy of the system clearly illustrates the desired spin-valley polarization in the TMD layer as well as the accompanying rotational phase transition in the magnetization. Numerical estimates utilizing the ${\mathrm{WS}}_2$ parameters as an example indicate a sharp transition in the spin-valley polarization over tens of percent at room temperature with only a modest change in the electrochemical potential of a few meV via electrostatic bias. Detection of the predicted phenomenon is expected to be straightforward through the corresponding modification in the TMD channel conductance.

The temperature $\left(T\right)$ as a function of pressure $\left(P\right)$ phase diagram is reported for the cubic Laves phase compound ${\mathrm{Au}}_2\text{Pb}$, which was recently proposed to support linearly dispersing topological bands, together with conventional quadratic bands. At ambient pressure, ${\mathrm{Au}}_2\text{Pb}$ exhibits several structural phase transitions at $T_1=97\text{K}$, $T_2=51\text{K}$, and $T_3=40\text{K}$ with superconductivity below $T_{\mathrm{c}}=1.2\text{K}$. Applied pressure results in a rich phase diagram where $T_1,T_2$, and $T_3$ evolve strongly with $P$ and a possible new phase is stabilized for $P>0.64$ GPa that also supports superconductivity below 1.1 K. These observations suggest that ${\mathrm{Au}}_2\text{Pb}$ is an ideal system in which to investigate the relationship between structural degrees of freedom, band topology, and resulting anomalous behaviors.

In order to remedy some of the shortcomings of the random phase approximation (RPA) within adiabatic connection fluctuation-dissipation (ACFD) density functional theory, we introduce a short-ranged, exchange-like kernel that is one-electron self-correlation free and exact for two-electron systems in the high-density limit. By tuning a free parameter in our model to recover an exact limit of the homogeneous electron gas correlation energy, we obtain a nonlocal, energy-optimized kernel that reduces the errors of RPA for both homogeneous and inhomogeneous solids. Using wave-vector symmetrization for the kernel, we also implement RPA renormalized perturbation theory for extended systems, and demonstrate its capability to describe the dominant correlation effects with a low-order expansion in both metallic and nonmetallic systems. The comparison of ACFD structural properties with experiment is also shown to be limited by the choice of norm-conserving pseudopotential.

Spin-flip scattering of charge carriers in metals with magnetic defects leads to the low-temperature saturation of the decoherence time ${\tau }_{\phi }$ of electrons at a value comparable to their spin relaxation time ${\tau }_s$. In two-dimensional (2D) conductors such a saturation can be lifted by an in-plane magnetic field $B_{\parallel }$, which polarizes spins of scatterers without affecting the orbital motion of free carriers. Here, we show that in 2D conductors with substantially different values of the $g$ factors of electrons ($g_e$) and magnetic defects ($g_i$), the decoherence time ${\tau }_{\phi }\left(B_{\parallel }\right)$ (reflected by the curvature of magnetoconductance) displays an anomaly: It first gets shorter, decaying on the scale $B_{\parallel }\sim \hslash /|g_i-g_e|{\mu }_B{\tau }_s$, before becoming longer at higher values of $B_{\parallel }$.

Repeated measurements that typically occur in two-time or multitime correlators rely on von Neumann's projection postulate, telling how to restart the system after an intermediate measurement. We invoke the principle of deferred measurement to describe an alternative procedure in which coevolving quantum memories extract system information through entanglement, combined with a final readout of the memories described by Born's rule. Our approach to repeated quantum measurements respects the unitary evolution of quantum mechanics during intermediate times, unifies the treatment of strong and weak measurements, and reproduces the projected and (anti)symmetrized correlators in the two limits. As an illustration, we apply our formalism to the calculation of the electron charge correlator in a mesoscopic physics setting, where single electron pulses assume the role of flying memory qubits. We propose an experimental setup that reduces the measurement of the time correlator to the measurement of currents and noise, exploiting the (pulsed) injection of electrons to cope with the challenge of performing short-time measurements.

We consider the braiding of Kramers pairs of Majorana bound states. We derive the most general transformation on the many-body ground state that is applied as the result of such a braiding process. The result is derived in the context of a toy model, but we will show that it has the most general form that is compatible with local and global conservation of electron parity. In accordance with earlier work, the resulting transformation turns out to be path dependent, which shows that Kramers pairs of Majorana bound states cannot be used for topological quantum computation. We also discuss under which conditions the result is path independent and corresponds to two independent exchanges of pairs of Majorana bound states.