We propose a fast, scalable all-optical design for arbitrary two-qubit operations for defect qubits in diamond (nitrogen-vacancy centers) and in silicon carbide, which are promising candidates for room temperature quantum computing. The interaction between qubits is carried out by microcavity photons. The approach uses constructive interference from higher energy excited states activated by optical control. In this approach the cavity mode remains off-resonance with the directly accessible optical transitions used for initialization and readout. All quantum operations are controlled by near-resonant narrow-bandwidth optical pulses. We perform full quantum numerical modeling of the proposed gates and show that high-fidelity operations can be obtained with realistic parameters.

We simulate scanning probe imaging of the local density of states related to scattering Fermi level wave functions inside a resonant cavity. We calculate the potential landscape within the cavity, taking into account the Coulomb charge of the probe and its screening by deformation of the two-dimensional electron gas using the local density approximation. Approximation of the tip potential by a Lorentz function is discussed. The electron transfer problem is solved with a finite difference approach. We look for stable work points for the extraction of the local density of states from conductance maps. We find that conductance maps are highly correlated with the local density of states when the Fermi energy level enters into Fano resonance with states localized within the cavity. Generally outside resonances the correlation between the local density of states and conductance maps is low.

We have performed diffusion quantum Monte Carlo calculations for nine isomers of 13-atom clusters of the $sp$-bonded elements Li, Na, Be, Mg, and Al and compared the energies with density functional theory (DFT) results. Taking the diffusion Monte Carlo (DMC) data as the benchmark, we find that the local spin-density approximation (LSDA) and Perdew-Burke-Ernzerhof (PBE) semilocal generalized gradient approximation functionals give good descriptions of the energy differences between the clusters for each of the elements and their cations. The errors in the energy differences between clusters are a little larger for the PBE functional, but on average the errors for both functionals are smaller than 10 meV per atom, except for Be${}_{13}$ clusters, which yield errors of 17 and 22 meV per atom from LSDA and PBE, respectively. We have, in addition, investigated using an inhomogeneous backflow transformation to improve the nodal surface of the trial wave functions used in the DMC calculations. We find that backflow lowers the DMC energies but does not affect the relative stabilities of the clusters.

In this paper, contactless microwave spectroscopy measurements of weak localization in as-grown and hydrogen intercalated quasi-free-standing graphene (QFSG) grown on SiC are presented. Delamination from the substrate is observed by the change from substrate dominated to grain boundaries dominated intervalley elastic scattering in QFSG comparing to epitaxial graphene. In the case of as-grown graphene, the finite coherence length at 0 K caused by an additional inelastic scattering is observed. This additional scattering mechanism vanishes for hydrogen intercalated QFSG, and the significant enhancement of coherence length comparing to as-grown QFSG and epitaxial graphene is observed. The coherence length is comparable to that observed in free-standing graphene. The conditions under which the quantum corrections produce weak localization or weak antilocalization behavior in conductivity are also discussed.

In this work, we extend recent inverse statistical-mechanical methods developed for many-particle systems to the case of spin systems. For simplicity, we focus in this initial study on the two-state Ising model with radial spin-spin interactions of finite range (i.e., extending beyond nearest-neighbor sites) on the square lattice under periodic boundary conditions. Our interest herein is to find the optimal set of shortest-range pair interactions within this family of Hamiltonians, whose corresponding ground state is a targeted spin configuration such that the difference in energies between the energetically closest competitor and the target is maximized. For an exhaustive list of competitors, this optimization problem is solved exactly using linear programming. The possible outcomes for a given target configuration can be organized into the following three solution classes: unique (nondegenerate) ground state (class I), degenerate ground states (class II), and solutions not contained in the previous two classes (class III). We have chosen to study a general family of striped-phase spin configurations comprised of alternating parallel bands of up and down spins of varying thicknesses and a general family of rectangular block checkerboard spin configurations with variable block size, which is a generalization of the classic antiferromagnetic Ising model. Our findings demonstrate that the structurally anisotropic striped phases, in which the thicknesses of up- and down-spin bands are equal, are unique ground states for isotropic short-ranged interactions. By contrast, virtually all of the block checkerboard targets are either degenerate or fall within class III solutions. The degenerate class II spin configurations are identified up to a certain block size. We also consider other target spin configurations with different degrees of global symmetries and order. Our investigation reveals that the solution class to which a target belongs depends sensitively on the nature of the target radial spin-spin correlation function. In the future, it will be interesting to explore whether such inverse statistical-mechanical techniques could be employed to design materials with desired spin properties.

The equation of state of $\alpha$-U has been measured at 300 K and 14 K up to 60 GPa, and calculated in similar conditions using density functional theory. We observe an unusual behavior of the experimental bulk modulus with temperature, which can be ascribed to the charge-density wave transition undergone in uranium at low temperature and low pressure. This results in a failure of the density functional theory with quasiharmonic thermodynamics to accurately predict the equation of state of $\alpha$-U in a moderate $P$-$T$ range (below 20 GPa and 300 K). However, these approximations should yield quantitative results in more extreme conditions.

We consider two quantum Ising chains initially prepared at thermal equilibrium but with different temperatures and coupled at a given time through one of their end points. In the long-time limit the system reaches a nonequilibrium steady state. We discuss properties of this nonequilibrium steady state and characterize the convergence to the steady regime. We compute the mean energy flux through the chain and show that the heat transport is ballistic. We derive also the large-deviation function for the quantum and thermal fluctuations of this energy transfer.

The quantum phases of one-dimensional spin $s=1/2$ chains are discussed for models with two parameters, frustrating exchange $g=J_2>0$ between second neighbors and normalized nonfrustrating power-law exchange with exponent $\alpha$ and distance dependence $r^{-\alpha }$. The ground state (GS) at $g=0$ has a long-range order (LRO) for $\alpha <2$ and long-range spin fluctuations for $\alpha >2$. The models conserve total spin $S=S_A+S_B$, have singlet GS for any $g$, $\alpha \ge 0$ and decouple at $1/g=0$ to linear Heisenberg antiferromagnets on sublattices $A$ and $B$ of odd- and even-numbered sites. Exact diagonalization of finite chains gives the sublattice spin $\langle S_A^2\rangle$, the magnetic gap $E_m$ to the lowest triplet state, and the excitation $E_{\sigma }$ to the lowest singlet with opposite inversion symmetry to the GS. An analytical model that conserves sublattice spin has a first-order quantum transition at $g_c=1/4\ln 2$ from a GS with perfect LRO to a decoupled phase with $S_A=S_B=0$ for $g\ge 4/{\pi }^2$ and no correlation between spins in different sublattices. The model with $\alpha =1$ has a first-order transition to a decoupled phase that closely resembles the analytical model. The bond order wave (BOW) phase and continuous quantum phase transitions of finite models with $\alpha \ge 2$ are discussed in terms of GS degeneracy where $E_{\sigma }\left(g\right)=0$, excited state degeneracy where $E_{\sigma }\left(g\right)=E_m\left(g\right)$, and $\langle S_A^2\rangle$. The decoupled phase at large frustration has nondegenerate GS for any exponent $\alpha$ and excited states related to sublattice excitations.

Angle resolved photoemission spectroscopy studies of the overdoped cuprate superconductor La${}_{2-x}$Sr${}_x$CuO${}_4$ find only small changes in the near nodal electron self-energy over a spectral range of several hundred meV as the doping increases from $x=0.2$ to $x=0.3$ and the superconducting transition temperature $T_c$ decreases from 32 to 0 K. These measurements put constraints on the structure of the electron-electron interaction. Here we show that a spin-fluctuation interaction leads to behavior which is consistent with these experimental results.

We study the gapped phase of Kitaev's honeycomb model (a $Z_2$ spin liquid) in the presence of lattice defects. We find that some dislocations and bond defects carry unpaired Majorana fermions. Physical excitations associated with these defects are (complex) fermion modes made out of two (real) Majorana fermions connected by a $Z_2$ gauge string. The quantum state of these modes is robust against local noise and can be changed by winding a $Z_2$ vortex around a dislocation. The exact solution respects gauge invariance and reveals a crucial role of the gauge field in the physics of Majorana modes.

Applying an external pressure to indirectly electron-doped 122-type (Ba${}_{1-x}$La${}_x$)Fe${}_2$As${}_2$ epitaxial films enhances the superconducting critical temperature ($T_{\mathrm{c}}$) up to 30.3 K. Different from the other family compounds, the $T_{\mathrm{c}}$ is enhanced not only in the underdoped region but also in the optimally doped and overdoped regions. Narrowing of the superconducting transition width and an increase in the carrier density take place simultaneously in the whole doping region, except at the heavily overdoped limit. This characteristic is unique to and is observed only in (Ba${}_{1-x}$La${}_x$)Fe${}_2$As${}_2$, in which the La doping is stabilized via nonequilibrium growth of the vapor phase epitaxy, among the 122-type iron-based superconductors, $A$Fe${}_2$As${}_2$ ($A$ $=$ Ba, Sr, and Ca).

Proton NMR measurements on the organic superconductor κ-(ET)${}_2$Cu[N(CN)${}_2$]Br [where ET represents bis(ethylenedithio)tetrathiafulvalene] $\left(T_C=11.6\mathrm{K}\right)$ exhibit stretched exponential spin-lattice relaxation below $T\approx 25\mathrm{K}$, suggestive of an inhomogeneous magnetic phase that develops in the normal state and coexists with superconductivity. The onset of this phase coincides approximately with a large normal-state Nernst signal reported previously. By contrast, the closely related superconductor κ-(ET)${}_2$Cu(NCS)${}_2$ $\left(T_C=10.5\mathrm{K}\right)$ shows single exponential spin-lattice relaxation and a conventional Nernst effect. The temperature range of $T_C$ < $T$ < 30 K encompasses several phenomena in the κ-(ET)${}_{2}X$ conductors, including changes in conduction-electron-spin resonance, electronic phase separation, and the onset of antiferromagnetic order. Analogous behavior in La${}_{2-x}$Sr${}_x$CuO${}_4$ suggests that a spin glass or density wave may develop in κ-(ET)${}_2$Cu[N(CN)${}_2$]Br.

SrRuO${}_3$ films have been grown in the tetragonal, structurally single-domain state under 1% of biaxial tensile strain. The angular dependencies of the magnetization and the magnetoresistance reveal an upright orientation of the tetragonal unit cell and biaxial magnetic in-plane anisotropy with 110${}_t$ easy axes. Reversible biaxial strain from piezoelectric Pb(Mg${}_{1/3}$Nb${}_{2/3}$)${}_{0.72}$Ti${}_{0.28}$O${}_3$ (PMN-PT) substrates has been applied to probe the direct strain response of the magnetization and the electrical resistance. At 1% tensile strain, the Curie temperature ($T_C$) and the ordered magnetic moment ($m_S$) at low temperatures are found to substantially decrease with further growing tensile strain. This suggests a suppression of $m_S$ resulting from distortions of the RuO${}_6$ octahedra, in line with reported density-functional calculations. Reversible strain has also been applied to a film under weak tensile strain revealing the opposite response, i.e., an enhancement of $T_C$ and $m_S$ with tensile strain. Structural and magnetic properties of SrRuO${}_3$ films in several static strain states (compressive, weak and strong tensile strain) are compared.

The angular momentum vector of a Heisenberg ferromagnet with isotropic exchange interaction is conserved, while under uniaxial crystalline anisotropy the projection of the total spin along the easy axis is a constant of motion. Using Noether's theorem, we prove that these conservation laws persist in the presence of dipole-dipole interactions. However, spin and orbital angular momentum are no longer conserved separately. We also define the linear momentum of ferromagnetic textures. We illustrate the general principles with special reference to spin transfer torques and identify the emergence of a nonadiabatic effective field acting on domain walls in ferromagnetic insulators.

We report the results of a muon spin rotation ($\mu$SR) study of the bulk of Bi${}_{2+x}$Sr${}_{2-x}$CaCu${}_2$O${}_{8+\delta }$, as well as pure and Ca-doped YBa${}_2$Cu${}_3$O${}_y$, which together with prior measurements reveal a universal inhomogeneous magnetic-field response of hole-doped cuprates extending to temperatures far above the critical temperature $T_c$. The primary features of our data are incompatible with the spatially inhomogeneous response being dominated by known charge-density-wave and spin-density-wave orders. Instead, the normal-state inhomogeneous line broadening is found to scale with the maximum value $T_c^{\max }$ for each cuprate family, indicating it is controlled by the same energy scale as $T_c$. Since the degree of chemical disorder varies widely among the cuprates we have measured, the observed scaling constitutes evidence for an intrinsic electronic tendency toward inhomogeneity above $T_c$.

We have used the ac magnetic susceptibility to investigate the vortex state in an optimally doped $p$-type Ba${}_{0.5}$K${}_{0.5}$Fe${}_2$As${}_2$ single crystal under various ac and dc fields. A peak effect is observed in the temperature dependence of the in-phase ac susceptibility, indicating an order-disorder transition on the vortex phase diagram. The peak effect displays an anomalous history effect compared with other type-II superconductors, which we ascribe to the strong pinning existing in the material. We observe the development of a small dissipation peak at the temperature $T_{p2}$ slightly below the peak effect region. Similar to the peak effect boundary, $T_{p2}$ delimits a region in the $H-T$ phase diagram which is independent on the ac field amplitude. We argue that this small peak may arise from the softening of the vortex lattice, leading to a collective pinning of the whole vortex lattice. This effect assists and further enhances the peak effect occurring in the Ba${}_{0.5}$K${}_{0.5}$Fe${}_2$As${}_2$ superconductor.

We here explore Majorana fermion states in an $s$-wave superfluid of cold atoms in the presence of spin-orbital coupling and an additional harmonic potential. The superfluid boundary is induced by a harmonic trap. Two locally separated Majorana fermion states are revealed numerically based on the self-consistent Bogoliubov-de Gennes equations. The local density of states is calculated, through which the signatures of Majorana excitations may be indicated experimentally.

We study the nonequilibrium dynamics of the Luttinger model after a quantum quench, when the initial state is a finite temperature thermal equilibrium state. The diagonal elements of the density matrix in the steady state show thermal features for high temperature initial states only, otherwise they retain highly nonthermal character. The time evolution of Uhlmann fidelity, which measures the distance between the time evolved and initial states, is evaluated for arbitrary initial temperatures and quench protocols. In the long time limit, the overlap between the time evolved and initial system decreases exponentially with the temperature with a universal prefactor. Within perturbation theory, the statistics of final total energy and work are numerically evaluated in the case of a sudden quench, which yield identical distributions at zero temperature. In both statistics, temperature effects are more significant in small systems. The Dirac-$\delta$ peak at the adiabatic ground state energy remains present in the probability distribution of the total energy, but disappears from the work distribution at nonzero initial temperatures.

We describe how to implement the time-dependent variational principle for matrix product states in the thermodynamic limit for nonuniform lattice systems. This is achieved by confining the nonuniformity to a (dynamically expandable) finite region with fixed boundary conditions. The suppression of nonphysical quasiparticle reflections from the boundary of the nonuniform region is also discussed. Using this algorithm we study the dynamics of localized excitations in infinite systems, which we illustrate in the case of the spin-1 antiferromagnetic Heisenberg model and the ${\varphi }^4$ model.

We study the polarization dynamics of exciton-polaritons propagating in sub-mm-thick (Cd,Zn)Te bulk crystals using polarimetric time-of-flight techniques. The application of a magnetic field in Faraday geometry leads to synchronous temporal oscillations of all Stokes parameters of an initially linearly or circularly polarized, spectrally broad optical pulse of 150-fs duration propagating through the crystal. Strong dispersion for photon energies close to the exciton resonance leads to stretching of the optical pulse to a duration of 200–300 ps and enhancement of magneto-optical effects such as the Faraday rotation and the nonreciprocal birefringence. The oscillation frequency of the exciton-polariton polarization increases with magnetic field $B$, reaching 10 GHz at $B\sim 5$ T. Surprisingly, the relative contributions of Faraday rotation and nonreciprocal birefringence undergo strong changes with photon energy, which is attributed to a nontrivial spectral dependence of Faraday rotation in the vicinity of the exciton resonance. This leads to polarization nutation of the transmitted optical pulse in the time domain. The results are well explained by a model that accounts for Faraday rotation and magnetospatial dispersion in zinc-blende crystals. We evaluate the exciton $g$ factor $|g_{\mathrm{exc}}|=0.2$ and the magnetospatial constant $\left|V\right|=5\times {10}^{-12}$ eV cm ${\text{T}}^{-1}$.