The nature of equilibrium states in disordered materials is often studied using an overlap function $P\left(q\right)$, the probability of two configurations having similarity $q$. Exact sampling simulations of a two-dimensional proxy for three-dimensional spin glasses indicate that common measures of $P\left(q\right)$ in smaller samples do not decide between theoretical pictures. Strong corrections result from $P\left(q\right)$ being an average over many scales, as seen in a toy droplet model. However, the median $\tilde{I}\left(q\right)$ of the integrals of sample-dependent $P\left(q\right)$ curves shows promise for deciding the thermodynamic behavior.

It has recently been proposed that spin-transfer torques in magnetic systems with anisotropic exchange can be strongly enhanced, reducing the characteristic current density with up to four orders of magnitude compared to conventional setups. Motivated by this, we analytically solve the equations of motion in a collective-coordinate framework for this type of anisotropic exchange system, to investigate the domain wall dynamics in detail. In particular, we obtain analytical expressions for the maximum attainable domain wall velocity of such a setup and also for the occurrence of Walker breakdown. Surprisingly, we find that, in contrast to the standard case with domain wall motion driven by the nonadiabatic torque, the maximum velocity obtained via the anisotropic exchange torque is completely independent of the nonadiabaticity parameter $\beta$, in spite of the torque itself being very large for small $\beta$. Moreover, the Walker breakdown threshold has an opposite dependence on $\beta$ in these two cases; i.e., for the anisotropic exchange torque scenario, the threshold value decreases monotonically with $\beta$. These findings are of importance to any practical application of the proposed giant spin-transfer torque in anisotropic exchange systems.

Co${}_2$FeSi, a Heusler alloy with the highest magnetic moment per unit cell and the highest Curie temperature, has largely been described theoretically as a half-metal. This conclusion, however, disagrees with point contact Andreev reflection (PCAR) spectroscopy measurements, which give much lower values of spin polarization, $P$. Here, we present the spin polarization measurements of Co${}_2$FeSi by the PCAR technique, along with a thorough computational exploration, within the DFT and a GGA+$U$ approach, of the Coulomb exchange $U$ parameters for Co and Fe atoms, taking into account spin-orbit coupling. We find that the orbital contribution ($m_o$) to the total magnetic moment ($m_T$) is significant, since it is at least 3 times greater than the experimental uncertainty of $m_T$. The account of $m_o$ radically affects the acceptable values of $U$. Specifically, we find no values of $U$ that would simultaneously satisfy the experimental values of the magnetic moment and result in the half-metallicity of Co${}_2$FeSi. On the other hand, the ranges of $U$ that we report as acceptable are compatible with spin polarization measurements (ours and the ones found in the literature), which all are within approximately the 40–60 $\%$ range. Thus, based on reconciling experimental and computational results, we conclude that (a) spin-orbit coupling cannot be neglected in calculating Co${}_2$FeSi magnetic properties, and (b) Co${}_2$FeSi Heusler alloy is not half-metallic. We believe that our approach can be applied to other Heusler alloys such as Co${}_2$FeAl.

We demonstrate rapid, first-order sideband transitions between a superconducting resonator and a frequency-modulated transmon qubit. The qubit contains a substantial asymmetry between its Josephson junctions leading to a linear portion of the energy band near the resonator frequency. The sideband transitions are driven with a magnetic flux signal of a few hundred MHz coupled to the qubit. This modulates the qubit splitting at a frequency near the detuning between the dressed qubit and resonator frequencies, leading to rates up to 85 MHz for exchanging quanta between the qubit and resonator.

The entanglement spectrum describing quantum correlations in many-body systems has been recently recognized as a key tool to characterize different quantum phases, including topological ones. Here we derive its analytically scaling properties in the vicinity of some integrable quantum phase transitions and extend our studies also to nonintegrable quantum phase transitions in one-dimensional spin models numerically. Our analysis shows that, in all studied cases, the scaling of the difference between the two largest nondegenerate Schmidt eigenvalues yields with good accuracy critical points and mass scaling exponents.

In symmetric quantum dots (QDs), it is well known that the spin hot spot (i.e., the cusplike structure due to the presence of degeneracy near the level or anticrossing point) is present for the pure Rashba case but is absent for the pure Dresselhaus case [Bulaev and Loss, Phys. Rev. Lett. 95, 076805 (2005)]. Since the Dresselhaus spin-orbit coupling dominates over the Rashba spin-orbit coupling in GaAs and GaSb QDs, it is important to find the exact location of the spin hot spot or the cusplike structure even for the pure Dresselhaus case. In this paper, we present analytical and numerical results that show that the spin hot spot can also be seen for the pure Dresselhaus spin-orbit coupling case by inducing large anisotropy through external gates. At or nearby the spin hot spot, the spin transition rate increases and the decoherence time decreases by several orders of magnitude compared to the case with no spin hot spot. Thus one should avoid such locations when designing QD spin based transistors for possible implementation in quantum logic gates, solid-state quantum computing, and quantum information processing. It is also possible to extract the exact experimental data [Amasha, MacLean, Radu, Zumbühl, Kastner, Hanson, and Gossard, Phys. Rev. Lett. 100, 046803 (2008] for the phonon mediated spin-flip rates from our developed theoretical model.

We experimentally studied the excitonic Rabi oscillations in self-assembled semiconductor quantum dots using two-pulse photon echo spectroscopy. We observed the change in the shape of the Rabi oscillations by the second pulse intensity. Our experimental results were systematically reproduced by numerical calculations taking into account the local field effect arising from the Coulomb interaction between photoinduced charge densities in a quantum dot. Our experimental findings indicated that the local field effect contributes considerably to the coherent optical responses of the excitons in quantum dots.

We use bosonization and renormalization group methods to determine the ground-state phase diagram of a one-dimensional frustrated Kondo-Heisenberg system consisting of a one-dimensional spin-$\frac{1}{2}$ Luttinger liquid coupled by a Kondo exchange interaction $J_K$ to a frustrated quantum antiferromagnetic Heisenberg chain, with a nearest-neighbor exchange coupling $J_1$ and a next-nearest-neighbor (frustrating) exchange interaction $J_2$. We analyze the interplay of quantum frustration in the antiferromagnetic chain with the Kondo exchange coupling $J_K$ with the Luttinger liquid. We discuss the structure of the phase diagram of this system as a function of the ratios $J_K/J_1$, $J_2/J_1$, and of the parameters of the Luttinger liquid. In particular, we discuss in detail the regimes in which a pair-density-wave state may be realized and its relation with the spin correlations in the frustrated antiferromagnetic chain.

We report an anomalous scaling of the D’yakonov-Perel’ spin relaxation with the momentum relaxation in semiconductor quantum wells under a strong magnetic field in the Voigt configuration. We focus on the case in which the external magnetic field is perpendicular to the spin-orbit-coupling–induced effective magnetic field and its magnitude is much larger than the latter one. It is found that the longitudinal spin relaxation time is proportional to the momentum relaxation time even in the strong-scattering limit, indicating that the D’yakonov-Perel’ spin relaxation demonstrates Elliott-Yafet-like behavior. Moreover, the transverse spin relaxation time is proportional (inversely proportional) to the momentum relaxation time in the strong- (weak-) scattering limit, both in the opposite trends against the well-established conventional D’yakonov-Perel’ spin relaxation behaviors. We further demonstrate that all the above anomalous scaling relations come from the unique form of the effective inhomogeneous broadening.

We present comprehensive investigations on a pyrochlore antiferromagnet Na${}_3$Co(CO${}_3$)${}_2$Cl. Performed dc magnetization indicates a broad maximum at around 4 K. The field dependence of the peak temperature of this maximum follows the de Almeida–Thouless line, suggesting a spin-glass-like phase transition. The ac susceptibility measurements determine the glassy transition temperature to be 4.5 K and reveal a frequency-independent peak at 17 K. The temperature dependence of the specific heat shows a sharp peak at 1.5 K and a broad hump at around 5 K, which are attributed to a long-range magnetic phase transition and a spin-glass-like freezing process, respectively. The average crystallographic structure of Na${}_3$Co(CO${}_3$)${}_2$Cl has been determined using neutron-powder diffraction. No obvious site disorder has been detected within the experimental resolution. The diffuse neutron scattering with polarization analysis indicates short-range spin correlations characterized by dominating antiferromagnetic coupling between nearest neighbors and weak ferromagnetic coupling between next-nearest neighbors. The long-range magnetic order below 1.5 K is evidenced by the magnetic reflections observed at 50 mK and can be well explained with an all-in–all-out spin configuration. Inelastic neutron scattering of Na${}_3$Co(CO${}_3$)${}_2$Cl exhibits collective magnetic excitations at 3.5 K, suggesting that the spin-glass-like transition temperature $T_{\mathrm{g}}$ $=$ 4.5 K does not correspond to a complete spin-glass freezing as expected in canonical spin glasses. The peak observed in magnetic susceptibility at 17 K is attributed to the onset of an intermediate partially ordered phase transition, qualitatively consistent with the theoretical predictions for pyrochlore antiferromagnets with weak ferromagnetic next-nearest-neighbor interactions.

We explore the exciton-polariton condensation in two degenerate orbital states. In the honeycomb lattice potential, at the third band we have two degenerate vortex-antivortex lattice states at inequivalent $K$- and $K^{\prime }$-points. We have observed energetically degenerate condensates within the linewidth $\sim$0.3 meV, and directly measured the vortex-antivortex lattice phase order of the order parameter. We have also observed the intensity anticorrelation between polariton condensates at the $K$- and $K^{\prime }$-points. We relate this intensity anticorrelation to the dynamical feature of polariton condensates induced by the stochastic relaxation from the common particle reservoir.

We present very low temperature (0.15 K) scanning tunneling microscopy and spectroscopy experiments in the layered superconductor LaSb${}_2$. We obtain topographic microscopy images with surfaces showing hexagonal and square atomic size patterns, and observe in the tunneling conductance a superconducting gap. We find well defined quasiparticle peaks located at a bias voltage comparable to the weak coupling $s$-wave BCS expected gap value (0.17 meV). The amount of states at the Fermi level is however large and the curves are significantly broadened. We find $T_c$ of 1.2 K by following the tunneling conductance with temperature.

We calculate the magnetic field above which vortices necessarily appear in a two-dimensional superconducting film in a nonuniform magnetic field applied near its center, as in two-coil measurements of superfluid density. Experiments suggest that free vortices appear when the Meissner screening current density is large enough that the free-energy barrier for separating tightly-bound vortex-antivortex (V-aV) pairs into free vortices is comparable to $k_{B}T$, not at the much lower thermodynamic critical field. Specifically, we calculate the applied magnetic field above which there exists no metastable state without vortices. There is a simple analogy with the appearance of phase-slip centers in a one-dimensional superconducting wire.

First-principles calculations of electric-field-driven superconductivity at the hydrogenated diamond (110) surface are presented. While the hydrogens on the surface effectively maintain the intrinsic $s{p}^3$ covalent nature of diamond, the hole carriers induced by an external negative electric field $(E$-field) lead to a metallic surface region. Importantly, the concentration of hole carriers, confined within a few carbon layers of thickness $\sim$5–10 Å below the surface, exceeds 10${}^{21}$ cm${}^{-3}$, which is larger than the critical hole density responsible for superconductivity in the boron-doped diamond, while the calculated electron-phonon coupling constants are comparable in magnitude, suggesting the possibility of superconductivity with enhanced critical field.

Strain engineering, which employs biaxial misfit strain to deform the crystal structure, is a powerful tool to tune the physical behavior of epitaxial thin films. Here we show that a 10-nm-thick BiFeO${}_3$ film is uniaxially strained by (110)-oriented LaAlO${}_3$ substrate, which exhibits a monoclinic lattice with a giant $c$/$a$ ∼ 1.24 and a unique stripe ferroelectric domain configuration, as revealed by high resolution synchrotron x-ray diffraction and piezoelectric force microscopy. A strain-phase diagram for BiFeO${}_3$ under uniaxial strain condition is predicted by first-principles calculations, suggesting that monoclinic $Pm$ phase with a large polarization of ∼130 μC/cm${}^2$ is the lowest-in-energy phase when strained by (110)-oriented LaAlO${}_3$ substrate. Our results provide a potential route to tune physical behavior of epitaxial ferroelectric thin films by uniaxial strain in (110) orientation, instead of widely investigated biaxial strain in (001) orientation.

Nonlocal charge correlations induced in two normal metals contacted separately to a superconductor have been studied intensively in the past few years. Here we investigate nonlocal correlations induced by the transfer of pure spin currents through a superconductor on a scale comparable to the superconducting coherence length. As with charge currents, two processes contribute to the nonlocal spin signal: crossed Andreev reflection (CAR), where an electron with spin-up injected from one normal metal into the superconductor results in a hole with spin-down being injected into the second normal metal, and elastic cotunneling (EC), where the electron with spin-up injected from the first normal metal results in an electron with spin-up being injected into the second normal metal. Unlike charge currents, however, the spin currents associated with CAR and EC add due to the fact that the bulk superconductor cannot sustain a net spin current.

Using first-principles calculation, we propose an interface structure for single triple-layer FeSe on the SrTiO${}_3$(001) surface, a high-$T_{\mathrm{c}}$ superconductor found recently. The key component of this structure is the oxygen deficiency on the top layer of the SrTiO${}_3$ substrate, as a result of Se etching used in preparing the high-$T_{\mathrm{c}}$ samples. The O vacancies strongly bind the FeSe triple layer to the substrate giving rise to a (2×1) reconstruction, as observed by scanning tunneling microscopy. The enhanced binding correlates to the significant increase of $T_{\mathrm{c}}$ observed in experiment. The O vacancies also serve as the source of electron doping, which modifies the Fermi surface of the first FeSe layer by filling the hole pocket near the center of the surface Brillouin zone, as suggested from angle-resolved photoemission spectroscopy measurement.

Twin boundaries are generic crystalline defects in noncentrosymmetric crystal structures. We study theoretically twin boundaries in time-reversal-symmetric noncentrosymmetric superconductors that admit parity-mixed Cooper pairing. Twin boundaries support spin currents as a consequence of this parity mixing. If the singlet and triplet components of the superconducting order parameter are of comparable magnitude, the superconducting state breaks spontaneously the bulk time-reversal symmetry locally near the twin boundary. By self-consistently evaluating the Bogoliubov-de Gennes equations and the gap functions, we find two distinct phases: First, time-reversal-symmetry breaking enhances the spin currents but does not lead to charge current. A secondary phase transition then triggers a spin magnetization and a finite charge current near the twin boundary.

We investigate the Landau-Zener transition in two- and three-level systems subject to a classical Gaussian noise. Two complementary limits of the noise being fast and slow compared to characteristic Landau-Zener tunnel times are discussed. The analytical solution of a density matrix (Bloch) equation is given for a long-time asymptotic of transition probability. It is demonstrated that the transition probability induced or assisted by the fast noise can be obtained through a procedure of Bloch's equation averaging with further reducing it to a master equation. In contrast to the case of fast noise, the transition probability for LZ transition induced, or assisted, by the slow classical noise can be obtained by averaging the solution of Bloch's equation over the noise realization. As a result, the transition probability is described by the activation Arrhenius law. The approximate solution of the Bloch's equation at finite times is written in terms of Fresnel's integrals and interpreted in terms of interference pattern. We discuss consequences of a local isomorphism between SU(2) and SO(3) groups and connections between Schrödinger and Bloch descriptions of spin dynamics. Based on this isomorphism, we establish the relations between $S=1/2$ and 1 transition probabilities influenced by the noise. A possibility to use the slow noise as a probe for tunnel time is discussed.

We study the ground-state phase diagram of the quantum spin-2 XXZ chain in the presence of on-site anisotropy using a matrix-product state based infinite-system density matrix renormalization group (iDMRG) algorithm. One of the interests in this system is in connecting the highly quantum-mechanical spin-1 phase diagram with the classical $S=\infty$ phase diagram. Several of the recent advances within DMRG make it possible to perform a detailed analysis of the whole phase diagram. We consider different types of on-site anisotropies, which allows us to establish the validity of the following statements: (1) the spin-2 model can be tuned into a phase, which is equivalent to the “topologically nontrivial” spin-1 Haldane phase, and (2) the spin-2 Haldane phase at the isotropic Heisenberg point is adiabatically connected to the “trivial” large-$D$ phase, with a continuous change of the Hamiltonian parameters. Furthermore, we study the spin-3 XXZ chain to help explain the development of the classical phase diagram. We present details on how to use the iDMRG method to map out the phase diagram and include an extensive discussion of the numerical methods.