Two electronic channels competing to screen a single impurity spin, as in the two-channel Kondo model, are expected to generate a ground state with a nontrivial entanglement structure. We exploit a spin-chain representation of the two-channel Kondo model to probe the ground-state block entropy, negativity, tangle, and Schmidt gap, using a density matrix renormalization group approach. In the presence of symmetric coupling to the two channels, we confirm field-theory predictions for the boundary entropy difference $ln(g_{\text{UV}}/g_{\text{IR}})=ln\left(2\right)/2$ between the ultraviolet and infrared limits and the leading $ln\left(x\right)/x$ impurity correction to the block entropy. The impurity entanglement $S_{\text{imp}}$ is shown to scale with the characteristic length ${\xi }_{2\text{CK}}$. We show that both the Schmidt gap and the entanglement of the impurity with one of the channels—as measured by the negativity—faithfully serve as order parameters for the impurity quantum phase transition appearing as a function of channel asymmetry, allowing for explicit determination of critical exponents, $\nu \approx 2$ and $\beta \approx 0.2$. Remarkably, we find the emergence of tripartite entanglement only in the vicinity of the critical channel-symmetric point.

We study the dynamics of optically induced nuclear spin polarization in a fluorine-doped ZnSe epilayer via time-resolved Kerr rotation. The nuclear polarization in the vicinity of a fluorine donor is induced by interaction with coherently precessing electron spins in a magnetic field applied in the Voigt geometry. It is detected by nuclei-induced changes in the electron spin coherence signal. This all-optical technique allows us to measure the longitudinal spin relaxation time $T_1$ of the ${}^{77}\mathrm{Se}$ isotope in a magnetic field range from 10 to 130 mT under illumination. We combine the optical technique with radio frequency methods to address the coherent spin dynamics of the nuclei and measure Rabi oscillations, Ramsey fringes, and the nuclear spin echo. The inhomogeneous spin dephasing time $T_2^{*}$ and the spin coherence time $T_2$ of the ${}^{77}\mathrm{Se}$ isotope are measured. While the $T_1$ time is on the order of several milliseconds, the $T_2$ time is several hundred microseconds. The experimentally determined condition $T_1\gg T_2$ verifies the validity of the classical model of nuclear spin cooling for describing the optically induced nuclear spin polarization.

A Markovian open quantum system which relaxes to a unique steady state ${\rho }_{\mathrm{ss}}$ of finite rank can be decomposed into a finite physically realizable ensemble (PRE) of pure states. That is, as shown by R. I. Karasik and H. M. Wiseman [Phys. Rev. Lett. 106, 020406 (2011)], in principle there is a way to monitor the environment so that in the long-time limit the conditional state jumps between a finite number of possible pure states. In this paper we show how to apply this idea to the dynamics of a double quantum dot arising from the feedback control of quantum transport, as previously considered by C. Pöltl, C. Emary, and T. Brandes [Phys. Rev. B 84, 085302 (2011)]. Specifically, we consider the limit where the system can be described as a qubit, and show that while the control scheme can always realize a two-state PRE, in the incoherent-tunneling regime there are infinitely many PREs compatible with the dynamics that cannot be so realized. For the two-state PREs that are realized, we calculate the counting statistics and see a clear distinction between the coherent and incoherent regimes.

The quantum spin Hall effect is endowed with topologically protected edge modes with a gapless Dirac spectrum. Applying a magnetic field locally along the edge leads to a gapped edge spectrum with the opposite parity for winding of spin texture for conduction and valence bands. Using Pancharatnam's prescription for the geometric phase it is shown that mismatch of this parity across a $p-n$ junction, which could be engineered into the edge by electrical gate induced doping, leads to a phase dependence in the two-terminal conductance which is quantized to either zero or $\pi$. It is further shown that application of a nonuniform magnetic field across the junction could lead to a nonquantized value of this geometric phase which is tunable between zero and $\pi$. A current asymmetry measurement which is shown to be robust against electron-electron interactions is proposed to infer the appearance of this Pancharatnam's geometric phase in transport across such junctions.

Polariton polarization can be described in terms of a pseudospin which can be oriented along the $x,y,$ or $z$ axis, similarly to electron and hole spin. Unlike electrons and holes where time-reversal symmetry requires that the spin-orbit interaction be odd in the momentum, the analog of the spin-orbit interaction for polaritons, the so-called TE-TM splitting, is even in the momentum. We calculate and compare spin transport of polariton, electron, and hole systems, in the diffusive regime of many scatterings. After dimensional rescaling diffusive systems with spatially uniform particle densities have identical dynamics, regardless of the particle type. Differences between the three particles appear in spatially nonuniform systems, with pumps at a specific localized point. We consider both oscillating pumps and transient (delta function) pumps. In such systems each particle type produces distinctive spin patterns. The particles can be distinguished by their differing spatial multipole character, their response and resonances in a perpendicular magnetic field, and their relative magnitude which is largest for electrons and weakest for holes. These patterns are manifested both in response to unpolarized pumps which produce in-plane and perpendicular spin signals, and to polarized pumps where the spin precesses from in-plane to out-of-plane and vice versa. These results will be useful for designing systems with large spin polarization signals, for identifying the dominant spin-orbit interaction and measuring subdominant terms in experimental devices, and for measuring the scattering time and the spin-orbit coupling's magnitude.

The understanding of orbital hybridization and spin polarization at the organic-ferromagnetic interface is essential in the search for efficient hybrid spintronic devices. Here, using first-principles calculations, we report a systematic study of spin-split hybrid states of ${\mathrm{C}}_{60}$ deposited on various ferromagnetic surfaces: bcc-Cr(001), bcc-Fe(001), bcc-Co(001), fcc-Co(001), and hcp-Co(0001). We show that the adsorption geometry of the molecule with respect to the surface crystallographic orientation of the magnetic substrate as well as the strength of the interaction play a crucial role in the spin polarization of the hybrid orbitals. We find that a large spin polarization in vacuum above the buckyball can only be achieved if the molecule is adsorbed upon a bcc-(001) surface by its pentagonal ring. Therefore, bcc-Cr(001), bcc-Fe(001), and bcc-Co(001) are the optimal candidates. Spin-polarized scanning tunneling spectroscopy measurements on single ${\mathrm{C}}_{60}$ adsorbed on Cr(001) and Co/Pt(111) also confirm that the symmetry both of the substrate and of the molecular conformation has a strong influence on the induced spin polarization. Our finding may give valuable insights for further engineering of spin filtering devices through single molecular orbitals.

Within a Kubo formalism, we study dc transport and ac optical properties of 3D Dirac and Weyl semimetals. Emphasis is placed on the approach to charge neutrality and on the differences between Dirac and Weyl materials. At charge neutrality, the zero-temperature limit of the dc conductivity is not universal and also depends on the residual scattering model employed. However, the Lorenz number $L$ retains its usual value $L_0$. With increasing temperature, the Wiedemann-Franz law is violated. At high temperatures, $L$ exhibits a new plateau at a value dependent on the details of the scattering rate. Such details can also appear in the optical conductivity, both in the Drude response and interband background. In the clean limit, the interband background is linear in photon energy and always extrapolates to the origin. This background can be shifted to the right through the introduction of a massless gap. In this case, the extrapolation can cut the axis at a finite photon energy as is observed in some experiments. It is also of interest to differentiate between the two types of Weyl semimetals: those with broken time-reversal symmetry and those with broken spatial-inversion symmetry. We show that, while the former will follow the same behavior as the 3D Dirac semimetals, for the zero magnetic field properties discussed here, the latter type will show a double step in the optical conductivity at finite doping and a single absorption edge at charge neutrality. The Drude conductivity is always finite in this case, even at charge neutrality.

We propose a kind of topological quantum state of semimetals in the quasi-one-dimensional (1D) crystal family ${\text{Ba}MX}_3$ ($M=\mathrm{V}$, Nb, or Ta; $X=\mathrm{S}$ or Se) by using symmetry analysis and first-principles calculation. We find that in ${\mathrm{BaVS}}_3$ the valence and conduction bands are degenerate in the $k_z=\pi /c$ plane ($c$ is the lattice constant along the $\widehat{z}$ axis) of the Brillouin zone (BZ). These nodal points form a node surface, and they are protected by a nonsymmorphic crystal symmetry consisting of a twofold rotation about the $\widehat{z}$ axis and a half-translation along the same $\widehat{z}$ axis. The band degeneracy in the node surface is lifted in ${\mathrm{BaTaS}}_3$ by including strong spin-orbit coupling (SOC) of Ta. The node surface is reduced into 1D node lines along the high-symmetry paths $k_x=0$ and $k_x=\pm \sqrt{3}{k}_y$ on the $k_z=\pi /c$ plane. These node lines are robust against SOC and guaranteed by the symmetries of the $P{6}_3/mmc$ space group. These node-line states are entirely different from previous proposals which are based on the accidental band touchings. We also propose a useful material design for realizing topological node-surface and node-line semimetals.

We theoretically study the thermoelectric transport properties of correlated T-shaped double quantum dots. The calculations are performed with the aid of the numerical renormalization group method. When each of the dots is occupied by a single electron, the system exhibits the two-stage Kondo effect. We identify the signatures of the two-stage screening in transport coefficients such as electrical and heat conductances, Seebeck coefficient, thermoelectric figure of merit, and the power factor. It is shown that the thermopower exhibits maxima for temperatures corresponding to the second stage of screening. Moreover, the normalized heat conductance and the electrical conductance are found to fulfill a modified Wiedemann-Franz law, which however becomes violated when the system is in the weak coupling regime. In addition, we also analyze the effects of external magnetic field, which gives rise to the occurrence of finite spin polarization of the conductance and a significant spin thermopower.

In this paper, we propose a selective sampling procedure to preferentially evaluate a potential energy surface (PES) in a part of the configuration space governing a physical property of interest. The proposed sampling procedure is based on a machine-learning method called the Gaussian process, which is used to construct a statistical model of the PES for identifying the region of interest in the configuration space. We demonstrate the efficacy of the proposed procedure for atomic diffusion and ionic conduction, specifically, the proton conduction in a well-studied proton-conducting oxide, barium zirconate $\left({\mathrm{BaZrO}}_3\right)$. The results of the demonstration study indicate that our procedure can efficiently identify the low-energy region characterizing the proton conduction in the host crystal lattice and that the descriptors used for the statistical PES model have a great influence on the performance.

We analyze the mathematical structure of equations for temperature $T_C$ of the superconductivity transition in a gas of interacting Fermi particles or at the phonon-mediated pairing in a metal in the case of nonadiabatic conditions ${\omega }_0\ge E_F$, i.e., when the characteristic phonon frequency ${\omega }_0$ is comparable or larger than the Fermi energy $E_F$. As the methods of calculating $T_C$ in common superconductors are not applicable in the nonadiabatic regime, the integral equations for $T_C$ are derived in the logarithmic approximation. The new equations contain no divergent terms in the antiadiabatic limit. The results can be immediately generalized to anisotropic band superconductors.

Many-body localization is characterized by a slow logarithmic growth of the entanglement entropy after a global quantum quench while the local memory of an initial density imbalance remains at infinite time. We investigate how much the proximity of a many-body localized phase can influence the dynamics in the delocalized ergodic regime where thermalization is expected. Using an exact Krylov space technique, the out-of-equilibrium dynamics of the random-field Heisenberg chain is studied up to $L=28$ sites, starting from an initially unentangled high-energy product state. Within most of the delocalized phase, we find a sub-ballistic entanglement growth $S\left(t\right)\propto t^{1/z}$ with a disorder-dependent exponent $z\ge 1$, in contrast with the pure ballistic growth $z=1$ of clean systems. At the same time, anomalous relaxation is also observed for the spin imbalance $\mathcal{I}\left(t\right)\propto t^{-\zeta }$ with a continuously varying disorder-dependent exponent $\zeta$, vanishing at the transition. This provides a clear experimental signature for detecting this nonconventional regime.

We investigate the strain dependence of the ferroelectric polarization and the structure of the ferroelectric domain walls in the layered perovskite-related barium fluorides, $\mathrm{Ba}M{\mathrm{F}}_4$ ($M=\mathrm{Mg}$, Zn). The unusual “geometric ferroelectricity” in these materials is driven by the softening of a single polar phonon mode consisting of rotations of the $M{\mathrm{F}}_6$ octahedra accompanied by polar displacements of the Ba cations, and in contrast to conventional ferroelectrics involves minimal electronic rehybridization. We therefore anticipate a different strain dependence of the polarization, and alternative domain wall structures compared with those found in conventional ferroelectric materials. Using first-principles calculations based on density functional theory (DFT) within the general gradient approximation (GGA), we calculate the variation of the crystal structure and the ferroelectric polarization under both compressive and tensile strain. We perform structural relaxations of neutral domain walls between oppositely oriented directions of the ferroelectric polarization and calculate their corresponding energies to determine which are most likely to form. We compare our results to literature values for conventional perovskite oxides to provide a source of comparison for understanding the ferroelectric properties of alternative nonoxide materials such as the barium fluorides.

Thermally driven spin-transfer torques have recently been reported in electrically insulating $\mathrm{ferromagnet}|\mathrm{normal}$-metal heterostructures. In this paper, we propose two physically distinct mechanisms for such torques. The first is a local effect: out-of-equilibrium, thermally activated magnons in the ferromagnet, driven by a spin Seebeck effect, exert a torque on the magnetization via magnon-magnon scattering with coherent dynamics. The second is a nonlocal effect which requires an additional magnetic layer to provide the symmetry breaking necessary to realize a thermal torque. The simplest structure in which to induce a nonlocal thermal torque is a spin valve composed of two insulating magnets separated by a normal metal spacer; there, a thermal flux generates a pure spin current through the spin valve, which results in a torque when the magnetizations of the layers are misaligned.

Using a substrate potential described by a pairwise sum of empirical ${}^4\mathrm{He}$-C interatomic potentials, we have performed path-integral Monte Carlo calculations for ${}^4\mathrm{He}$ adatoms on graphite. It is found that a second-layer commensurate structure is not stable above an incommensurate first-layer triangular solid. This is consistent with the conclusion of the previous theoretical study of Corboz et al. [Phys. Rev. B 78, 245414 (2008)] that was based on a laterally averaged one-dimensional potential. On the other hand, we observe a new stable ${\mathrm{C}}_{7/12}$ commensurate solid in the first ${}^4\mathrm{He}$ layer at the areal density of 0.111 ${\AA }^{-2}$, which is close to the second-layer promotion density. This high-density commensurate solid exhibits a $\sqrt{12}\times \sqrt{12}$ structure registered to the graphite surface that is not disrupted by the development of the second ${}^4\mathrm{He}$ layer. Furthermore, a second-layer 4/7 commensurate structure relative to the first-layer ${\mathrm{C}}_{7/12}$ solid is found to be at least metastable, opening the possibility of two-dimensional supersolidity.

While there have been recent reports of zero-energy modes in single-particle tunneling density of states, their identity as Majorana modes has not been unequivocally established thus far. We make predictions for the local compressibility ${\kappa }_{\mathrm{loc}}$, tuned by changing the chemical potential $\mu$ in a semiconducting nanowire with strong spin-orbit coupling and in a Zeeman field in proximity to a superconductor, which has been proposed as a candidate system for observing Majorana modes. We show that in the center of the wire, the topological phase transition is signaled by a divergence of ${\kappa }_{\mathrm{loc}}$ as a function of $\mu$, an important diagnostic of the phase transition. We also find that a single strong impurity potential can lead to a local negative compressibility at the topological phase transition. The origin of such anomalous behavior can be traced to the formation of Andreev bound states close to topological phase transitions. Measurable by a gate-tunable scanning electron transistor, the compressibility includes contributions from both single-particle states and collective modes and is therefore a complementary probe from scanning tunneling spectroscopy, which is sensitive to only the single-particle density of states.

We study the spinful fermionic Haldane-Hubbard model at half-filling using a combination of quantum cluster methods: cluster perturbation theory, the variational cluster approximation, and cluster dynamical mean-field theory. We explore possible zero-temperature phases of the model as a function of onsite repulsive interaction strength and next-nearest-neighbor hopping amplitude and phase. Our approach allows us to access the regime of intermediate interaction strength, where charge fluctuations are significant and effective spin model descriptions may not be justified. Our approach also improves upon mean-field solutions of the Haldane-Hubbard model by retaining local quantum fluctuations and treating them nonperturbatively. We find a correlated topological Chern insulator for weak interactions and a topologically trivial Néel antiferromagnetic insulator for strong interactions. For intermediate interactions, we find that topologically nontrivial Néel antiferromagnetic insulating phases and/or a topologically nontrivial nonmagnetic insulating phase may be stabilized.

Femtosecond laser-induced volume nanograting formation is numerically investigated. The developed model solves nonlinear Maxwell's equations coupled with multiple rate free carrier density equations in the presence of randomly distributed inhomogeneities in fused silica. As a result of the performed calculations, conduction band electron density is shown to form nanoplanes elongated perpendicular to the laser polarization. Two types of nanoplanes are identified. The structures of the first type have a characteristic period of the laser wavelength in glass and are attributed to the interference of the incident and the inhomogeneity-scattered light waves. Field components induced by coherent multiple scattering in directions perpendicular to the laser polarization are shown to be responsible for the formation of the second type of structures with a subwavelength periodicity. In this case, the influence of the inhomogeneity concentration on the period of nanoplanes is shown. The calculation results not only help to identify the physical origin of the self-organized nanogratings, but also explain their period and orientation.

We theoretically study, through combining the density functional theory and an unfolding technique, the electronic band structure and the charge doping effects for the deposition of potassium on multilayer FeSe films grown on ${\mathrm{SrTiO}}_3$ (001) surface. These results form a theoretical baseline for further detailed studies of low-temperature electronic properties and their multiway quantum engineering of FeSe thin films. We explain the Fermi-surface topology observed in experiment and formulate the amount of doped electrons as a function of atomic K coverage. We show that the atomic K deposition efficiently dopes electrons to the top layer of FeSe. Both checkerboard- and pair-checkerboard-antiferromagnetic (AFM) FeSe layers show electron pockets at the $M$ point and no Fermi pocket at the $\mathrm{\Gamma }$ point with moderate atomic K coverage. The electron transfer from the K adsorbate to the FeSe film introduces a strong electric field, which leads to a double-Weyl-cone structure at the $M$ point in the Brillouin zone of checkerboard-AFM FeSe. We demonstrate that with experimentally accessible heavy-electron doping, an electronlike Fermi pocket will emerge at the $\mathrm{\Gamma }$ point, which should manifest itself in modulating the high-temperature superconductivity of FeSe thin films.