We investigate superconductivity in the candidate topological superconductor ${\mathrm{Nb}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ from $x=0$ to 1. We observe superconductivity for all $x>0$ with a superconducting transition temperature $T_C\sim$ 3 K similar to that reported in Cu- and Sr-intercalated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Structurally, for small $x$ we find introduction of Nb modifies the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ separation while for larger $x$ the system is driven to the BiSe structure. While all compounds are found to be superconducting, two distinct behaviors for the upper critical field $H_{c2}$ are observed. From photoemission spectroscopy we find that the enhanced $H_{c2}$ corresponds to compounds with a low surface Dirac point energy ($-0.67$ eV) relative to the Fermi level; in absolute scale this value is lower than previously reported superconductors in this materials family. We comment on the utility of this system for the study of topological superconductivity in a new structure type combining a doping (${\mathrm{Bi}}_2$ type) layer and layers with topologically protected gapless state (${\mathrm{Bi}}_2{\mathrm{Se}}_3$ type).

Lattice thermal conductivity ${\kappa }_{\mathrm{lat}}$ of MgO at high pressures $P$ and temperatures $T$ up to 150 GPa and 4000 K are determined using lattice dynamics calculations and the linearized phonon Boltzmann transport equation (BTE) beyond the relaxation time approximation (RTA) from first principles. It is found that the complete solution of the linearized BTE substantially corrects values of ${\kappa }_{\mathrm{lat}}$ calculated with the RTA by $\sim 30\%$, from $\sim 42$ to $\sim 54\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ under ambient conditions. The calculated values of ${\kappa }_{\mathrm{lat}}$ are in good agreement with those from the existing experiments. At conditions representative of the Earth's core-mantle boundary $(P=136\mathrm{GPa}$ and $T=3800\mathrm{K})$, ${\kappa }_{\mathrm{lat}}$ is predicted to be $\sim 32$ and $\sim 40\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ by RTA and the full solution of BTE, respectively. We report a detailed comparison of our study with earlier theoretical studies.

Below the phase transition temperature $T_{\text{c}}\simeq {10}^{-3}\mathrm{K}$ ${}^3\mathrm{He}$-B has a mixture of normal and superfluid components. Turbulence in this material is carried predominantly by the superfluid component. We explore the statistical properties of this quantum turbulence, stressing the differences from the better known classical counterpart. To this aim we study the time-honored Hall-Vinen-Bekarevich-Khalatnikov coarse-grained equations of superfluid turbulence. We combine pseudospectral direct numerical simulations with analytic considerations based on an integral closure for the energy flux. We avoid the assumption of locality of the energy transfer which was used previously in both analytic and numerical studies of the superfluid ${}^3\mathrm{He}$-B turbulence. For $T<0.37T_{\text{c}}$, with relatively weak mutual friction, we confirm the previously found “subcritical” energy spectrum $E\left(k\right)$, given by a superposition of two power laws that can be approximated as $E\left(k\right)\propto k^{-x}$ with an apparent scaling exponent $\frac{5}{3}<x\left(k\right)<3$. For $T>0.37T_{\text{c}}$ and with strong mutual friction, we observed numerically and confirmed analytically the scale-invariant spectrum $E\left(k\right)\propto k^{-x}$ with a $(k$-independent) exponent $x>3$ that gradually increases with the temperature and reaches a value $x\sim 9$ for $T\approx 0.72T_{\text{c}}$. In the near-critical regimes we discover a strong enhancement of intermittency which exceeds by an order of magnitude the corresponding level in classical hydrodynamic turbulence.

The fully self-consistent $\mathit{GW}$ approximation is an established method for electronic structure calculations. Its most serious deficiency is known to be an incorrect prediction of the dielectric response. In this work, we examine the $\mathit{GW}$ approximation for the homogeneous electron gas and find that problems with the dielectric response are drastically improved by enforcing the particle-number conservation law in the polarization function. We also find that previously reported data for the ground-state energy contradict each other well outside of reported error bounds. Some of these results created a false impression of how accurate the fully self-consistent $\mathit{GW}$ approximation is. Our two independent implementations of the $GW$ method agree with the data plotted in X.-Z. Yan [Phys. Rev. E 84, 016706 (2011)], thus confirming only that data set. We also present values for other key Fermi-liquid properties.

We have investigated the spin-valley degree of freedom characterized by the product of spin and valley indices in zigzag-edge graphene ribbons with a net magnetization by using a tight-binding model. We found that the $CT$ invariance leads to the spin-valley current which is neither spin polarized nor valley polarized but fully spin-valley polarized, where $C$ and $T$ are the charge conjugation and the time-reversal operation, respectively. In the quantum Hall regime, the gapless edge states protected by the $CPT$ invariance carry the spin-valley degree of freedom leading to a dissipationless spin-valley current, where $P$ is the inversion operation.

We present an absolute extraction method of optical constants of metal from the measured reflection electron energy loss spectroscopy spectra with the help of a recently developed reverse Monte Carlo technique. The method is based on a direct physical modeling of electron transportation with an optimization procedure of the energy loss function (ELF). The optical constants and the electron inelastic mean free path were obtained after verifying the accuracy of the derived ELF with the f- and $ps$-sum rules. This approach provides a valid and universal tool to investigate intrinsic properties of metals by using the electron energy loss spectroscopy technique.

We perform density-matrix renormalization group studies of a two-dimensional electron gas in a high perpendicular magnetic field and with an anisotropic band mass. At half filling in the lowest Landau level, such a system is a Fermi liquid of composite fermions. By measuring the Fermi surface of these composite fermions, we determine a relationship between the anisotropy of composite fermion dispersion ${\alpha }_{CF}$ and the original anisotropy ${\alpha }_F$ of the fermion dispersion at zero magnetic field. For systems where the electrons interact via a Coulomb interaction, we find ${\alpha }_{CF}=\sqrt{{\alpha }_F}$ within our numerical accuracy. The same result has been found concurrently in recent experiments. We also find that the relationship between the anisotropies is dependent on the form of the electron-electron interaction.

Synchrotron x-ray studies of single-layer $\mathrm{TiS}{\mathrm{e}}_2$ reveal displacements of the Ti and Se atoms as a function of temperature. The measurements, with a high sensitivity of 0.001 Å, show a (2 × 2) charge-density-wave (CDW) structure at temperatures below a critical temperature of $T_{\mathrm{C}1}=233\mathrm{K}$. The temperature dependence follows a BCS-like second-order mean-field behavior. A five-layer $\mathrm{TiS}{\mathrm{e}}_2$ film also exhibits a CDW transition of the same character but at a lower transition temperature of $T_{\mathrm{C}5}=204\mathrm{K}$, which is the same as that for bulk $\mathrm{TiS}{\mathrm{e}}_2$. The results demonstrate that lattice distortion is an integral part of the CDW transition that must also involve renormalization of the electronic structure.

The binary intermetallic compound TiAu is distinguished by an extremely sharp and narrow density of states peak at the Fermi level that has been proposed, via scattering between mirrored van Hove singularities, to be the mechanism for antiferromagnetic ordering versus the more fundamental ferromagnetic Stoner instability. Here we study, using density functional theory methods, magnetic tendencies and effects of doping, the latter within the virtual crystal approximation (VCA). Ferromagnetic tendencies are quantified using the fixed spin moment approach, illustrating the strong Stoner instability that does not, however, provide the ground state. Use of VCA results allows the identification of the value of the Stoner exchange constant $\mathcal{I}=0.74\mathrm{eV}$ for Ti. Magnetic fluctuations not included with semilocal density functionals are quantified with the procedure provided by Ortenzi and coauthors, with alloy concentrations corresponding to the quantum critical points reduced by a factor of three to five. Our results provide useful guidelines for experimental doping studies of TiAu.

In condensed matter physics, one of the major topics is to find out and classify interesting novel topological matters and phenomena. Topologically nontrivial systems can also be achieved by using periodical driving fields. On the other hand, ratchet effect can be used to collect useful work from fluctuations or realize directed transport in periodically driven systems. It is highly desirable to realize directed transport in the absence of magnetic field or magnetization for high-performance applications. Here, we promote a dimer chain by applying two mutually perpendicular ac electric fields, and obtain an effective two-dimensional Hamiltonian in the low-frequency regime. We thereby derive quantum Floquet anomalous Hall conductance and then find a quantized ratchet effect in the resulting current along the chain. This directed electric current, when averaged over one period, is quantized in units of the Hall conductance times the frequency of the main ac field. Our further analysis shows that these originate from the electric fields only. This lengthwise quantized ratchet effect without magnetic field or magnetization should be useful to designing novel robust low-energy-consumption electronic applications.

We show that correlated two-particle backscattering can induce fractional charge oscillations in a quantum dot built at the edge of a two-dimensional topological insulator by means of magnetic barriers. The result nicely complements recent works where those fractional oscillations were predicted in the strong-coupling regime. Moreover, since by rotating the magnetization of the barriers a fractional charge can be trapped in the dot via the Jackiw-Rebbi mechanism, the system we analyze offers the opportunity to study the interplay between this noninteracting charge fractionalization and the fractionalization due to two-particle backscattering. We demonstrate that the number of fractional oscillations of the charge density depends on the magnetization angle. In fact, a rotating magnetization can add or subtract fractional charges from the dot continuously. Finally, we address the renormalization induced by two-particle backscattering on the spin density, which is characterized by a dominant oscillation with a length twice as large as the charge-density oscillations.

Electrons in two-dimensional graphene sheets behave as interacting chiral Dirac fermions and have unique screening properties due to their symmetry and reduced dimensionality. By using a combination of scanning tunneling spectroscopy measurements and theoretical modeling we have characterized how graphene's massless charge carriers screen individual charged calcium atoms. A backgated graphene device configuration has allowed us to directly visualize how the screening length for this system can be tuned with carrier density. Our results provide insight into electron-impurity and electron-electron interactions in a relativistic setting with important consequences for other graphene-based electronic devices.

While long spin coherence times and efficient single-qubit quantum control have been implemented successfully in nitrogen-vacancy (NV) centers in diamond, the controlled coupling of remote NV spin qubits remains challenging. Here, we propose and analyze a controlled-phase (cphase) gate for the spins of two NV centers embedded in a common optical cavity and driven by two off-resonant lasers. In combination with previously demonstrated single-qubit gates, cphase allows for arbitrary quantum computations. The coupling of the NV spin to the cavity mode is based upon Raman transitions via the NV excited states and can be controlled with the laser intensities and relative phase. We find characteristic laser frequencies at which the scattering amplitude of a laser photon into the cavity mode is strongly dependent on the NV center spin. A scattered photon can be reabsorbed by another selectively driven NV center and can generate a conditional phase (cphase) gate. Gate times around 200 ns are within reach, nearly two orders of magnitude shorter than typical NV spin coherence times of around 10 $\mu \mathrm{s}$. The separation between the two interacting NV centers is limited only by the extension of the cavity.

Electric field manipulation of magnetic properties has attracted a lot of research interest recently in solid-state physics. However, ferroelectric strain modulation of antiferromagnetic (AFM) layer is rarely studied in ferromagnet/antiferromagnet/ferroelectric heterostructures. In this paper, we prepared a Ni/NiO(001) heterostructure on ferroelectric $\mathrm{Pb}{\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right)}_{0.7}\mathrm{T}{\mathrm{i}}_{0.3}{\mathrm{O}}_3\left(001\right)$ substrates and observed an out-of-plane electric field modulation of exchange bias and magnetic anisotropy in the Ni layer. The exchange bias was easily eliminated by an electric field cycle, which was due to the AFM domain switching induced by piezoelectric strain in the NiO layer. Synchrotron x ray linear dichroism results confirmed the AFM moment alignment induced by ferroelectric strain as well. Our work showed a promising strategy to manipulate AFM moments and domains, serving the blooming AFM spintronics.

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline ${\mathrm{NH}}_4\left[{\left({\mathrm{V}}_2{\mathrm{O}}_3\right)}_2{(4,4^{\prime }-bpy)}_2\left({\mathrm{H}}_2{\mathrm{PO}}_4\right){\left({\mathrm{PO}}_4\right)}_2\right]·0.5{\mathrm{H}}_2\mathrm{O}$ by means of x-ray diffraction, magnetic susceptibility, electron spin resonance, and ${}^{31}\mathrm{P}$ nuclear magnetic resonance measurements. Temperature-dependent magnetic susceptibility could be described well using a weakly coupled spin-$\frac{1}{2}$ dimer model with an excitation gap $\mathrm{\Delta }/k_{\mathrm{B}}\simeq 26.1$ K between the singlet ground state and triplet excited states and a weak interdimer exchange coupling $J^{\prime }/k_{\mathrm{B}}\simeq 4.6$ K. A gapped chain model also describes the data well with a gap of about 20 K. The electron spin resonance intensity as a function of temperature traces the bulk susceptibility nicely. The isotropic Landé $g$ factor is estimated to be about $g\simeq 1.97$, at room temperature. We are able to resolve the ${}^{31}\mathrm{P}$ NMR signal as coming from two inequivalent P sites in the crystal structure. The hyperfine coupling constant between ${}^{31}\mathrm{P}$ nucleus and ${\mathrm{V}}^{4+}$ spins is calculated to be $A_{\mathrm{hf}}\left(1\right)\simeq 2963$ Oe/${\mu }_{\mathrm{B}}$ and $A_{\mathrm{hf}}\left(2\right)\simeq 1466$ Oe/${\mu }_{\mathrm{B}}$ for the P(1) and P(2) sites, respectively. Our NMR shift and spin-lattice relaxation rate for both the ${}^{31}\mathrm{P}$ sites show an activated behavior at low temperatures, further confirming the singlet ground state. The estimated value of the spin gap from the NMR data measured in an applied field of $H=9.394$ T is consistent with the gap obtained from the magnetic susceptibility analysis using the dimer model. Because of a relatively small spin gap, ${\mathrm{NH}}_4\left[{\left({\mathrm{V}}_2{\mathrm{O}}_3\right)}_2{(4,4^{\prime }-bpy)}_2\left({\mathrm{H}}_2{\mathrm{PO}}_4\right){\left({\mathrm{PO}}_4\right)}_2\right]·0.5{\mathrm{H}}_2\mathrm{O}$ is a promising compound for further experimental studies under high magnetic fields.

In light of recent results obtained for the prototype helimagnet MnSi, we examine the possible magnetic structures of compounds of the same family, consistent with the crystal symmetries when the magnetic propagation vector is parallel to the [001] axis. The analysis of a published muon spin rotation spectrum recorded in MnGe [N. Martin et al., Phys. Rev. B 93, 174405 (2016)] shows no deviation from the canonical helimagnetic structure, unlike in MnSi. This qualitative difference calls for further theoretical works on chiral magnets.

We describe an approach, based on direct numerical solution of the Usadel equation, to finding stationary points of the free energy of superconducting nanorings. We consider both uniform (equilibrium) solutions and the critical droplets that mediate activated transitions between them. For the uniform solutions, we compute the critical current as a function of the temperature, thus obtaining a correction factor to Bardeen's 1962 interpolation formula. For the droplets, we present a metastability chart that shows the activation energy as a function of the temperature and current. A comparison of the activation energy for a ring to experimental results for a wire connected to superconducting leads reveals a discrepancy at large currents. We discuss possible reasons for it. We also discuss the nature of the bifurcation point at which the droplet merges with the uniform solution.

Motivated by the need to understand current-induced magnetization dynamics at the nanoscale, we have developed a formalism, within the framework of Keldysh Green function approach, to study the current-induced dynamics of a ferromagnetic (FM) nanoisland overlayer on a spin-orbit-coupling (SOC) Rashba plane. In contrast to the commonly employed classical micromagnetic LLG simulations the magnetic moments of the FM are treated quantum mechanically. We obtain the density matrix of the whole system consisting of conduction electrons entangled with the local magnetic moments and calculate the effective damping rate of the FM. We investigate two opposite limiting regimes of FM dynamics: (1) The precessional regime where the magnetic anisotropy energy (MAE) and precessional frequency are smaller than the exchange interactions and (2) the local spin-flip regime where the MAE and precessional frequency are comparable to the exchange interactions. In the former case, we show that due to the finite size of the FM domain, the “Gilbert damping” does not diverge in the ballistic electron transport regime, in sharp contrast to Kambersky's breathing Fermi surface theory for damping in metallic FMs. In the latter case, we show that above a critical bias the excited conduction electrons can switch the local spin moments resulting in demagnetization and reversal of the magnetization. Furthermore, our calculations show that the bias-induced antidamping efficiency in the local spin-flip regime is much higher than that in the rotational excitation regime.