Compounds of Fe, Ti, and Sb were prepared using arc melting and vacuum annealing. ${\mathrm{Fe}}_2\mathrm{TiSb}$, expected to be a full Heusler compound crystallizing in the $\mathrm{L}{2}_1$ structure, was shown by XRD and SEM analyses to be composed of weakly magnetic grains of nominal composition ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$ with iron-rich precipitates in the grain boundaries. FeTiSb, a composition consistent with the formation of a half-Heusler compound, also decomposed into ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$ grains with Ti-Sb rich precipitates and was weakly magnetic. The dominant ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$ phase appears to crystallize in a defective $\mathrm{L}{2}_1$-like structure with iron vacancies. Based on this finding, a first-principles DFT-based binary cluster expansion of Fe and vacancies on the Fe sublattice of the $\mathrm{L}{2}_1$ structure was performed. Using the cluster expansion, we computationally scanned $>{10}^3$ configurations and predict a novel, stable, nonmagnetic semiconductor phase to be the zero-temperature ground state. This new structure is an ordered arrangement of Fe and vacancies, belonging to the space group $R3m$, with composition ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$, i.e., between the full- and half-Heusler compositions. This phase can be visualized as alternate layers of $\mathrm{L}{2}_1$ phase ${\mathrm{Fe}}_2\mathrm{TiSb}$ and $\mathrm{C}{1}_b$ phase FeTiSb, with layering along the [111] direction of the original cubic phases. Our experimental results on annealed samples support this predicted ground-state composition, but further work is required to confirm that the $R3m$ structure is the ground state.

We numerically investigate the electronic structures around a vortex core in a bilayer superconducting system, with $s$-wave pairing, Rashba spin-orbit coupling, and Zeeman magnetic field, with the use of the quasiclassical Green's function method. The BCS phase and the so-called pair-density-wave (PDW) phase appear in the temperature-magnetic-field phase diagram in a bulk uniform system [T. Yoshida et al., Phys. Rev. B 86, 134514 (2012)]. In the low magnetic field perpendicular to the layers, the zero-energy vortex bound states in the BCS phase are split by the Zeeman magnetic field. On the other hand, the PDW state appears in the high magnetic field, and the sign of the order parameter is opposite between the layers. We find that the vortex core suddenly shrinks and the zero-energy bound states appear by increasing the magnetic field through the BCS-PDW transition. We discuss the origin of the change in the vortex-core structure between the BCS and PDW states by clarifying the relation between the vortex bound states and the bulk energy spectra. In the high-magnetic-field region, the PDW state and vortex bound states are protected by the spin-orbit coupling. These characteristic behaviors in the PDW state can be observed by scanning tunneling microscopy/spectroscopy.

Motivated by the impressive recent advance in manipulating cold ytterbium atoms, we explore and substantiate the feasibility of realizing the Coqblin-Schrieffer model in a gas of cold fermionic ${}^{173}\mathrm{Yb}$ atoms. Making use of different AC polarizabillity of the electronic ground state (electronic configuration ${}^1\mathrm{S}_0$) and the long lived metastable state (electronic configuration ${}^3\mathrm{P}_0$), it is substantiated that the latter can be localized and serve as a magnetic impurity while the former remains itinerant. The exchange mechanism between the itinerant ${}^1\mathrm{S}_0$ and the localized ${}^3\mathrm{P}_0$ atoms is analyzed and shown to be antiferromagnetic. The ensuing SU(6) symmetric Coqblin-Schrieffer Hamiltonian is constructed, and, using the calculated exchange constant $J$, perturbative renormalization group (RG) analysis yields the Kondo temperature $T_K$ that is experimentally accessible. A number of thermodynamic measurable observables are calculated in the weak-coupling regime $T>T_K$ (using perturbative RG analysis) and in the strong-coupling regime $T<T_K$ (employing known Bethe ansatz techniques).

We investigate the strong influence of the ${\mathrm{\Theta }}_{II}$-loop-current order on both unidirectional and bidirectional $d$-wave charge-density-wave/pair-density-wave (CDW/PDW) composite orders along axial momenta $(\pm Q_0,0)$ and $(0,\pm Q_0)$ that emerge in an effective hot-spot model departing from the three-band Emery model relevant to the phenomenology of the cuprate superconductors. This study is motivated by the compelling evidence that the ${\mathrm{\Theta }}_{II}$-loop-current order described by this model may explain groundbreaking experiments such as spin-polarized neutron scattering performed in these materials. Here, we demonstrate, within a saddle-point approximation, that the ${\mathrm{\Theta }}_{II}$-loop-current order clearly coexists with bidirectional (i.e., checkerboard) $d$-wave CDW and PDW orders along axial momenta, but is visibly detrimental to the unidirectional (i.e., stripe) case. This result has potentially far-reaching implications for the physics of the cuprates and agrees well with very recent x-ray experiments on YBCO that indicate that at higher dopings the CDW order has indeed a tendency to be bidirectional.

We demonstrate an advanced spin-system design using a hybrid spin consisting of a strongly coupled metal ion and verdazyl radical. Ab initio calculation, magnetization, and ESR measurements evidenced the first realization of an $S=2$ ferromagnetic-antiferromagnetic alternating chain with Ising anisotropy in a Mn-verdazyl complex $\left[{\mathrm{Mn}\left(\mathrm{hfac}\right)}_2\right]·(o-\mathrm{Py}-\mathrm{V})$ [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate; o-Py-V = 3-(2-pyridyl)-1,5-diphenylverdazyl]. Furthermore, we find an anomalous change in magnetization at 1/4 of the saturation value, which is probably a manifestation of the quantum nature of the system.

Semiconductor-superconductor heterostructures represent a promising platform for the detection of Majorana zero modes and subsequently the processing of quantum information using their exotic non-Abelian statistics. Theoretical modeling of such low-dimensional heterostructures is generally based on phenomenological effective models. However, a more microscopic understanding of the band structure and, especially, of the spin-orbit coupling of electrons in these devices is important for optimizing their parameters for applications in quantum computing. In this paper, we approach this problem by first obtaining a highly accurate effective tight-binding model of bulk InSb from ab initio calculations. This model is symmetrized and correctly reproduces both the band structure and the wave function character. It is then used to simulate slabs of InSb in external electric fields. The results of this simulation are used to determine a growth direction for InSb nanowires that optimizes the conditions for the experimental realization of Majorana zero modes.

Unconventional optical Tamm surface states have been demonstrated in transmission and reflectance spectra of three-dimensional opal photonic crystals coated by thin metal films. These states appear in registry with diffraction resonances and localize the electromagnetic energy in asymmetric resonators formed by stacks of lattice planes and metal semishells. Tamm defect states provide the bypass for light at the edges of the Bragg diffraction resonances and thus reduce the diffraction efficiency. Despite the hidden nature of this effect, its magnitude is comparable to the extraordinary transmission associated with the surface-plasmon polaritons that are simultaneously excited at the surfaces of the corrugated metal films.

We study fully hexagonal boron nitride (hBN) encapsulated graphene spin valve devices at room temperature. The device consists of a graphene channel encapsulated between two crystalline hBN flakes: thick-hBN flake as a bottom gate dielectric substrate which masks the charge impurities from ${\mathrm{SiO}}_2/\mathrm{Si}$ substrate and single-layer thin-hBN flake as a tunnel barrier. Full encapsulation prevents the graphene from coming in contact with any polymer/chemical during the lithography and thus gives homogeneous charge and spin transport properties across different regions of the encapsulated graphene. Further, even with the multiple electrodes in-between the injection and the detection electrodes which are in conductivity mismatch regime, we observe spin transport over $12.5-\mu \mathrm{m}$-long distance under the thin-hBN encapsulated graphene channel, demonstrating the clean interface and the pinhole-free nature of the thin hBN as an efficient tunnel barrier.

We correct erroneous conclusions from our previous article [Phys. Rev. B 90, 241112(R) (2014)] regarding the values of various critical exponents, calculated to two-loop order. Furthermore, from a three-loop renormalization group flow equation, we argue, that $\epsilon$-expansion near two spatial dimensions, with $\epsilon =d-2$, may not be reliable to address the critical properties of the disorder-driven Dirac semimetal-metal quantum phase transition in $d=3$.

We define a “hyperconductor” to be a material whose electrical and thermal dc conductivities are infinite at zero temperature and finite at any nonzero temperature. The low-temperature behavior of a hyperconductor is controlled by a quantum critical phase of interacting electrons that is stable to all potentially gap-generating interactions and potentially localizing disorder. In this paper, we compute the low-temperature dc and ac electrical and thermal conductivities in a one-dimensional hyperconductor, studied previously by the present authors, in the presence of both disorder and umklapp scattering. We identify the conditions under which the transport coefficients are finite, which allows us to exhibit examples of violations of the Wiedemann-Franz law. The temperature dependence of the electrical conductivity, which is characterized by the parameter ${\mathrm{\Delta }}_X$, is a power law, $\sigma \propto 1/T^{1-2(2-{\mathrm{\Delta }}_X)}$ when ${\mathrm{\Delta }}_X\ge 2$, down to zero temperature when the Fermi surface is commensurate with the lattice. There is a surface in parameter space along which ${\mathrm{\Delta }}_X=2$ and ${\mathrm{\Delta }}_X\approx 2$ for small deviations from this surface. In the generic (incommensurate) case with weak disorder, such scaling is seen at high temperatures, followed by an exponential increase of the conductivity $ln\sigma \sim 1/T$ at intermediate temperatures and, finally, $\sigma \propto 1/T^{2-2(2-{\mathrm{\Delta }}_X)}$ at the lowest temperatures. In both cases, the thermal conductivity diverges at low temperatures.

It has been known since the pioneering work of Bernal, Fowler, and Pauling that common, hexagonal (Ih) water ice is the archetype of a frustrated material: a proton-bonded network in which protons satisfy strong local constraints (the “ice rules”) but do not order. While this proton disorder is well established, there is now a growing body of evidence that quantum effects may also have a role to play in the physics of ice at low temperatures. In this paper, we use a combination of numerical and analytic techniques to explore the nature of proton correlations in both classical and quantum models of ice Ih. In the case of classical ice Ih, we find that the ice rules have two, distinct, consequences for scattering experiments: singular “pinch points,” reflecting a zero-divergence condition on the uniform polarization of the crystal, and broad, asymmetric features, coming from its staggered polarization. In the case of the quantum model, we find that the collective quantum tunneling of groups of protons can convert states obeying the ice rules into a quantum liquid, whose excitations are birefringent, emergent photons. We make explicit predictions for scattering experiments on both classical and quantum ice Ih, and show how the quantum theory can explain the “wings” of incoherent inelastic scattering observed in recent neutron scattering experiments [Bove et al., Phys. Rev. Lett. 103, 165901 (2009)]. These results raise the intriguing possibility that the protons in ice Ih could form a quantum liquid at low temperatures, in which protons are not merely disordered, but continually fluctuate between different configurations obeying the ice rules.

A topological superconducting wire with an effective time-reversal symmetry is known to have a ${\mathbb{Z}}_8$ topological classification in the presence of interactions. The topological index $\left|n\right|\le 4$ counts the number of Majorana end states, negative $n$ corresponding to end states that are odd under time reversal. If such a wire is weakly coupled to a normal-metal lead, interactions induce a Kondo-like correlated state if $\left|n\right|=4$. We show that the Kondo-like state manifests itself in an anomalous temperature dependence of the zero-bias conductance and by an anomalous Fano factor for the zero-temperature normally reflected current at finite bias. We also consider the splitting of the effective Kondo resonance for weak symmetry-breaking perturbations.

We report on the observation of a terahertz radiation-induced photon drag effect in epitaxially grown $n$- and $p$-type $\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right){}_2{\mathrm{Te}}_3$ three-dimensional topological insulators with different antimony concentrations $x$ varying from 0 to 1. We demonstrate that the excitation with polarized terahertz radiation results in a dc electric photocurrent. While at normal incidence a current arises due to the photogalvanic effect in the surface states, at oblique incidence it is outweighed by the trigonal photon drag effect. The developed microscopic model and theory show that the photon drag photocurrent can be generated in surface states. It arises due to the dynamical momentum alignment by time- and space-dependent radiation electric field and implies the radiation-induced asymmetric scattering in the electron momentum space. We show that the photon drag current may also be generated in the bulk. Both surface states and bulk photon drag currents behave identically upon variation of such macroscopic parameters as radiation polarization and photocurrent direction with respect to the radiation propagation. This fact complicates the assignment of the trigonal photon drag effect to a specific electronic system.

We study the stationary Josephson current in a junction between a topological and an ordinary (topologically trivial) superconductor. Such an S-TS junction hosts a Majorana zero mode that significantly influences the current-phase relation. The presence of the Majorana state is intimately related with the breaking of the time-reversal symmetry in the system. We derive a general expression for the supercurrent for a class of short topological junctions in terms of the normal-state scattering matrix. The result is strongly asymmetric with respect to the superconducting gaps in the ordinary $\left({\mathrm{\Delta }}_0\right)$ and topological $\left({\mathrm{\Delta }}_{\mathrm{top}}\right)$ leads. We apply the general result to a simple model of a nanowire setup with strong spin-orbit coupling in an external magnetic field and proximity-induced superconductivity. The system shows parametrically strong suppression of the critical current $I_c\propto {\mathrm{\Delta }}_{\mathrm{top}}/R_N^2$ in the tunneling limit $(R_N$ is the normal-state resistance). This is in strong contrast with the Ambegaokar-Baratoff relation applicable to junctions with preserved time-reversal symmetry. We also consider the case of a generic junction with a random scattering matrix and obtain a more conventional scaling law $I_c\propto {\mathrm{\Delta }}_{\mathrm{top}}/R_N$.

We performed diffusion Monte Carlo (DMC) calculations of the spectroscopic properties of a large set of molecules, assessing the effect of different approximations. In systems containing elements with large atomic numbers, we show that the errors associated with the use of nonlocal mean-field-based pseudopotentials in DMC calculations can be significant and may surpass the fixed-node error. We suggest practical guidelines for reducing these pseudopotential errors, which allow us to obtain DMC-computed spectroscopic parameters of molecules and equation of state properties of solids in excellent agreement with experiment.

Implementing complex sequences of gates is crucial for any quantum computing architecture to become practical. This requires long-lived qubits which can be manipulated many times without errors. Here we propose a scheme to process hybrid qubits consisting of spin ensembles coupled to superconducting resonators in a cavity-protection regime, which enhances their coherence time by orders of magnitude. We perform numerical experiments for the quantum simulation of the $XY$ model and the quantum Fourier transform, by including all the main decoherence mechanisms and assuming system parameters that are guaranteed by present technology.

We study the Ruderman-Kittel-Kasuya-Yosida interaction in three-dimensional Dirac semimetals. Using retarded Green's functions in real space, we obtain and analyze asymptotic expressions for the interaction, with magnetic impurities at different distances and relative angle with respect to high symmetry directions on the lattice. We show that the Fermi velocity anisotropy in these materials produces a strong renormalization of the magnitude of the interaction, as well as a correction to the frequency of oscillation in real space. Hybridization of the impurities to different conduction electron orbitals are shown to result in interesting anisotropic spin-spin interactions which can generate spiral spin structures in doped samples.

We derive a so-called gap equation for superconductivity in a quasi-one-dimensional (Q1D) layered conductor in a magnetic field, which is parallel to both its conducting plane and its conducting axis. This equation demonstrates that the orbital destructive effects against superconductivity cannot destroy it, if the perpendicular to the plane coherence length is less than the interplane distance. On the basis of our results, we suggest arguments that some triplet superconducting phase was indeed discovered in ultrahigh magnetic fields in the Q1D superconductor ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ [see X. Xu et al., Phys. Rev. Lett. 102, 206602 (2009)]. We also discuss a possibility that the above-mentioned superconducting phase can be stable at arbitrary high magnetic fields.

The synthesis and crystallographic, thermodynamic, and transport properties of single crystalline ${\mathrm{Rh}}_9{\mathrm{In}}_4{\mathrm{S}}_4$ were studied. The resistivity, magnetization, and specific heat measurements all clearly indicate bulk superconductivity with a critical temperature, $T_c\sim 2.25$ K. The Sommerfeld coefficient $\gamma$ and the Debye temperature $\left({\mathrm{\Theta }}_{\text{D}}\right)$ were found to be 34 mJ ${\mathrm{mol}}^{-1} {\mathrm{K}}^{-2}$ and 217 K, respectively. The observed specific heat jump, $\mathrm{\Delta }C/\gamma T_c=1.66$, is larger than the expected BCS weak coupling value of 1.43. Ginzburg-Landau (GL) ratio of the low-temperature GL-penetration depth, ${\lambda }_{\text{GL}}\approx 5750$ Å, to the GL-coherence length, ${\xi }_{\text{GL}}\approx 94$ Å, is large: $\kappa \sim 60$. Furthermore, we observed a peak effect in the resistivity measurement as a function of both temperature and magnetic field.

Measurements of the resistivity, Hall coefficient, and Raman spectroscopy are performed on a Rashba semiconductor BiTeCl single crystal at high pressures up to 50 GPa. We find that applying pressure first induces a theoretically predicted insulating state, followed by a superconducting phase with an insulating normal state. Upon heavy compression, another different superconducting phase is entered into with a metallic normal state. A domelike evolution of the superconducting transition temperature with pressure is obtained with a crossover from the electron to hole carriers across the boundary of the two superconducting phases. These findings imply the possible realization of a topological state of the insulating and superconducting phases in this material.