A continuous deformation of a Hamiltonian possessing at low energy two Dirac points of opposite chiralities can lead to a gap opening by merging the two Dirac points. In two dimensions, the critical Hamiltonian possesses a semi-Dirac spectrum: linear in one direction but quadratic in the other. We study the transport properties across such a transition, from a Dirac semimetal through a semi-Dirac phase toward a gapped phase. Using both a Boltzmann approach and a diagrammatic Kubo approach, we describe the conductivity tensor within the diffusive regime. In particular, we show that both the anisotropy of the Fermi surface and the Dirac nature of the eigenstates combine to give rise to anisotropic transport times, manifesting themselves through an unusual matrix self-energy.

We present results of electronic structure calculations of the bulk properties of the Weyl semimetal TaAs. The emergence of Weyl (massless) fermions in TaAs, due to its electronic band structure, is indicative of a new state of matter in the condensed phase that is of great interest for fundamental physics and possibly new applications. Many of the physical properties of the material, however, are unknown. We have calculated the structural parameters, dielectric function, elastic constants, phonon dispersion, electronic band structure, and Born effective charges using density functional theory within the generalized gradient approximation, including spin-orbit coupling where necessary. Our results provide essential information on the material; and our calculations agree well with the relatively small number of experimental data available. Moreover, we have determined the relative stability of the ground state body-centered tetragonal phase with respect to other common binary phases as a function of pressure at the athermal limit, predicting a transition to the CsCl cubic structure at 23.3 GPa. Finally, we have determined the band structure using an unbiased hybrid density functional that includes 25% exact exchange, in order to refine the previously determined positions in $k$ space of the Weyl points.

We demonstrate that the phenomenon of current confinement along graphene $n-p$ junctions at high magnetic fields can be used to form an Aharonov-Bohm interferometer. The interference system exploits a closed $n-p$ junction that can be induced by a floating gate within the sample, and coupling of the junction currents with the edge currents in the quantum Hall regime. Operation of the device requires current splitting at the edge and the $n-p$ junction contacts which is found for armchair ribbons at low Fermi energy.

Ultrathin metallic silicide nanowires with extremely high aspect ratios can be easily grown, e.g., by deposition of rare earth elements on semiconducting surfaces. These wires play a pivotal role in fundamental research and open intriguing perspectives for CMOS applications. However, the electronic properties of these one-dimensional systems are extremely sensitive to atomic-sized defects, which easily alter the transport characteristics. In this study, we characterized comprehensively ${\mathrm{TbSi}}_2$ wires grown on Si(100) and correlated details of the atomic structure with their electrical resistivities. Scanning tunneling microscopy (STM) as well as all transport experiments were performed in situ using a four-tip STM system. The measurements are complemented by local spectroscopy and density functional theory revealing that the silicide wires are electronically decoupled from the Si template. On the basis of a quasiclassical transport model, the size effect found for the resistivity is quantitatively explained in terms of bulk and surface transport channels considering details of atomic-scale roughness. Regarding future applications the full wealth of these robust nanostructures will emerge only if wires with truly atomically sharp interfaces can be reliably grown.

This paper gives a theoretical formulation of the electromagnetic response of the quasi-two-dimensional crystals suitable for investigation of optical activity and polariton modes. The response to external electromagnetic field is described by current-current response tensor ${\mathrm{\Pi }}_{\mu \nu }$ calculated by solving the Dyson equation in the random phase approximation, where current-current interaction is mediated by the photon propagator $D_{\mu \nu }$. The irreducible current-current response tensor ${\mathrm{\Pi }}_{\mu \nu }^0$ is calculated from the ab initio Kohn-Sham orbitals. The accuracy of ${\mathrm{\Pi }}_{\mu \nu }^0$ is tested in the long-wavelength limit where it gives correct Drude dielectric function and conductivity. The theory is applied to the calculation of optical absorption and conductivity in pristine and doped single-layer graphene and successfully compared with previous calculations and measurements.

Mott insulating ultracold gases possess a unique whole-atom exchange interaction which enables large quantum fluctuations between the Zeeman sublevels of each atom. By strengthening this interaction—either through the use of large-spin atoms or by tuning the particle-particle interactions via optical Feshbach resonance—one may enhance fluctuations and facilitate the appearance of the long-sought-after quantum spin liquid phase—all in the highly tunable environment of cold atoms. To illustrate the relationship between the spin magnitude, interaction strength, and resulting magnetic phases, we present and solve a mean-field theory for bosons optically confined to the one-particle-per-site Mott state, using both analytic and numerical methods. We find on square and triangular lattices for bosons of hyperfine spin $f>2$ that making the repulsive $s$-wave scattering length through the singlet channel small—relative to the higher-order scattering channels—accesses a short-range resonating valence bond (s-RVB) spin liquid phase.

The magnetocrystalline contribution to magnetic anisotropy was studied in the canted antiferromagnetic state of layered ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ single crystals. We performed torque measurements in magnetic fields up to 9 T under various magnetic field orientations. The strong dependence of torque on the magnetic field revealed that the magnetic easy axis is along the in-plane direction and that the observed field-induced weak ferromagnetic order is attributed only to the in-plane component of the external magnetic field. The dependence of torque on the angle produces a twofold symmetric sawtoothlike shape. A simple model consisting of canted antiferromagnetic and magnetic induction terms showed good agreement with the measured torque. These results show that magnetic anisotropy is closely related to the anisotropy of the exchange between antiferromagnetic moments whose canting is mediated by the Dzyaloshinsky-Moriya interaction. Our study demonstrates that torque magnetometry can be extended to the investigation of the magnetic anisotropy of complex magnetic phases.

Recent experiments have shown that magnetic skyrmion bubbles can be generated and injected at room temperature in thin films [W. Jiang et al., Science 349, 283 (2015)]. Here, we demonstrate, using micromagnetic modeling, that such skyrmions can be generated by an inhomogeneous spin Hall torque in the presence of Dzyaloshinskii-Moriya interactions (DMIs). In the experimental Ta/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ thin films, the DMI is rather small; nevertheless, the skyrmion bubbles are stable, or at least metastable on observational time scales.

We study the persistent current in a ring consisting of $N\gg 1$ Josephson junctions threaded by the magnetic flux. When the dynamics of the ring is dominated by the capacitances of the superconducting islands the system is equivalent to the $xy$ spin system in $1+1$ dimensions at the effective temperature $T^{*}=\sqrt{2JU}$, with $J$ being the Josephson energy of the junction and $U$ being the charging energy of the superconducting island. The numerical problem is challenging due to the absence of thermodynamic limit and slow dynamics of the Kosterlitz-Thouless transition. It is investigated on lattices containing up to $\times {10}^6$ sites. At $T^{*}\ll J$ the quantum phase slips are frozen. The low-$T^{*}$ dependence of the persistent current computed numerically agrees quantitatively with the analytical formula provided by the spin-wave approximation. The high-$T^{*}$ behavior depends strongly on the magnetic flux and on the number of superconducting islands $N$. We present a detailed numerical study of the unbinding of vortex-antivortex pairs responsible for the phase slips, the superconductor-insulator transition, and evolution of the persistent current in a finite-size system.

In order to identify the pairing symmetry with chirality, we study site-selective NMR in chiral $p$-wave superconductors. We calculate local nuclear relaxation rate $T_1^{-1}$ in the vortex lattice state by Eilenberger theory, including the applied magnetic field dependence. We find that $T_1^{-1}$ in the NMR resonance line shape is different between two chiral states $p_{\pm }(=p_x\pm i{p}_y)$, depending on whether the chirality is parallel or antiparallel to the vorticity. Anomalous suppression of $T_1^{-1}$ occurs around the vortex core in the chiral $p_{-}$ wave due to the negative coherence term coming from the odd-frequency $s$-wave Cooper pair induced around the vortex with Majorana state.

We show that a chain of nonmagnetic impurities deposited on a fully gapped two- or three-dimensional superconductor can become a topological one-dimensional superconductor with protected Majorana bound states at its end. A prerequisite is that the pairing potential of the underlying superconductor breaks the spin-rotation symmetry, as it is generically the case in systems with strong spin-orbit coupling. We illustrate this mechanism for a spinless triplet-superconductor $\left(p_x+i{p}_y\right)$ and a time-reversal symmetric Rashba superconductor with a mixture of singlet and triplet pairing. For the latter, we show that the impurity chain can be topologically nontrivial even if the underlying superconductor is topologically trivial.

We report on local magnetization measurements showing the presence of an unexpectedly strong transverse Meissner effect in the superconducting ${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$ single crystals. We show that for tilted magnetic fields $\left(H_a\right)$ vortices remain aligned with the $ab$ planes up to field $H_k\sim H_p/sin{\theta }_H$ (where ${\theta }_H$ is the angle between the applied field and the $ab$ plane and $H_p$ the first penetration field) and that for $H_a>H_k$, the field dependence of the vortex direction ${\theta }_B\left(H_a\right)$ can be well described assuming that vortices remain partially locked in the planes forming a staircase structure of average direction ${\theta }_B\ne {\theta }_H$. These results indicate the existence of a strong modulation of the vortex core energy along the $c$ axis but its origin remains unclear.

We investigated phonon-phonon interactions in photoexcited single-crystalline graphite by ultrafast electron diffraction. Transient electron diffraction profiles from a 35 nm graphite film were observed following laser excitation. Changes in intensities of diffraction spots revealed a two-exponential relaxation process with <0.5 ps and ∼8 ps time scales. Furthermore, the transient thermal diffuse scattering signal showed evidence for population of phonons across the Γ$K$ and Γ$M$ branches of the phonon dispersion spectrum on a <2 ps time scale. Based on these dynamics, we conclude that the fast relaxation process corresponds to decay of strongly coupled optical phonons, and the slow relaxation process to redistribution of phonon energy to the equilibrium thermal distribution.

We report neutron scattering and muon spin relaxation measurements ($\mu \mathrm{SR}$) on the pyrochlore antiferromagnet ${\mathrm{Yb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$. Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the $J=\frac{7}{2}{\mathrm{Yb}}^{3+}$ ion in this environment. The crystal electric field ground state doublet in ${\mathrm{Yb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ corresponds primarily to $m_J=\pm \frac{1}{2}$ with local XY anisotropy, consistent with an $S_{\text{eff}}=\frac{1}{2}$ description for the Yb moments. $\mu \mathrm{SR}$ measurements reveal the presence of an ordering transition at $T_N=0.57$ K with persistent weak dynamics in the ordered state. Finally, we present neutron diffraction measurements that reveal a clear phase transition to the $\mathbf{k}=\left(000\right){\mathrm{\Gamma }}_5$ ground state with an ordered magnetic moment of $0.3\left(1\right){\mu }_B$ per Yb ion. We compare and contrast this phenomenology with the low-temperature behavior of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, the prototypical $S_{\text{eff}}=\frac{1}{2}$ XY pyrochlore magnets.

We investigate partially disordered antiferromagnetism in ${\mathrm{CoCl}}_2$-2SC(${\mathrm{NH}}_2$)${}_2$, in which $ab$-plane hexagonal layers are staggered along the $c$ axis rather than stacked. A robust 1/3 state forms in applied magnetic fields in which the spins are locked, varying as a function of neither temperature nor field. By contrast, in zero field and applied fields at higher temperatures, partial antiferromagnetic order occurs, in which free spins are available to create a Curie-like magnetic susceptibility. We report measurements of the crystallographic structure and the specific heat, magnetization, and electric polarization down to $T=50\mathrm{mK}$ and up to ${\mu }_{0}H=60\mathrm{T}$. The ${\mathrm{Co}}^{2+}\mathbf{S}=3/2$ spins are Ising-like and form distorted hexagonal layers. The Ising energy scale is well separated from the magnetic exchange, and both energy scales are accessible to the measurements, allowing us to cleanly parametrize them. In transverse fields, a quantum Ising phase transition can be observed at 2 T. Finally, we find that magnetic exchange striction induces changes in the electric polarization up to $3\mu \mathrm{C}/{\mathrm{m}}^2$, and single-ion magnetic anisotropy effects induce a much larger electric polarization change of $300\mu \mathrm{C}/{\mathrm{m}}^2$.

We report a study of the superconducting and normal-state properties of the filled-skutterudite system ${\mathrm{PrPt}}_4{\mathrm{Ge}}_{12-x}{\mathrm{Sb}}_x$. Polycrystalline samples with Sb concentrations up to $x=5$ were synthesized and investigated by means of x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements. We observed a suppression of superconductivity with increasing Sb substitution up to $x=4$, above which no signature of superconductivity was observed down to 140 mK. The Sommerfeld coefficient, $\gamma$, of superconducting specimens decreases with increasing $x$ up to $x=3$, suggesting that superconductivity may depend on the density of electronic states in this system. The specific heat for $x=0.5$ exhibits an exponential temperature dependence in the superconducting state, reminiscent of a nodeless superconducting energy gap. We observed evidence for a weak “rattling” mode associated with the Pr ions, characterized by an Einstein temperature ${\mathrm{\Theta }}_{\mathrm{E}}\sim 60$ K for $0\le x\le 5$; however, the rattling mode may not play any role in suppressing superconductivity.

A magnetic skyrmion observed experimentally in chiral magnets is a topologically protected spin texture. For their unique properties, such as high mobility under current drive, skyrmions have a huge potential for applications in next-generation spintronic devices. Defects naturally occurring in magnets have profound effects on the static and dynamical properties of skyrmions. In this work we study the effect of an atomic defect on a skyrmion by performing the first-principles calculations within the density functional theory, taking MnSi as an example. By substituting one site of Mn or Si with different elements, we can tune the pinning energy. The effects of pinning by an atomic defect can be understood qualitatively within a phenomenological model.

We discuss the dissipative preparation of $p\text{-wave}$ superconductors in number-conserving one-dimensional fermionic systems. We focus on two setups: the first one entails a single wire coupled to a bath, whereas in the second one the environment is connected to a two-leg ladder. Both settings lead to stationary states which feature the bulk properties of a $p\text{-wave}$ superconductor, identified in this number-conserving setting through the long-distance behavior of the proper $p\text{-wave}$ correlations. The two schemes differ in the fact that the steady state of the single wire is not characterized by topological order, whereas the two-leg ladder hosts Majorana zero modes, which are decoupled from damping and exponentially localized at the edges. Our analytical results are complemented by a numerical study based both on an exact representation of the density matrix and on a matrix-product-density-operator one. With these tools we characterize the steady-state properties of the protocols, their asymptotic decay rate, and their robustness.