The magnetic phases of a triangular-lattice antiferromagnet ${\mathrm{CuCrO}}_2$ were investigated in magnetic fields along the $c$ axis, $H\parallel \left[001\right]$, up to 120 T. Faraday rotation and magnetoabsorption spectroscopy were used to unveil the rich physics of magnetic phases. An up-up-down (UUD) magnetic structure phase was observed around 90–105 T at temperatures around 10 K. Additional distinct anomalies adjacent to the UUD phase were uncovered and the $\mathsf{Y}$-shaped and the $\mathsf{V}$-shaped phases are proposed to be viable candidates. These ordered phases emerged as a result of the interplay of geometrical spin frustration, single-ion anisotropy, and thermal fluctuations in an environment of extremely high magnetic fields.

The discovery of new families of unconventional superconductors is important both experimentally and theoretically, especially if it challenges current models and thinking. By using muon spin relaxation in zero field, time-reversal symmetry breaking has been observed in ${\mathrm{Re}}_6\mathrm{Hf}$. Moreover, the temperature dependence of the superfluid density exhibits $s$-wave superconductivity with an enhanced electron-phonon coupling. This, coupled with the results from isostructural ${\mathrm{Re}}_6\mathrm{Zr}$, shows that the ${\mathrm{Re}}_{6}X$ family is indeed a new and important group of unconventional superconductors.

Understanding the domain wall dynamics is an important issue in modern magnetism. Here we present results of domain wall displacement in curved cylindrical nanowires at a constant magnetic field. We show that the average velocity of a transverse domain wall increases with curvature. Contrary to what is observed in stripes, in a curved wire the transverse domain wall oscillates along and rotates around the nanowire with the same frequency. These results open the possibility of new oscillation-based applications.

We review the dimensional reduction procedure in the group cohomology classification of bosonic SPT phases with finite Abelian unitary symmetry group. We then extend this to include general reductions of arbitrary dimensions and also extend the procedure to fermionic SPT phases described by the Gu-Wen supercohomology model. We then show that we can define topological invariants as partition functions on certain closed orientable/spin manifolds equipped with a flat connection. The invariants are able to distinguish all phases described within the respective models. Finally, we establish a connection to invariants obtained from braiding statistics of the corresponding gauged theories.

In this paper we present a comparative investigation of the electronic structures of ${\mathrm{NbO}}_2$ and ${\mathrm{VO}}_2$ obtained within a combination of density functional theory and cluster-dynamical mean-field theory calculations. We investigate the role of dynamic electronic correlations on the electronic structure of the metallic and insulating phases of ${\mathrm{NbO}}_2$ and ${\mathrm{VO}}_2$, with a focus on the mechanism responsible for the gap opening in the insulating phases. For the rutile metallic phases of both oxides, we obtain that electronic correlations lead to a strong renormalization of the $t_{2g}$ subbands, as well as the emergence of incoherent Hubbard subbands, signaling that electronic correlations are also important in the metallic phase of ${\mathrm{NbO}}_2$. Interestingly, we find that nonlocal dynamic correlations do play a role in the gap formation of the [body-centered-tetragonal (bct)] insulating phase of ${\mathrm{NbO}}_2$, by a similar physical mechanism as that recently proposed by us in the case of the monoclinic ($M_1$) dimerized phase of ${\mathrm{VO}}_2$ [Phys. Rev. Lett. 117, 056402 (2016)]. Although the effect of nonlocal dynamic correlations in the gap opening of bct phase is less important than in the ($M_1$ and $M_2$) monoclinic phases of ${\mathrm{VO}}_2$, their presence indicates that the former is not a purely Peierls-type insulator, as it was recently proposed.

The electronic structure and chemical bonding in reactively magnetron sputtered $\mathrm{Zr}{\mathrm{H}}_x$ ($x=0.15$, 0.30, 1.16) thin films with oxygen content as low as 0.2 at.% are investigated by 4d valence band, shallow 4p core-level, and 3d core-level x-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity toward the H 1s–Zr 4d hybridization region at $\sim 6\mathrm{eV}$ below the Fermi level. For low hydrogen content ($x=0.15$, 0.30), the films consist of a superposition of hexagonal closest-packed metal ($\alpha$ phase) and understoichiometric $\delta \text{−}\mathrm{Zr}{\mathrm{H}}_x$ ($\mathrm{Ca}{\mathrm{F}}_2$-type structure) phases, while for $x=1.16$, the films form single-phase $\mathrm{Zr}{\mathrm{H}}_x$ that largely resembles that of stoichiometric $\delta \text{−}\mathrm{Zr}{\mathrm{H}}_2$ phase. We show that the cubic $\delta \text{−}\mathrm{Zr}{\mathrm{H}}_x$ phase is metastable as thin film up to $x=1.16$, while for higher H contents the structure is predicted to be tetragonally distorted. For the investigated $\mathrm{Zr}{\mathrm{H}}_{1.16}$ film, we find chemical shifts of 0.68 and 0.51 eV toward higher binding energies for the Zr $4p_{3/2}$ and $3d_{5/2}$ peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as a function of hydrogen content is discussed in relation to the charge transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.

We analyze the control of Majorana zero-energy states by mapping the fermionic system onto a chain of Ising spins. Although the topological protection is lost for the Ising system, the mapping provides additional insight into the nature of the quantum states. By controlling the local magnetic field, one can separate the Ising chain into ferromagnetic and paramagnetic phases, corresponding to topological and nontopological sections of the fermionic system. In this paper we propose (topologically nonprotected) protocols performing the braiding operation, and in fact also more general rotations. We first consider a $T$-junction geometry, but we also propose a protocol for a purely one-dimensional (1D) system. Both setups rely on an extra spin-$\frac{1}{2}$ coupler. By including the extra spin in the $T$-junction geometry, we overcome limitations due to the 1D character of the Jordan–Wigner transformation. In the 1D geometry the coupler, which controls one of the Ising links, should be manipulated once the ferromagnetic (topological) section of the chain is moved far away. We also propose experimental implementations of our scheme. One is based on a chain of flux qubits which allows for all needed control fields. We also describe how to translate our scheme for the 1D setup to a chain of superconducting wires hosting each a pair of Majorana edge states.

Shortly after the discovery of Weyl semimetals, properties related to the topology of their bulk band structure have been observed, e.g., signatures of the chiral anomaly and Fermi arc surface states. These essentially single particle phenomena are well understood, but whether interesting many-body effects due to interactions arise in Weyl systems remains much less explored. Here, we investigate the effect of interactions in a microscopic model of a type-II Weyl semimetal in a strong magnetic field. We identify a charge density wave (CDW) instability even for weak interactions stemming from the emergent nesting properties of the type-II Weyl Landau level dispersion. We map out the dependence of this CDW on magnetic field strength. Remarkably, as a function of decreasing temperature, a cascade of CDW transitions emerges and we predict characteristic signatures for experiments.

The possible realization of dissipationless chiral edge current in a topological insulator/magnetic insulator heterostructure is based on the condition that the magnetic proximity exchange coupling at the interface is dominated by the Dirac surface states of the topological insulator. Here we report a polarized neutron reflectometry observation of Dirac-electron-mediated magnetic proximity effect in a bulk-insulating topological insulator $(\mathrm{B}{\mathrm{i}}_{0.2}\mathrm{S}{\mathrm{b}}_{0.8}{)}_2\mathrm{T}{\mathrm{e}}_3$/magnetic insulator EuS heterostructure. We are able to maximize the proximity-induced magnetism by applying an electrical back gate to tune the Fermi level of topological insulator to be close to the Dirac point. A phenomenological model based on diamagnetic screening is developed to explain the suppressed proximity-induced magnetism at high carrier density. Our work paves the way to utilize the magnetic proximity effect at the topological insulator/magnetic insulator heterointerface for low-power spintronic applications.

We show that a smooth interface between two insulators of opposite topological ${\mathbb{Z}}_2$ indices possesses multiple surface states, both massless and massive. While the massless surface state is nondegenerate, chiral and insensitive to the interface potential, the massive surface states only appear for a sufficiently smooth heterojunction. The surface states are particle-hole symmetric and a voltage drop reveals their intrinsic relativistic nature, similarly to Landau bands of Dirac electrons in a magnetic field. We discuss the relevance of the massive Dirac surface states in recent angle-resolved photoemission spectroscopy and transport experiments.

Recent photoemission spectroscopy measurements (T. J. Reber et al., arXiv:1509.01611) on cuprate superconductors have inferred that over a wide range of doping, the imaginary part of the electron self-energy scales as ${\mathrm{\Sigma }}^{″}\sim {({\omega }^2+{\pi }^2{T}^2)}^a$ with $a=1$ in the overdoped Fermi-liquid state and $a<0.5$ in the optimal to underdoped regime. We show that this non-Fermi-liquid scaling behavior can naturally be explained by the presence of a scale-invariant state of matter known as unparticles. We evaluate analytically the electron self-energy due to interactions with fermionic unparticles. We find that, in agreement with experiments, the imaginary part of the self-energy scales with respect to temperature and energy as ${\mathrm{\Sigma }}^{″}\sim T^{2+2\alpha }$ and ${\omega }^{2+2\alpha }$, where $\alpha$ is the anomalous dimension of the unparticle propagator. In addition, the calculated occupancy and susceptibility of fermionic unparticles, unlike those of normal fermions, have significant spectral weights even at high energies. This unconventional behavior is attributed to the branch cut in the unparticle propagator which broadens the unparticle spectral function over a wide energy range and nontrivially alters the scattering phase space by enhancing (suppressing) the intrinsic susceptibility at low energies for negative (positive) $\alpha$. Our work presents evidence suggesting that unparticles might be important low-energy degrees of freedom in strongly coupled systems such as the cuprate superconductors.

The extended Falicov-Kimball model is analyzed exactly in the ground state at half filling in the limit of large dimensions. In the model the on-site and the intersite density-density interactions between all particles are included. We determined the model's phase diagram and found a discontinuous transition between two different charge-ordered phases. Our analytical calculations show that the ground state of the system is insulating for any nonzero values of the interaction couplings. We also show that the dynamical mean-field theory and the static broken-symmetry Hartree-Fock mean-field approximation give the same results for the model at zero temperature. In addition, we prove using analytical expressions that at infinitesimally small, but finite, temperatures the system can be metallic.

Single crystals of $\mathrm{EuMnS}{\mathrm{b}}_2$ were successfully grown and their structural and electronic properties were investigated systematically. The material crystallizes in an orthorhombic-layered structure (space group: Pnma, No. 62) comprising a periodic sequence of -MnSb/Eu/Sb/Eu/- layers ($\sim 1\mathrm{nm}$ in thickness), and massless fermions are expected to emerge in the Sb layer, by analogy of the candidate Dirac materials $\mathrm{EuMnB}{\mathrm{i}}_2$ and $A\mathrm{Mn}P{n}_2$ ($A=\mathrm{Ca}$ or Sr or Ba, $Pn=\mathrm{Sb}$ or Bi). The magnetic and specific heat measurements of $\mathrm{EuMnS}{\mathrm{b}}_2$ suggest an antiferromagnetic ordering of Eu moments near 20 K. A characteristic hump appears in the temperature-dependent electrical resistivity curve at $\sim 25\mathrm{K}$. A spin-flop transition of Eu moments with an onset magnetic field of $\sim 15\mathrm{kOe}$ (at 2 K) was observed. Interestingly, $\mathrm{EuMnS}{\mathrm{b}}_2$ shows a negative magnetoresistance (up to $-95\%$) in contrast to the positive magnetoresistances observed for $\mathrm{EuMnB}{\mathrm{i}}_2$ and $A\mathrm{Mn}P{n}_2$ ($A=\mathrm{Ca}$ or Sr or Ba, $Pn=\mathrm{Sb}$ or Bi), providing a unique opportunity to study the correlation between electronic and magnetic properties in this class of materials.

We have used optical spectroscopy to observe spectral broadening of ${\mathrm{WS}}_2$ exciton reflectance peaks in heterostructures of monolayer ${\mathrm{WS}}_2$ capped with mono- to few-layer graphene. The broadening is found to be similar for the A and B excitons and on the order of 5–10 meV. No strong dependence on the number of graphene layers was observed within experimental uncertainty. The broadening can be attributed to charge- and energy-transfer processes between the two materials, providing an observed lower bound for the corresponding time scales of 65 fs.

We present the structural evolution of Bi on Au(111) from monolayer to multilayer regimes explored mainly by scanning tunneling microscopy. At the monolayer regime, Bi clusters distribute homogeneously to make an array with $5\times 5$ periodicity. Further increase of the coverage converts these clusters to the two-dimensional periodic $\left(\sqrt{37}\times \sqrt{37}\right)\mathrm{R}25.3^{\circ }$ and $\left(\mathrm{p}\times \sqrt{3}\right)$ structures. We propose a model of the $\left(\sqrt{37}\times \sqrt{37}\right)\mathrm{R}25.3^{\circ }$ structure based on the STM measurements and density-functional theory calculations. In the multilayer regime, superstructures at first appear with long-range periodicities arising from the moiré structures due mainly to the stacking of Bi layers. Then, the thicker films grow whose lattice constants are close to those of the (110) surface of rhombohedral Bi crystal. The Bi(110) films of more than 60 layers grow stably. Thus, this system provides a good stage for further investigation of the peculiar electronic properties of Bi(110).

We experimentally demonstrate room-temperature storage of quantum entanglement using two nuclear spins weakly coupled to the electronic spin carried by a single nitrogen-vacancy center in diamond. We realize universal quantum gate control over the three-qubit spin system and produce entangled states in the decoherence-free subspace of the two nuclear spins. By injecting arbitrary collective noise, we demonstrate that the decoherence-free entangled state has coherence time longer than that of other entangled states by an order of magnitude in our experiment.

Planar ferromagnetic channels have been shown to theoretically support a long-range ordered and coherently precessing state where the balance between local spin injection at one edge and damping along the channel establishes a dissipative exchange flow, sometimes referred to as a spin superfluid. However, realistic materials exhibit in-plane anisotropy, which breaks the axial symmetry assumed in current theoretical models. Here, we study dissipative exchange flows in a ferromagnet with in-plane anisotropy from a dispersive hydrodynamic perspective. Through the analysis of a boundary value problem for a damped sine-Gordon equation, dissipative exchange flows in a ferromagnetic channel can be excited above a spin current threshold that depends on material parameters and the length of the channel. Symmetry-broken dissipative exchange flows display harmonic overtones that redshift the fundamental precessional frequency and lead to a reduced spin pumping efficiency when compared to their symmetric counterpart. Micromagnetic simulations are used to verify that the analytical results are qualitatively accurate, even in the presence of nonlocal dipole fields. Simulations also confirm that dissipative exchange flows can be driven by spin transfer torque in a finite-sized region. These results delineate the important material parameters that must be optimized for the excitation of dissipative exchange flows in realistic systems.