Quantum oscillations in the binary antiferromagnetic metal FeAs are presented and compared to theoretical predictions for the electronic band structure in the anomalous spin density wave state of this material. Demonstrating a method for growing single crystals out of Bi flux, we utilize the highest quality FeAs to perform torque magnetometry experiments up to 35 T, using rotations of field angle in two planes to provide evidence for one electron and one hole band in the magnetically ordered state. The resulting picture agrees with previous experimental evidence for multiple carriers at low temperatures, but the exact Fermi surface shape differs from predictions, suggesting that correlations play a role in deviation from ab initio theory and cause up to a fourfold enhancement in the effective carrier mass.

The ultrafast response to an optical pulse excitation of the spin-spin exchange interaction in transition metal antiferromagnets is studied within the framework of the time-dependent spin-density functional theory. We propose a formulation for the full dynamical exchange interaction, which is nonlocal in space, and it is derived starting from ab initio arguments. Then, we investigate the effect of the laser pulse on the onset of the dynamical process. It is found that we can distinguish two types of excitations, both activated immediately after the action of the laser pulse. While the first one can be associated to a Stoner-like excitation and involves the transfer of spin from one site to another, the second one is related to the ultrafast modification of a Heisenberg-like exchange interaction and can trigger the formation of spin waves in the first few hundred femtoseconds of the time evolution.

Antiferromagnetic insulators on a diamond lattice are candidate materials to host exotic magnetic phenomena ranging from spin-orbital entanglement to degenerate spiral ground states and topological paramagnetism. Compared to other three-dimensional networks of magnetic ions, such as the geometrically frustrated pyrochlore lattice, the investigation of diamond-lattice magnetism in real materials is less mature. In this work, we characterize the magnetic properties of model $A$-site spinels ${\mathrm{CoRh}}_2{\mathrm{O}}_4$ (cobalt rhodite) and ${\mathrm{CuRh}}_2{\mathrm{O}}_4$ (copper rhodite) by means of thermomagnetic and neutron-scattering measurements, and we perform group theory analysis, Rietveld refinement, mean-field theory, and spin-wave theory calculations to analyze the experimental results. Our investigation reveals that cubic ${\mathrm{CoRh}}_2{\mathrm{O}}_4$ is a canonical $S=3/2$ diamond-lattice Heisenberg antiferromagnet with a nearest-neighbor exchange $J=0.63$ meV and a Néel ordered ground state below a temperature of 25 K. In tetragonally distorted ${\mathrm{CuRh}}_2{\mathrm{O}}_4$, competing exchange interactions between up to third-nearest-neighbor spins lead to the development of an incommensurate spin helix at 24 K with a magnetic propagation vector ${\mathbf{k}}_{\mathrm{m}}=(0,0,0.79)$. Strong reduction of the ordered moment is observed for the $S=1/2$ spins in ${\mathrm{CuRh}}_2{\mathrm{O}}_4$ and captured by our $1/S$ corrections to the staggered magnetization. Our work identifies ${\mathrm{CoRh}}_2{\mathrm{O}}_4$ and ${\mathrm{CuRh}}_2{\mathrm{O}}_4$ as reference materials to guide future work searching for exotic quantum behavior in diamond-lattice antiferromagnets.

We demonstrate an enhanced, bidirectional, in-plane magnetic field tuning of the gyrotropic resonance frequency of a magnetic vortex within a ferromagnetic disk by introducing a flat edge. When the core is in its vicinity, the flat edge locally reduces the core's directional dynamic stiffness for movement parallel to the edge. This strongly reduces the net dynamic core stiffness, leading to the gyrotropic frequency being significantly less than when the core is centered (or located near the round edge). This leads to the measurable range of gyrotropic frequencies being more than doubled and also results in a clear chirality-mediated bistability of the gyrotropic resonance frequency due to what is effectively a chirality dependence of the core's confining potential.

Recently it was shown that an impurity exchanging orbital angular momentum with a surrounding bath can be described in terms of the angulon quasiparticle [Phys. Rev. Lett. 118, 095301 (2017)]. The angulon consists of a quantum rotor dressed by a many-particle field of boson excitations and can be formed out of, for example, a molecule or a nonspherical atom in superfluid helium or out of an electron coupled to lattice phonons or a Bose condensate. Here we develop an approach to the angulon based on the path-integral formalism, which sets the ground for a systematic, perturbative treatment of the angulon problem. The resulting perturbation series can be interpreted in terms of Feynman diagrams, from which, in turn, one can derive a set of diagrammatic rules. These rules extend the machinery of the graphical theory of angular momentum—well known from theoretical atomic spectroscopy—to the case where an environment with an infinite number of degrees of freedom is present. In particular, we show that each diagram can be interpreted as a ‘skeleton’, which enforces angular momentum conservation, dressed by an additional many-body contribution. This connection between the angulon theory and the graphical theory of angular momentum is particularly important as it allows us to systematically and substantially simplify the analytical representation of each diagram. In order to exemplify the technique, we calculate the one- and two-loop contributions to the angulon self-energy, the spectral function, and the quasiparticle weight. The diagrammatic theory we develop paves the way to investigate next-to-leading order quantities in a more compact way compared to the variational approaches.

We study the energy spectrum and the electromagnetic response of Andreev bound states in short Josephson junctions made of semiconducting nanowires. We focus on the joint effect of Zeeman and spin-orbit coupling on the Andreev level spectra. Our model incorporates the penetration of the magnetic field in the proximitized wires, which substantially modifies the spectra. We pay special attention to the occurrence of fermion-parity switches at increasing values of the field and to the magnetic field dependence of the absorption strength of microwave-induced transitions. Our calculations can be used to extract quantitative information from microwave and tunneling spectroscopy experiments, such as the recently reported measurements in Van Woerkom et al. [Nat. Phys. (2017), doi:10.1038/nphys4150].

The method based on fast Fourier transforms proposed by G. Román-Pérez and J. M. Soler [Phys. Rev. Lett. 103, 096102 (2009)], which allows for a computationally fast implementation of the nonlocal van der Waals (vdW) functionals, has significantly contributed to making the vdW functionals popular in solid-state physics. However, the Román-Pérez-Soler method relies on a plane-wave expansion of the electron density; therefore it cannot be applied readily to all-electron densities for which an unaffordable number of plane waves would be required for an accurate expansion. In this work, we present the results for the lattice constant and binding energy of solids that were obtained by applying a smoothing procedure to the all-electron density calculated with the linearized augmented plane-wave method. The smoothing procedure has the advantages of being very simple to implement, basis-set independent, and allowing the calculation of the potential. It is also shown that the results agree very well with those from the literature that were obtained with the projector augmented wave method.

The magnetic structure of the in-plane skyrmions in epitaxial MnSi/Si(111) thin films is probed in three dimensions by the combination of polarized neutron reflectometry (PNR) and small-angle neutron scattering (SANS). We demonstrate that skyrmions exist in a region of the phase diagram above a temperature of 10 K. PNR shows the skyrmions are confined to the middle of the film due to the potential well formed by the surface twists. However, SANS shows that there is considerable disorder within the plane indicating that the magnetic structure is a two-dimensional skyrmion glass.

Using Brillouin light scattering microscopy and micromagnetic simulations, we study the propagation and transformation of magnetostatic spin waves across uniformly biased curved magnonic waveguides. Our results demonstrate that the spin wave transmission through the bend can be enhanced or weakened by modifying the distribution of the inhomogeneous internal magnetic field spanning the structure. Our results open up the possibility of optimally molding the flow of spin waves across networks of magnonic waveguides, thereby representing a step forward in the design and construction of the more complex magnonic circuitry.

We present a comprehensive first-principles study of ${\mathrm{BaTiO}}_3$ ultrathin films epitaxially grown on Ge(001). We recently reported on the experimental realization of this system and analyzed the $2\times 1$ structural distortions in the BTO thin film which may give rise to technologically relevant functional properties [D. P. Kumah et al., Phys. Rev. Lett. 116, 106101 (2016)]. In this work, we describe the structural and electronic properties of the experimentally observed interface configuration, as well as a distinct metastable interface configuration with a higher out-of-plane polarization. We show that these two distinct interface structures can be made energetically degenerate by choosing a top electrode with an appropriate work function, thus enabling, in principle, an epitaxial ferroelectric thin film oxide. We analyze the interface chemistry and electronic structure and show that in the two polarization states the bands align differently, indicating a strong ferroelectric field effect. We also show that, surprisingly, in the intrinsic limit for the semiconductor, switching the oxide polarization state can cause the dominant charge carrier to switch between electrons and holes. The coupling of ferroelectric switching in the oxide with charge carrier type modulation in the semiconductor may have novel technological applications.

The purpose of the paper is to gain deeper insight into microscopic formation of the Dzyaloshinskii-Moriya interaction (DMI). The paper aims at the development of the physical picture able to address apparently contradicting conclusions of recent studies concerning the location of the DMI energy in the real and reciprocal spaces as well as the relation between values of the atomic moments and the DMI strength. The main tools of our study are the first-principles calculations of the energies of the spiral magnetic states with opposite chiralities. We suggest a method of the calculation of the spiral structures with account for the spin-orbit coupling (SOC). It is based on the application of the generalized Bloch theorem and generalized Bloch functions and allows to reduce the consideration of arbitrary incommensurate spiral to small chemical unit cell. The method neglects the anisotropy in the plane orthogonal to the rotation axis of the spirals that does not influence importantly the DMI energy. For comparison, the supercell calculation with full account for the SOC is performed. The concrete calculations are performed for the Co/Pt bilayer. We consider the distribution of the DMI energy in both real and reciprocal spaces and the dependence of the DMI on the number of electrons. The results of the calculations reveal a number of energy compensations in the formation of the DMI. Thus, the partial atomic contributions as functions of the spiral wave vector $\mathbf{q}$ are nonmonotonic and have strongly varying slopes. However, in the total DMI energy these atom-related features compensate each other, resulting in a smooth $\mathbf{q}$ dependence. The reason for the peculiar form of the partial DMI contributions is a $\mathbf{q}$-dependent difference in the charge distribution between $\mathbf{q}$ and $-\mathbf{q}$ spirals. The strongly $\mathbf{q}$-dependent relation between atomic contributions shows that the real-space distribution of the DMI energy obtained for a selected $\mathbf{q}$ value cannot be considered as a general characteristic of a given material. Our study shows that it is physically most consistent to consider the electronic hybridization as a primary effect reflecting the nature of the DMI whereas the $\mathbf{q}$-dependent real-space distribution of the DMI energy is a consequence of the complex processes in the electronic structure including the charge transfer process. The physical process of the DMI formation is connected with the difference in the hybridization of the Co and Pt states for $\mathbf{q}$ and $-\mathbf{q}$ spirals under the influence of the SOC and broken spatial inversion. It depends sensitively on details of the electronic structure. The calculations with constraints on the values of the Co and Pt atomic moments show that there is no direct relation between these atomic quantities and the DMI strength since the details of the electronic structure crucial for the DMI are not reflected in these integral characteristics. The application of the method to the calculation of the magnon energies in systems with DMI is briefly addressed.

Two-dimensional electron gases with strong spin-orbit coupling covered by a superconducting layer offer a flexible and potentially scalable platform for Majorana networks. We predict Majorana bound states (MBSs) to appear for experimentally achievable parameters and realistic gate potentials in two designs: either underneath a narrow stripe of a superconducting layer (S stripes) or where a narrow stripe has been removed from a uniform layer (N stripes). The coupling of the MBSs can be tuned for both types in a wide range ($<1\mathrm{neV}$ to $>10\mu \mathrm{eV}$) using gates placed adjacent to the stripes. For both types, we numerically compute the local density of states for two parallel Majorana-stripe ends as well as Majorana trijunctions formed in a tuning-fork geometry. The MBS coupling between parallel Majorana stripes can be suppressed below 1 neV for potential barriers in the meV range for separations of about 200 nm. We further show that the MBS couplings in a trijunction can be gate controlled in a range similar to the intrastripe coupling while maintaining a sizable gap to the excited states (tens of $\mu \mathrm{eV}$). Altogether, this suggests that braiding can carried out on a time scale of 10–100 ns.

We operate a resonant exchange qubit in a highly symmetric triple-dot configuration using IQ-modulated rf pulses. We find that the qubit splitting is an order of magnitude less sensitive to all relevant control voltages, compared to the conventional operating point, but we observe no significant improvement in the quality of Rabi oscillations. For weak driving this is consistent with Overhauser field fluctuations modulating the qubit splitting. For strong driving we infer that effective voltage noise modulates the coupling strength between rf drive and the qubit, thereby quickening Rabi decay. Application of CPMG dynamical decoupling sequences consisting of up to 32 $\pi$ pulses significantly prolongs qubit coherence, leading to marginally longer dephasing times in the symmetric configuration. This is consistent with dynamical decoupling from low frequency noise, but quantitatively cannot be explained by effective gate voltage noise and Overhauser field fluctuations alone. Our results inform recent strategies for the utilization of symmetric configurations in the operation of triple-dot qubits.

At small momenta, the Girvin-MacDonald-Platzman (GMP) mode in the fractional quantum Hall (FQH) effect can be identified with gapped nematic fluctuations in the isotropic FQH liquid. This correspondence would be exact as the GMP mode softens upon approach to the putative point of a quantum phase transition to a FQH nematic. Motivated by these considerations as well as by suggestive evidence of an FQH nematic in tilted field experiments, we have sought evidence of such a nematic FQHE in a microscopic model of interacting electrons in the lowest Landau level at filling factor 1/3. Using a family of anisotropic Laughlin states as trial wave functions, we find a continuous quantum phase transition between the isotropic Laughlin liquid and the FQH nematic. Results of numerical exact diagonalization also suggest that rotational symmetry is spontaneously broken, and that the phase diagram of the model contains both a nematic and a stripe phase.

A Weyl semimetal with broken time-reversal symmetry has a minimum of two species of Weyl fermions, distinguished by their opposite chirality, in a pair of Weyl cones at opposite momenta $\pm K$ that are displaced in the direction of the magnetization. Andreev reflection at the interface between a Weyl semimetal in the normal state (N) and a superconductor (S) that pairs $\pm K$ must involve a switch of chirality, otherwise it is blocked. We show that this “chirality blockade” suppresses the superconducting proximity effect when the magnetization lies in the plane of the NS interface. A Zeeman field at the interface can provide the necessary chirality switch and activate Andreev reflection.

We show that $(3+1)$-dimensional topological phases of matter generically support loop excitations with topological degeneracy. The loops carry “Cheshire charge”: topological charge that is not the integral of a locally defined topological charge density. Cheshire charge has previously been discussed in non-Abelian gauge theories, but we show that it is a generic feature of all (3+1)-D topological phases (even those constructed from an Abelian gauge group). Indeed, Cheshire charge is closely related to nontrivial three-loop braiding. We use a dimensional reduction argument to compute the topological degeneracy of loop excitations in the $(3+1)$-dimensional topological phases associated with Dijkgraaf-Witten gauge theories. We explicitly construct membrane operators associated with such excitations in soluble microscopic lattice models in ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ Dijkgraaf-Witten phases and generalize this construction to arbitrary membrane-net models. We explain why these loop excitations are the objects in the braided fusion 2-category $Z\left(\mathbf{2}{\mathbf{Vect}}_G^{\omega }\right)$, thereby supporting the hypothesis that 2-categories are the correct mathematical framework for $(3+1)$-dimensional topological phases.

We study the finite-size spectrum of the O($N$)-symmetric Wilson-Fisher conformal field theory (CFT) on the ($d=2$)-spatial-dimension torus using the expansion in $\epsilon =3-d$. This is done by deriving a set of universal effective Hamiltonians describing fluctuations of the zero-momentum modes. The effective Hamiltonians take the form of $N$-dimensional quantum anharmonic oscillators, which are shown to be strongly coupled at the critical point for small $\epsilon$. The low-energy spectrum is solved numerically for $N=1,2,3,4$. Using exact diagonalization, we also numerically study explicit lattice models known to be in the O(2) and O(3) universality class, obtaining estimates of the low-lying critical spectrum. The analytic and numerical results show excellent agreement and the critical low-energy torus spectra are qualitatively different among the studied CFTs, identifying them as a useful fingerprint for detecting the universality class of a quantum critical point.

In the context of the relaxation time approximation to Boltzmann transport theory, we examine the behavior of the Hall number $n_H$ of a metal in the neighborhood of a Lifshitz transition from a closed Fermi surface to open sheets. We find a universal nonanalytic dependence of $n_H$ on the electron density in the high-field limit, but a nonsingular dependence at low fields. The existence of an assumed nematic transition produces a doping dependent $n_H$ similar to that observed in recent experiments in the high-temperature superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$.

The many-body localized (MBL) phase is characterized by a complete set of quasilocal integrals of motion and area-law entanglement of excited eigenstates. We study the effect of non-Abelian continuous symmetries on MBL, considering the case of $\text{SU}\left(2\right)$ symmetric disordered spin chains. The $\text{SU}\left(2\right)$ symmetry imposes strong constraints on the entanglement structure of the eigenstates, precluding conventional MBL. We construct a fixed-point Hamiltonian, which realizes a nonergodic (but non-MBL) phase characterized by eigenstates having logarithmic scaling of entanglement with the system size, as well as an incomplete set of quasilocal integrals of motion. We study the response of such a phase to local symmetric perturbations, finding that even weak perturbations induce multispin resonances. We conclude that the nonergodic phase is generally unstable and that $\text{SU}\left(2\right)$ symmetry implies thermalization. The approach introduced in this Rapid Communication can be used to study dynamics in disordered systems with non-Abelian symmetries, and provides a starting point for searching nonergodic phases beyond conventional MBL.

We used resonant inelastic x-ray scattering (RIXS) at the Ni $L_3$ edge to measure the dispersion of spin waves in NiO thin films along the [101], [001], and [111] directions. Samples with tensile and compressive in-plane strain show identical dispersion within the experimental uncertainty. The fitting of the data with a linear spin wave model applied to a three-dimensional Heisenberg antiferromagnetic lattice provides a leading superexchange parameter $J^{\prime }=18\mathrm{meV}$. The magnon energy at the Brillouin zone boundary and the value of $J^{\prime }$ are $5\%$ smaller than those determined by inelastic neutron scattering on bulk single crystals. This discrepancy is likely induced by the strain or other structural differences between bulk and epitaxially grown samples. These results demonstrate the capabilities of high-resolution RIXS in the study of the magnetic structure of thin films and heterostructures for which neutron scattering is not sensitive enough.