We calculate the wave-vector-dependent electronic spin susceptibility ${\chi }_{\alpha \beta }(\mathbf{q},{\mathbf{H}}_0)$ of a $d$-wave superconductor in uniform magnetic field ${\mathbf{H}}_0$ with Pauli pair-breaking. We find that the transverse component of the susceptibility tensor can be greater than its normal state value; the longitudinal component also slightly increases but in a very limited range of $q$'s. We identify several wave vectors $\{{\mathbf{q}}_{\perp },{\mathbf{q}}_{\parallel }\}$ that correspond to the maxima of either ${\chi }_{\perp }$ or ${\chi }_{\parallel }$. We compare our results with available data on the high-field phase in heavy-fermion CeCoIn${}_5$.

We develop a self-consistent relativistic disordered local moment (RDLM) scheme aimed at describing finite-temperature magnetism of itinerant metals from first principles. Our implementation in terms of the Korringa-Kohn-Rostoker multiple-scattering theory and the coherent potential approximation allows us to relate the orientational distribution of the spins to the electronic structure, thus a self-consistent treatment of the distribution is possible. We present applications for bulk bcc Fe, $L$1${}_0$-FePt, and FeRh ordered in the CsCl structure. The calculations for Fe show significant variation of the local moments with temperature, whereas according to the mean-field treatment of the spin fluctuations the Curie temperature is overestimated. The magnetic anisotropy of FePt alloys is found to depend strongly on intermixing between nominally Fe and Pt layers, and it shows a power-law behavior as a function of magnetization for a broad range of chemical disorder. In the case of FeRh we construct a lattice constant vs temperature phase diagram and determine the phase line of metamagnetic transitions based on self-consistent RDLM free-energy curves.

A novel state is predicted for a superconductor/ferromagnet (S/F) heterostructure. It is shown that the equilibrium S/F bilayer can be in a bistable state: one state is ground and the other is metastable. One of these states is always normal and the other is superconducting. One can switch between these states by an external control parameter, for example, by an applied magnetic field. This phenomenon can manifest itself through the hysteresis behavior upon varying the applied magnetic field or, even, temperature. It can also lead to transition of the heterostructure into the superconducting state upon increase of the temperature.

A path-integral Monte Carlo method based on the worm algorithm has been developed to compute the chemical potential of interacting bosonic quantum fluids. By applying it to finite-sized systems of liquid ${}^4\mathrm{He}$, we have confirmed that the chemical potential scales inversely with the number of particles to lowest order. The introduction of a simple scaling form allows for the extrapolation of the chemical potential to the thermodynamic limit, where we observe excellent agreement with known experimental results for ${}^4\mathrm{He}$ at saturated vapor pressure. We speculate on future applications of the proposed technique, including its use in studies of mixtures and confined quantum fluids.

The negatively charged silicon-vacancy (SiV${}^{-}$) center in diamond is a promising single-photon source for quantum communications and information processing. However, the center's implementation in such quantum technologies is hindered by contention surrounding its fundamental properties. Here we present optical polarization measurements of single centers in bulk diamond that resolve this state of contention and establish that the center has a $\langle 111\rangle$ aligned split-vacancy structure with ${\mathrm{D}}_{3\mathrm{d}}$ symmetry. Furthermore, we identify an additional electronic level and evidence for the presence of dynamic Jahn-Teller effects in the center's 738-nm optical resonance.

Motivated by the recent synthesis of $\beta$-Li${}_2$IrO${}_3$—a spin-orbit entangled $j=1/2$ Mott insulator with a three-dimensional lattice structure of the Ir${}^{4+}$ ions—we consider generalizations of the Kitaev model believed to capture some of the microscopic interactions between the iridium moments on various trivalent lattice structures in three spatial dimensions. Of particular interest is the so-called hyperoctagon lattice—the premedial lattice of the hyperkagome lattice, for which the ground state is a gapless quantum spin liquid where the gapless Majorana modes form an extended two-dimensional Majorana Fermi surface. We demonstrate that this Majorana Fermi surface is inherently protected by lattice symmetries and discuss possible instabilities. We thus provide the first example of an analytically tractable microscopic model of interacting SU(2) spin-$\frac{1}{2}$ degrees of freedom in three spatial dimensions that harbors a spin liquid with a two-dimensional spinon Fermi surface.

We report x-ray synchrotron experiments on epitaxial films of uranium, deposited on niobium and tungsten seed layers. Despite similar lattice parameters for these refractory metals, the uranium epitaxial arrangements are different and the strains propagated along the orthorhombic a axis of the uranium layers are of opposite sign. At low temperatures these changes in epitaxy result in dramatic modifications to the behavior of the charge-density wave in uranium. The differences are explained with the current theory for the electron-phonon coupling in the uranium lattice. Our results emphasize the intriguing possibilities of producing epitaxial films of elements that have complex structures like the light actinides uranium to plutonium.

By means of the first-principles calculations, we found that the early characterized 3D topological Dirac semimetal $P$6${}_3$/$mmc$-${\mathrm{Na}}_3\mathrm{Bi}$ is dynamically unstable at the ground state due to the presence of the large imaginary phonon frequencies around the $K$ point. Alternatively, our calculations suggested a new ground-state phase crystalizing in a $P$$\overline{3}$$c$1 structure with buckled graphite-like Na/Bi sheets, which is both energetically and dynamically stable. Moreover, the calculations also uncovered that the $P$$\overline{3}$$c$1 phase is a 3D topological Dirac semimetal, with exactly the same electronic states of the metastable $P$6${}_3$/$mmc$ phase.

In the crater function approach to the erosion of a solid surface by a broad ion beam, the average crater produced by the impact of an ion is used to compute the constant coefficients in the continuum equation of motion for the surface. We extend the crater function formalism so that it includes the dependence of the crater on the curvature of the surface at the point of impact. We then demonstrate that our formalism yields the correct coefficients for the Sigmund model of ion sputtering if terms up to second order in the spatial derivatives are retained. In contrast, if the curvature dependence of the crater is neglected, the coefficients can deviate substantially from their exact values. Our results show that accurately estimating the coefficients using craters obtained from molecular dynamics simulations will require significantly more computational power than was previously thought.

The structural properties of three compositions of Ge-Te liquids (${\mathrm{Ge}}_{10}{\mathrm{Te}}_{90}$, ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$, ${\mathrm{Ge}}_{20}{\mathrm{Te}}_{80}$) are studied from a combination of density functional based molecular dynamics simulations and neutron scattering experiments. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, neighbor distributions and compare our results with experimental findings. Most noticeable is the good agreement found in the reproduction of the structure in real and reciprocal space, resulting from the incorporation of dispersion forces in the simulation. This leads to Ge and Te coordination numbers which are lower than in previous studies and which can now be followed with temperature, while also strongly depending on the chosen cutoff distance. Results show a gradual conversion of higher coordinated species (Te${}^{\text{IV}}$, Ge${}^{\text{V}}$) into lower coordinated ones at lower temperature, while leaving anticipated coordinations from the octet rule (Te${}^{\text{II}}$ and Ge${}^{\text{IV}}$) nearly unchanged. Structural correlations are characterized as a function of temperature and composition. The vibrational density of states is also measured from inelastic neutron scattering for different compositions and temperatures, and compared to the simulated counterpart which exhibits a reasonable agreement at low frequency.

We present specific-heat and neutron-scattering results for the $S=\mathrm{1/2}$ quantum antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)${\mathrm{CuBr}}_4$. The material orders magnetically at ${\mathit{T}}_N=1.99\left(2\right)$ K, and magnetic excitations are accompanied by an energy gap of 0.30(2) meV due to spin anisotropy. The system is best described as coupled two-leg spin-1/2 ladders with the leg exchange $J_{\mathrm{leg}}=0.60\left(2\right)$ meV, rung exchange $J_{\mathrm{rung}}=0.64\left(9\right)$ meV, interladder exchange $J_{\mathrm{int}}=0.19\left(2\right)$ meV, and an interaction-anisotropy parameter $\lambda =0.93\left(2\right)$, according to inelastic neutron-scattering measurements. In contrast to most spin ladders reported to date, the material is a rare example in which the interladder coupling is very near the critical value required to drive the system to a Néel-ordered phase without the assistance of a magnetic field.

The presence of topological defects in magnetic media often leads to normal modes with zero frequency (zero modes). Such modes are crucial for long-time behavior, describing, for example, the motion of a domain wall as a whole. Conventional numerical methods to calculate the spin-wave spectrum in magnetic media are either inefficient or they fail for systems with zero modes. We present a new efficient computational scheme that reduces the magnetic normal-mode problem to a generalized Hermitian eigenvalue problem also in the presence of zero modes. We apply our scheme to several examples, including two-dimensional domain walls and Skyrmions, and show how the effective masses that determine the dynamics can be calculated directly. These systems highlight the fundamental distinction between the two types of zero modes that can occur in spin systems, which we call special and inertial zero modes. Whereas the inertial modes are generic Goldstone modes related to a broken continuous symmetry, the special modes arise naturally when two broken continuous symmetries coexist. Our method is suitable for both conservative and dissipative systems. For the latter case, we present a perturbative scheme to take into account damping, which can also be used to calculate dynamical susceptibilities.

Domains in noncentrosymmetric materials represent regions of different crystal structure and spin-orbit coupling. Twin boundaries separating such domains display unusual properties in noncentrosymmetric superconductors (NCSs), where magnetoelectric effects influence the local lower and upper critical magnetic fields. As a model system, we investigate NCSs with tetragonal crystal structure and Rashba spin-orbit coupling (RSOC), and with twin boundaries parallel to their basal planes. There, we report that there are two types of such twin boundaries which separate domains of opposite RSOC. In a magnetic field parallel to the basal plane, magnetoelectric coupling between the spin polarization and supercurrents induces an effective magnetic field at these twin boundaries. We show that this leads to unusual effects in such superconductors, and in particular to the modification of the upper and lower critical fields, in ways that depend on the type of twin boundary, as analyzed in detail, both analytically and numerically. Experimental implications of these effects are discussed.

London penetration depth, $\lambda \left(T\right)$, was measured in single crystals of ${\mathrm{K}}_{1-x}$${\mathrm{Na}}_x$${\mathrm{Fe}}_2$${\mathrm{As}}_2$, $x=0$ and 0.07, down to temperatures of 50 mK, $\sim T_c/50$. Isovalent substitution of Na for K significantly increases impurity scattering, with $\rho \left(T_c\right)$ rising from 0.2 to 2.2 $\mu \Omega$ cm, and leads to a suppression of $T_c$ from 3.5 to 2.8 K. At the same time, a close to $T$-linear $\Delta \lambda \left(T\right)$ in pure samples changes to almost $T^2$ in the substituted samples. The behavior never becomes exponential as expected for the accidental nodes, as opposed to $T^2$ dependence in superconductors with symmetry imposed line nodes. The superfluid density in the full temperature range follows a simple clean and dirty $d$-wave dependence, for pure and substituted samples, respectively. This result contradicts suggestions of multiband scenarios with strongly different gap structure on four sheets of the Fermi surface.

We perform numerical simulation of quantum turbulence at zero temperature generated by an oscillating sphere. In this simulation, we injected vortices on the sphere to generate turbulence. Although we prepare injected vortex loops of identical length, they are extended by the oscillating sphere to form a tangle through numerous reconnections. The resulting tangle around the sphere is anisotropic and affected by the oscillation. The vortex tangle continues to emit vortex loops, which leave the sphere. The statistics of emitted loops differs significantly from those of the original injected vortices. First, the sizes of the emitted loops are widely distributed, ranging from smaller to much larger than the size of the initial injected loop. Second, the propagation direction of the emitted loops exhibits anisotropy: Small loops move away almost isotropically, but large ones do so anisotropically along the oscillation direction of the sphere. Thus, the oscillating object stirs the initial injected vortices to reproduce a group of vortices with different statistics. Such physics is compared with the experiments of vibrating objects.

We use the quasiclassical Eilenberger theory to study the topological superconducting proximity effects between a segment of a nanowire with a $p$-wave order parameter and a metallic segment. This model faithfully represents key qualitative features of an experimental setup, where only a part of a nanowire is in immediate contact with a bulk superconductor, inducing topological superconductivity. It is shown that the Eilenberger equations represent a viable alternative to the Bogoliubov–de Gennes theory of the topological superconducting heterostructures and provide a much simpler quantitative description of some observables. For our setup, we obtain exact analytical solutions for the quasiclassical Green's functions and the density of states as a function of position and energy. The correlations induced by the boundary involve terms associated with both $p$-wave and odd-frequency pairing, which are intertwined and contribute to observables on an equal footing. We recover the signatures of the standard Majorana mode near the end of the superconducting segment, but find no such localized mode induced in the metallic segment. Instead, the zero-bias feature is spread out across the entire metallic part in accordance with the previous works. In shorter wires, the Majorana mode and delocalized peak split up away from zero energy. For long metallic segments, nontopological Andreev bound states appear and eventually merge together, giving rise to a gapless superconductor.

Inelastic neutron scattering (INS) experiments under applied magnetic field at low temperatures show detailed low-lying magnetic excitations in the so-called tridiminshed iron icosahedron magnetic molecule. The magnetic molecule consists of nine iron ${\mathrm{Fe}}^{3+}$ ($s=5/2$) and three phosphorous atoms that are situated on the 12 vertices of a nearly perfect icosahedron. The three phosphorous atoms form a plane that separates the iron cluster into two weakly coupled three- and six-ion fragments, $\left\{{\mathrm{Fe}}_3\right\}$ and $\left\{{\mathrm{Fe}}_6\right\}$, respectively. The magnetic field INS results exhibit an $S=1/2$ ground state expected from a perfect equilateral triangle of the $\left\{{\mathrm{Fe}}_3\right\}$ triad with a powder averaged $g$ value $=2.00$. Two sets of triplet excitations whose temperature and magnetic field dependence indicate an $S=0$ ground state with two nondegenerate $S=1$ states are attributed to the $\left\{{\mathrm{Fe}}_6\right\}$ fragment. The splitting may result from a finite coupling between the two fragments, single-ion anisotropy, antisymmetric exchange couplings, or from magnetic frustration of its triangular building blocks.

We investigate the nonresonant Raman response for iron-based superconductors using the framework of an effective $S_4$ model that was recently proposed to capture the essential electronic and magnetic properties of iron-based superconductors. We compute the momentum matrix elements and the resulting Raman vertices exactly for different polarization geometries and find that a simple $cos{k}_{x}cos{k}_y$ superconducting gap function is in good agreement with experimental data measured in both iron pnictides and iron chalcogenides. The Raman peaks are also matched quantitatively with the measured superconducting gaps by other experimental techniques. The result strengthens the validity of the $S_4$ model and the dominance of the $s$-wave pairing for iron-based superconductors.

Group-14 elements, including C, Si, Ge, and Sn, can form various stable and metastable structures. Finding new metastable structures of group-14 elements with desirable physical properties for new technological applications has attracted a lot of interest. Using a genetic algorithm, we discovered a new low-energy metastable distorted $s{p}^3$-hybridized framework structure of the group-14 elements. It has $P$$4_2/\mathit{mnm}$ symmetry with 12 atoms per unit cell. The void volume of this structure is as large as ${139.7\AA }^3$ for Si $P$$4_2/\mathit{mnm}$, and it can be used for gas or metal-atom encapsulation. Band-structure calculations show that $P$$4_2/\mathit{mnm}$ structures of Si and Ge are semiconducting with energy band gaps close to the optimal values for optoelectronic or photovoltaic applications. With metal-atom encapsulation, the $P$$4_2/\mathit{mnm}$ structure would also be a candidate for rattling-mediated superconducting or used as thermoelectric materials.