The low-energy magnetic excitations of the spin-$1/2$ kagome antiferromagnet ${\mathrm{CaCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2·0.6{\mathrm{H}}_2\mathrm{O}$ (Ca-kapellasite) have been investigated by a ${}^{35}\mathrm{Cl}$ NMR experiment in fields up to 18.9 T. Recently, Ca-kapellasite was found to be one of the most promising candidates to explore the intrinsic magnetic properties in kagome antiferromagnets, because magnetic defects, which deteriorate the intrinsic magnetic properties, are minimized by choosing Ca ions with a large ionic radius as the counterions. From our nuclear spin-lattice relaxation rate $1/T_1$ measurement, we found a power-law temperature dependence below the magnetic ordering temperature $T_M=7.2$ K, which leads us to suggest that gapless magnetic excitations survive even in the ordered state. This power-law behavior is suppressed in high magnetic fields. We discuss a possible magnetic state that can generate gapless excitations at low fields.

The question of the number of thermodynamic states present in the low-temperature phase of the three-dimensional Edwards-Anderson Ising spin glass is addressed by studying spin- and link-overlap distributions using population annealing Monte Carlo simulations. We consider overlaps between systems with the same boundary condition, which are the usual quantities measured, and also overlaps between systems with different boundary conditions, both for the full systems and also within a smaller window within the system. Our results appear to be fully compatible with a single pair of pure states such as in the droplet-scaling picture. However, our results for whether or not domain walls induced by changing boundary conditions are space filling or not are also compatible with scenarios having many thermodynamic states, such as the chaotic-pairs picture and the replica-symmetry-breaking picture. The differing results for spin overlaps in the same and different boundary conditions suggest that finite-size effects are very large for the system sizes currently accessible in low-temperature simulations.

In-plane resistivity measurements as a function of temperature and magnetic field up to 35 T with precise orientation within the crystallographic $ac$ plane were used to study the upper critical field $H_{c2}$ of the hole-doped iron-based superconductor ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. Compositions of the samples studied spanned from under- doped $x=0.17$ $(T_c=12$ K) and $x=0.22$ $(T_c=20$ K), both in the coexistence range of stripe magnetism and superconductivity, through optimal doping $x=0.39$ $(T_c=38.4$ K) and $x=0.47$ $(T_c=37.2$ K), to overdoped $x=0.65$ $(T_c=22$ K) and $x=0.83$ $(T_c=10$ K). We find notable doping asymmetry of the shapes of the anisotropic $H_{c2}\left(T\right)$, suggesting the important role of paramagnetic limiting effects in the $H\parallel a$ configuration in overdoped compositions and multiband effects in underdoped compositions.

We use unpolarized and polarized neutron scattering to study the temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped ${\mathrm{NaFe}}_{0.9785}{\mathrm{Co}}_{0.0215}\mathrm{As}$, with coexisting superconductivity $(T_{\mathrm{c}}\approx 19$ K) and weak antiferromagnetic order $(T_{\mathrm{N}}\approx 30$ K, ordered moment $\approx 0.02{\mu }_{\mathrm{B}}/\mathrm{Fe})$. A single spin resonance mode with intensity tracking the superconducting order parameter is observed, although energy of the mode only softens slightly upon approaching $T_{\mathrm{c}}$. Polarized neutron scattering reveals that the single resonance is mostly isotropic in spin space, similar to overdoped ${\mathrm{NaFe}}_{0.935}{\mathrm{Co}}_{0.045}\mathrm{As}$ but different from optimal electron-, hole-, and isovalently doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ compounds, all featuring an additional prominent anisotropic component. Spin anisotropy in ${\mathrm{NaFe}}_{0.9785}{\mathrm{Co}}_{0.0215}\mathrm{As}$ is instead present at energies below the resonance, which becomes partially gapped below $T_{\mathrm{c}}$, similar to the situation in optimally doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.9}$. Our results indicate that anisotropic spin fluctuations in ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ appear in the form of a resonance in the underdoped regime, become partially gapped below $T_{\mathrm{c}}$ near optimal doping, and disappear in overdoped compounds.

We introduce the concept of “hidden ladders” for the bilayer Ruddlesden-Popper-type compounds: While the crystal structure is bilayer, $d_{xz}$ ($d_{yz}$) orbitals in the relevant $t_{2g}$ sector of the transition metal form a two-leg ladder along $x$ ($y$) since the $d_{xz}$ ($d_{yz}$) electrons primarily hop in the leg ($x,y$) direction along with the rung ($z$) direction. This leads us to propose that ${\mathrm{Sr}}_3{\mathrm{Mo}}_2{\mathrm{O}}_7$ and ${\mathrm{Sr}}_3{\mathrm{Cr}}_2{\mathrm{O}}_7$ are candidates for the hidden-ladder material, with the correct position of $E_F$ from a first-principles band calculation. Based on the analysis of Eliashberg equation, we predict the possible occurrence of high-temperature superconductivity in these noncopper materials arising from the interband pair-scattering processes between a wide band and an “incipient” narrow band on the ladder.

We develop a unified theoretical picture for excitations in Mott systems, portraying both the heavy quasiparticle excitations and the Hubbard bands as features of an emergent Fermi liquid state formed in an extended Hilbert space, which is nonperturbatively connected to the physical system. This observation sheds light on the fact that even the incoherent excitations in strongly correlated matter often display a well-defined Bloch character, with pronounced momentum dispersion. Furthermore, it indicates that the Mott point can be viewed as a topological transition, where the number of distinct dispersing bands displays a sudden change at the critical point. Our results, obtained from an appropriate variational principle, display also remarkable quantitative accuracy. This opens an exciting avenue for fast realistic modeling of strongly correlated materials.

Defects between gapped boundaries provide a possible physical realization of projective non-Abelian braid statistics. A notable example is the projective Majorana/parafermion braid statistics of boundary defects in fractional quantum Hall/topological insulator and superconductor heterostructures. In this paper, we develop general theories to analyze the topological properties and projective braiding of boundary defects of topological phases of matter in two spatial dimensions. We present commuting Hamiltonians to realize defects between gapped boundaries in any $(2+1)D$ untwisted Dijkgraaf-Witten theory, and use these to describe their topological properties such as their quantum dimension. By modeling the algebraic structure of boundary defects through multifusion categories, we establish a bulk-edge correspondence between certain boundary defects and symmetry defects in the bulk. Even though it is not clear how to physically braid the defects, this correspondence elucidates the projective braid statistics for many classes of boundary defects, both amongst themselves and with bulk anyons. Specifically, three such classes of importance to condensed matter physics/topological quantum computation are studied in detail: (1) a boundary defect version of Majorana and parafermion zero modes, (2) a similar version of genons in bilayer theories, and (3) boundary defects in $\mathfrak{D}\left(S_3\right)$.

Obtaining strong magnetoelectric couplings in bulk materials and heterostructures is an ongoing challenge. We demonstrate that manganite heterostructures of the form (Insulator) $/{\left({\mathrm{LaMnO}}_3\right)}_{\mathrm{n}}/\mathrm{Interface}/{\left({\mathrm{CaMnO}}_3\right)}_{\mathrm{n}}/\left(\mathrm{Insulator}\right)$ show strong multiferroicity in magnetic manganites where ferroelectric polarization is realized by charges leaking from ${\mathrm{LaMnO}}_3$ to ${\mathrm{CaMnO}}_3$ due to repulsion. Here, an effective nearest-neighbor electron-electron (electron-hole) repulsion (attraction) is generated by cooperative electron-phonon interaction. Double exchange, when a particle virtually hops to its unoccupied neighboring site and back, produces magnetic polarons that polarize antiferromagnetic regions. Thus a striking giant magnetoelectric effect ensues when an external electrical field enhances the electron leakage across the interface.

${\mathbb{Z}}_4$ parafermions can be realized in a strongly interacting quantum spin Hall Josephson junction or in a spin Hall Josephson junction strongly coupled to an impurity spin. In this paper, we study a system that has both features, but with weak (repulsive) interactions and a weakly coupled spin. We show that for a strongly anisotropic exchange interaction, at low temperatures the system enters a strong coupling limit in which it hosts two ${\mathbb{Z}}_4$ parafermions, characterizing a fourfold degeneracy of the ground state. We construct the parafermion operators explicitly and show that they facilitate fractional $e/2$ charge tunneling across the junction. The dependence of the effective low-energy spectrum on the superconducting phase difference reveals an $8\pi$ periodicity of the supercurrent.

The $\mathit{ab}$-plane optical properties of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ single crystals have been measured over a wide temperature and frequency range. Below $T^{*}\simeq 155$ K, where this material undergoes an incoherent-coherent crossover, a new coherent response emerges in the optical conductivity. A spectral weight analysis suggests that this feature originates from high-energy bound states. Below $\simeq 75$ K the scattering rate for this feature is quadratic in temperature, indicating a Fermi-liquid response. Theoretical calculations suggest this crossover is dominated by the $d_{xy}$ orbital. Our results indicate Kondo-type screening is the likely mechanism for the incoherent-coherent crossover in hole-overdoped ${\mathrm{KFe}}_2{\mathrm{As}}_2$.

Spin-orbit coupled semiconducting nanowires with proximity-induced superconductivity are expected to host Majorana zero modes at their endpoints when a sufficiently strong magnetic field is applied. The resulting phase would be a one-dimensional topological superconductor. However, while a variety of experiments have been performed observing a zero-bias conductance peak (suggestive of Majorana zero modes), the topological nature of these physical systems is still a subject of debate. Here we suggest a quantitative test of the degree to which a system displaying a zero bias peak may be considered topological. The experiment is similar to previous measurements of conductance but is performed with the aid of a quantum dot at the wire's end. We arrive at the surprising result that the nonlocal behavior of a topological system may be tested through a local measurement.

We describe the nonequilibrium quench dynamics of the Sachdev-Ye-Kitaev models of fermions with random all-to-all interactions. These provide tractable models of the dynamics of quantum systems without quasiparticle excitations. The Kadanoff-Baym equations show that, at long times, the fermion two-point function has a thermal form at a final temperature determined by energy conservation, and the numerical analysis is consistent with a thermalization rate proportional to this temperature. We also obtain an exact analytic solution of the quench dynamics in the large $q$ limit of a model with $q$ fermion interactions: in this limit, the thermalization of the two-point function is instantaneous.

We numerically study the spin-$\frac{1}{2}$ breathing kagome lattice. In this variation of the kagome Heisenberg antiferromagnet, the spins belonging to upward and downward facing triangles have different coupling strengths. Using the density matrix renormalization group (DMRG) method and exact diagonalization, we show that the kagome antiferromagnet spin liquid is extremely robust to this anisotropy. Materials featuring this anisotropy—and especially the recently studied vanadium compound $[{\mathrm{NH}}_4{]}_2\left[{\mathbf{C}}_7{\mathbf{H}}_{14}\mathbf{N}\right]\left[{\mathbf{V}}_7{\mathbf{O}}_6{\mathbf{F}}_{18}\right]$ (DQVOF)—may thus be very good candidates to realize the much studied kagome spin liquid. Further, we closely examine the limit of strong breathing anisotropy and find indications of a transition to a nematic phase.

We report on temperature-dependent magnetotransport experiments of ${\mathrm{SmB}}_6$ single crystals in high magnetic fields up to 33 T. Above the low-temperature plateau region in the zero-field resistivity, we find two distinct gapped regimes. The first is characterized by a temperature-independent gap ${\mathrm{\Delta }}_2$ that closes in the presence of a high magnetic field while the second regime, above $\sim 8\mathrm{K}$, features an energy gap ${\mathrm{\Delta }}_1$ that is temperature dependent. In the entire temperature range, we observe an overall negative magnetoresistance that does not depend on the orientation of the crystalline plane with respect to the applied magnetic field with a maximum value at $T\sim 6.2\mathrm{K}$. The regime of in-gap states is characterized by a low mobility, an anomalously high carrier concentration, and an enhanced nonlocal resistance that exceeds the expected classical value for a two-dimensional metallic conductor. Overall, our data can be interpreted in terms of a transition from bulk to in-gap dominated transport, but evidence of topological surface states is not observed.

Density functional theory calculations are performed to explore the nature of the contact between metal electrodes and defected monolayer ${\mathrm{MoSe}}_2$. Partial Fermi level pinning is observed at perfect ${\mathrm{MoSe}}_2$/metal interfaces. Both As- and Br-substituted ${\mathrm{MoSe}}_2$ will induce extra bands in valence band and conduction band, respectively, which exerts influence on the Schottky barrier height. An enhanced partial Fermi level pinning occurs when As- and Br-substituted ${\mathrm{MoSe}}_2$ make contacts with metal electrodes. Se vacancy in the ${\mathrm{MoSe}}_2$ layer can induce a large amount of interfacial states in the band gap of the ${\mathrm{MoSe}}_2$ layer. As a result, nearly complete Fermi level pinning is observed in Se-vacancy ${\mathrm{MoSe}}_2$/metal contacts. Our work offers insight into the Fermi level pinning at the interfaces between two-dimensional materials and metal electrodes, which is important for the applications of two-dimensional materials in nanoelectronic devices with good performance.

Measurement of the Hugoniot and sound velocity provides information on the bulk modulus and Grüneisen parameter of a material at extreme conditions. The capability to launch multilayered (copper/aluminum) flyer plates at velocities in excess of 20 km/s with the Sandia Z accelerator has enabled high-precision sound-velocity measurements at previously inaccessible pressures. For these experiments, the sound velocity of the copper flyer must be accurately known in the multi-Mbar regime. Here we describe the development of copper as an absolutely calibrated sound-velocity standard for high-precision measurements at pressures in excess of 400 GPa. Using multilayered flyer plates, we performed absolute measurements of the Hugoniot and sound velocity of copper for pressures from 500 to 1200 GPa. These measurements enabled the determination of the Grüneisen parameter for dense liquid copper, clearly showing a density dependence above the melt transition. Combined with earlier data at lower pressures, these results constrain the sound velocity as a function of pressure, enabling the use of copper as a Hugoniot and sound-velocity standard for pressures up to 1200 GPa.

We propose a first-principles framework for longitudinal spin fluctuations (LSFs) in disordered paramagnetic (PM) multicomponent alloy systems and apply it to investigate the influence of LSFs on the temperature dependence of two elastic constants of PM austenitic stainless steel Fe15Cr15Ni. The magnetic model considers individual fluctuating moments in a static PM medium with first-principles-derived LSF energetics in conjunction with describing chemical disorder and randomness of the transverse magnetic component in the single-site alloy formalism and disordered local moment (DLM) picture. A temperature-sensitive mean magnetic moment is adopted to accurately represent the LSF state in the elastic-constant calculations. We make evident that magnetic interactions between an LSF impurity and the PM medium are weak in the present steel alloy. This allows gaining accurate LSF energetics and mean magnetic moments already through a perturbation from the static DLM moments instead of a tedious self-consistent procedure. We find that LSFs systematically lower the cubic shear elastic constants $c^{\prime }$ and $c_{44}$ by $\sim 6\mathrm{GPa}$ in the temperature interval 300–1600 K, whereas the predominant mechanism for the softening of both elastic constants with temperature is the magneto-volume coupling due to thermal lattice expansion. We find that non-negligible local magnetic moments of Cr and Ni are thermally induced by LSFs, but they exert only a small influence on the elastic properties. The proposed framework exhibits high flexibility in accurately accounting for finite-temperature magnetism and its impact on the mechanical properties of PM multicomponent alloys.

The study of the Berezinskii-Kosterlitz-Thouless transition in two-dimensional ${\left|\phi \right|}^4$ models can be performed in several representations, and the amplitude-phase (AP) Madelung parametrization is a natural way to study the contribution of density fluctuations to nonuniversal quantities. We introduce a functional renormalization group scheme in AP representation where amplitude fluctuations are integrated first to yield an effective sine-Gordon model with renormalized superfluid stiffness. By a mapping between the lattice $XY$ and continuum ${\left|\phi \right|}^4$ models, our method applies to both on equal footing. Our approach correctly reproduces the existence of a line of fixed points and of universal thermodynamics and it allows to estimate universal and nonuniversal quantities of the two models, finding good agreement with available Monte Carlo results. The presented approach is flexible enough to treat parameter ranges of experimental relevance.

The magnetic properties of ${\mathrm{Li}}_2{\mathrm{Cu}}_2\mathrm{O}{\left({\mathrm{SO}}_4\right)}_2$ are investigated in the framework of density functional theory. In its high-temperature tetragonal structure, this compound appears as a rare material realization of a frustrated spin-1/2 two-leg ladder, where magnetic frustration arises from competing nearest and next-nearest interactions along the legs. Through a large magnetoelastic coupling, the triclinic distortion occurring around 125 K is shown to induce the formation of a staggered dimer structure, lifting most of the magnetic frustration.