We investigate electronic excitations in ${\mathrm{La}}_{2-x}{(\mathrm{Br},\mathrm{Sr})}_x{\mathrm{CuO}}_4$ using resonant inelastic x-ray scattering (RIXS) at the oxygen $K$ edge. RIXS spectra of the hole-doped cuprates show clear momentum dependence below 1 eV. The broad spectral weight exhibits positive dispersion and shifts to higher energy with increasing hole concentration. Theoretical calculation of the dynamical charge structure factor on oxygen orbitals in a three-band Hubbard model is consistent with the experimental observation of the momentum and doping dependence, and therefore the momentum-dependent spectral weight is ascribed to intraband charge excitations which have been observed in electron-doped cuprates. Our results confirm that the momentum-dependent charge excitations exist on the order of the transfer energy ($t$), and the broad spectral line shape indicates damped and incoherent character of the charge excitations at the energy range in the doped Mott insulators.

We use resonant inelastic x-ray scattering (RIXS) at the Cu ${\mathrm{L}}_3$ edge to measure the charge and spin excitations in the “half-stuffed” Cu-O planes of the cuprate antiferromagnet ${\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_4{\mathrm{Cl}}_2$. The RIXS line shape reveals distinct contributions to the $dd$ excitations from the two structurally inequivalent Cu sites, which have different out-of-plane coordinations. The low-energy response exhibits magnetic excitations. We find a spin-wave branch whose dispersion follows the symmetry of a ${\mathrm{CuO}}_2$ sublattice, similar to the case of the “fully stuffed” planes of tetragonal CuO (T-CuO). Its bandwidth is closer to that of a typical cuprate material, such as ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2$, than it is to that of T-CuO. We interpret this result as arising from the absence of the effective four-spin inter-sublattice interactions that act to reduce the bandwidth in T-CuO.

Freezing of water is arguably one of the most common phase transitions on Earth and almost always happens heterogeneously. Despite its importance, we lack a fundamental understanding of what makes substrates efficient ice nucleators. Here we address this by computing the ice nucleation (IN) ability of numerous model hydroxylated substrates with diverse surface hydroxyl (OH) group arrangements. Overall, for the substrates considered, we find that neither the symmetry of the OH patterns nor the similarity between a substrate and ice correlate well with the IN ability. Instead, we find that the OH density and the substrate-water interaction strength are useful descriptors of a material's IN ability. This insight allows the rationalization of ice nucleation ability across a wide range of materials and can aid the search and design of novel potent ice nucleators in the future.

The random phase approximation (RPA) for total energies has previously been shown to provide a qualitatively correct description of static correlation in molecular systems, where density functional theory (DFT) with local functionals are bound to fail. This immediately poses the question of whether the RPA is also able to capture the correct physics of strongly correlated solids such as Mott insulators. Due to strong electron localization, magnetic interactions in such systems are dominated by superexchange, which in the simplest picture can be regarded as the analog of static correlation for molecules. In this paper, we investigate the performance of the RPA for evaluating both superexchange and direct exchange interactions in the magnetic solids NiO, MnO, ${\mathrm{Na}}_3{\mathrm{Cu}}_2{\mathrm{SbO}}_6,{\mathrm{Sr}}_2{\mathrm{CuO}}_3,{\mathrm{Sr}}_2{\mathrm{CuTeO}}_6$, and a monolayer of ${\mathrm{CrI}}_3$, which were chosen to represent a broad variety of magnetic interactions. It is found that the RPA can accurately correct the large errors introduced by Hartree-Fock, independent of the input orbitals used for the perturbative expansion. However, in most cases, accuracies similar to RPA can be obtained with DFT+U, which is significantly simpler from a computational point of view.

Time periodic modulations of the transverse field in the closed $XY$ spin-$\frac{1}{2}$ chain generate a very rich dynamical phase diagram, with a hierarchy of ${\mathbb{Z}}_n$ topological phases characterized by differing numbers of Floquet-Majorana modes. This rich phase diagram survives when the system is coupled to dissipative end reservoirs. Circumventing the obstacle of preparing and measuring quasienergy configurations endemic to Floquet-Majorana detection schemes, we show that stroboscopic heat transport and spin density are robust observables to detect both the dynamical phase transitions and Majorana modes in dissipative settings. We find that the heat current provides very clear signatures of these Floquet topological phase transitions. In particular, we observe that the derivative of the heat current, with respect to a control parameter, changes sign at the boundaries separating topological phases with differing nonzero numbers of Floquet-Majorana modes. We present a simple scheme to directly count the number of Floquet-Majorana modes in a phase from the Fourier transform of the local spin density profile. Our results are valid provided the anisotropies are not strong and can be easily implemented in quantum engineered systems.

We study the coupling between conventional (Maxwell) and emergent electrodynamics in quantum spin ice, a 3+1-dimensional $\mathrm{U}\left(1\right)$ quantum spin liquid. We find that a uniform electric field can be used to tune the properties of both the ground state and excitations of the spin liquid. In particular, it induces emergent birefringence, rendering the speed of the emergent light anisotropic and polarization-dependent. A sufficiently strong electric field triggers a quantum phase transition into new $\mathrm{U}\left(1\right)$ quantum spin liquid phases, which trap emergent electric $\pi$ fluxes. The flux patterns of these new phases depend on the direction of the electric field. Strikingly, some of the canonical pinch points in the spin structure factor, characteristic of classical spin ice, emerge near the phase transition, while they are absent in the quantum spin liquid phases. Estimating the electric field strength required, we find that this transition is potentially accessible experimentally. Finally, we propose a minimal mechanism by which an oscillating electric field can generate emergent radiation inside a quantum spin ice material with non-Kramers spin doublets.

Even at zero temperature dissipation reduces quantum fluctuations and tends to localize particles. A notable exception is the nonlinear dissipation due to quasiparticle tunneling in a Josephson junction. It is well known that quasiparticle dissipation does not suppress tunneling of the superconducting phase difference between next-nearest metastable phase states even though tunneling to the nearest phase state is suppressed. The reason is that the dissipative action admits an instanton solution, the so-called Korshunov instanton. Here, we analyze this model at elevated bias current $I$. We find that besides the known regime where the logarithm of the tunneling rate scales as $I^{2/3}$ there is a novel regime with a scaling $I^2$. We argue that the increased tunneling rate that derives from the elevated bias current is favorable for experimental verification of the Korshunov instantons.

Ultrafast optical pump–optical probe and optical pump–terahertz probe spectroscopy were performed on vanadium dioxide (${\mathrm{VO}}_2$) and vanadium sesquioxide (${\mathrm{V}}_2{\mathrm{O}}_3$) thin films over a wide temperature range. A comparison of the experimental data from these two different techniques and two different vanadium oxides, in particular a comparison of the spectral weight oscillations generated by the photoinduced longitudinal acoustic modulation, reveals the strong electron-phonon coupling that exists in both materials. The low-energy Drude response of ${\mathrm{V}}_2{\mathrm{O}}_3$ appears more amenable than ${\mathrm{VO}}_2$ to ultrafast strain control. Additionally, our results provide a measurement of the temperature dependence of the sound velocity in both systems, revealing a four- to fivefold increase in ${\mathrm{VO}}_2$ and a three- to fivefold increase in ${\mathrm{V}}_2{\mathrm{O}}_3$ across the insulator-to-metal phase transition. Our data also confirm observations of strong damping and phonon anharmonicity in the metallic phase of ${\mathrm{VO}}_2$, and suggest that a similar phenomenon might be at play in the metallic phase of ${\mathrm{V}}_2{\mathrm{O}}_3$. More generally, our simple table-top approach provides relevant and detailed information about dynamical lattice properties of vanadium oxides, paving the way to similar studies in other complex materials.

We bring together magnetic circular dichroism, photoconductivity, and complementary first-principles calculations in order to unravel spin-charge interactions in the high Curie temperature magnet ${\mathrm{NiFe}}_2{\mathrm{O}}_4$. Analysis uncovers a massive set of well-isolated spin-down states, a metamagnetic transition involving spin on the Ni center that switches the electronic structure of this system, and photoconductivity that depends on the magnetic field. These findings open the door for the creation and control of spin-polarized excitations from minority channel charge transfer in spinel ferrites.

We present a simple and fast method to simulate spin-torque driven magnetization dynamics in nanopillar spin-valve structures. The approach is based on the coupling between a spin transport code based on random matrix theory and a micromagnetics finite-elements software. In this way the spatial dependence of both spin transport and magnetization dynamics is properly taken into account. Our results are compared with experiments. The excitation of the spin-wave modes, including the threshold current for steady-state magnetization precession and the nonlinear frequency shift of the modes are reproduced correctly. The giant magneto resistance effect and the magnetization switching also agree with experiment. The similarities with recently described spin-caloritronics devices are also discussed.

Lead-doped barium bismuthate is intriguing because of its relatively high critical temperature and the nature of its superconductivity in the vicinity of a charge-density-wave-ordered topological insulator. Here we present bilayers of barium bismuthate and barium leadate showing emergent superconductivity as a function of the barium leadate top layer thickness. Magnetotransport studies allow for the characterization of the superconducting properties in these bilayers to be two-dimensional. In addition current-voltage characteristics of superconducting bilayers reveal signatures of a Berezinskiĭ-Kosterlitz-Thouless transition. The particular observed dependence of the superconducting transition temperature on the thickness of the barium leadate top layer suggests the formation of the superconducting state originating from the interface.

We demonstrate sharp thermally induced switching between ferromagnetic and antiferromagnetic RKKY (Ruderman-Kittel-Kasuya-Yosida) exchange in a spin-valve with the spacer incorporating a thin diluted ferromagnetic layer as the core. We illustrate the mechanism behind the effect as being due to a change in the effective thickness of the spacer induced by the Curie transition into its paramagnetic state. The ability to switch between ferromagnetic and antiferromagnetic states in a magnetic multilayer by a slight change in temperature may lead to new types of spin-thermoelectronic devices for use in such applications as memory or oscillators.

${\mathrm{Cs}}_2{\mathrm{FeCl}}_5·{\mathrm{D}}_2\mathrm{O}$ has been identified as a linear magnetoelectric material, although the correlation of this property with the magnetic structures of this compound has not been adequately studied. We have used single-crystal and powder neutron diffraction to obtain detailed information about its nuclear and magnetic structures. From the nuclear structure analysis, we describe the occurrence of a phase transition related to the reorganization of the ${[\mathrm{FeCl}}_5·{\mathrm{D}}_2{\mathrm{O}]}^{-2}$ ions and the ${\mathrm{Cs}}^{+}$ counterion. The magnetic structure was determined at zero magnetic field at 1.8 K using single-crystal diffraction and its temperature evolution was recorded using powder diffraction. The symmetry analysis of the magnetic structure is compatible with the occurrence of the magnetoelectric effect. Moreover, the evolution of the magnetic structure as a function of the external magnetic field has also been studied. The reorientation of the magnetic moments under applied external field along the easy axis ($b$ axis at low temperature) is compatible with the occurrence of a spin-flop transition. The application of a magnetic field below $T_N$ compels the magnetic moments to flip from the $b$ axis to the $ac$ plane (with a small induced component along the $b$ axis), for a critical magnetic field of ca. 1.2 T.

We investigated magnetic and thermodynamic properties of $S=\frac{1}{2}$ quasi-one-dimensional antiferromagnet ${\mathrm{KCuMoO}}_4\left(\mathrm{OH}\right)$ through single-crystalline magnetization and heat capacity measurements. At zero field, it behaves as a uniform $S=\frac{1}{2}$ Heisenberg antiferromagnet with $J=238\mathrm{K}$, and exhibits a canted antiferromagnetism below $T_{\mathrm{N}}=1.52\mathrm{K}$. In addition, a magnetic field $H$ induces the anisotropy in magnetization and opens a gap in the spin-excitation spectrum. These properties are understood in terms of an effective staggered field induced by staggered $g$ tensors and Dzyaloshinsky-Moriya (DM) interactions. Temperature dependencies of the heat capacity and their field variations are consistent with those expected for quantum sine-Gordon model, indicating that spin excitations consist of soliton, antisoliton, and breather modes. From field dependencies of the soliton mass, the staggered field normalized by the uniform field $c_{\mathrm{s}}$ is estimated as 0.041, 0.174, and 0.030, for $H\parallel a$, $b$, and $c$, respectively. Such a large variation of $c_{\mathrm{s}}$ is understood as the combination of staggered $g$ tensors and DM interactions which induce the staggered field in the opposite direction for $H\parallel a$ and $c$ but almost the same direction for $H\parallel b$ at each Cu site.

We use exact diagonalization to study the eigenstate thermalization hypothesis (ETH) in the quantum dimer model on the square and triangular lattices. Due to the nonergodicity of the local plaquette-flip dynamics, the Hilbert space, which consists of highly constrained close-packed dimer configurations, splits into sectors characterized by topological invariants. We show that this has important consequences for ETH: We find that ETH is clearly satisfied only when each topological sector is treated separately, and only for moderate ratios of the potential and kinetic terms in the Hamiltonian. By contrast, when the spectrum is treated as a whole, ETH breaks down on the square lattice and, apparently, also on the triangular lattice. These results demonstrate that quantum dimer models have interesting thermalization dynamics.

Ab initio atomic relaxations often take large numbers of steps and long times to converge, especially when the initial atomic configurations are far from the local minimum or there are curved and narrow valleys in the multidimensional potentials. An atomic relaxation method based on on-the-flight force learning and a corresponding curved-line search algorithm is presented to accelerate this process. Results demonstrate the superior performance of this method for metal and magnetic clusters when compared with the conventional conjugate-gradient method.

We report a study of the magnetic field dependence of photoluminescence of ${\mathrm{NV}}^{-}$ centers (negatively charged nitrogen vacancy centers) in diamond single crystals. In such a magnetic field dependence characteristic sharp features are observed, which are coming from level anticrossings (LACs) in a coupled electron-nuclear spin system. For studying such LAC lines we use sensitive lock-in detection to measure the photoluminescence intensity. This experimental technique allows us to obtain new LAC lines. Additionally, a remarkably strong dependence of the LAC lines on the modulation frequency is found. Specifically, upon decrease of the modulation frequency from 12 kHz to 17 Hz the amplitude of the LAC lines increases by approximately two orders of magnitude. To take a quantitative account for such effects, we present a theoretical model, which describes the spin dynamics in a coupled electron-nuclear spin system under the action of an oscillating external magnetic field. Good agreement between experiments and theory allows us to conclude that the observed effects are originating from coherent spin polarization exchange in a coupled spin system comprising the spin-polarized ${\mathrm{NV}}^{-}$ center. Our results are of great practical importance allowing one to optimize the experimental conditions for probing LAC-derived lines in diamond crystals comprising ${\mathrm{NV}}^{-}$ centers and for indirect detection and identification of other paramagnetic defect centers.

We report on the results of a de Haas–van Alphen (dHvA) measurement performed on the recently discovered antiferromagnet ${\mathrm{URhIn}}_5$ ($T_N=98$ K), a $5f$ analog of the well studied heavy fermion antiferromagnet ${\mathrm{CeRhIn}}_5$. We observe two small Fermi surfaces: a roughly spherical pocket $\beta$, with $F_{\beta }\simeq 0.3$ kT, and a pillow-shaped closed surface, $\alpha$, with $F_{\alpha }\simeq 1.1$ kT. In addition, we observe two higher frequencies ${\gamma }_1$ with $F_{{\gamma }_1}\simeq 3.2$ kT and ${\gamma }_2$ with $F_{{\gamma }_2}\simeq 3.5$ kT that are seen only near the $c$ axis, and that may arise on cylindrical Fermi surfaces. The measured cyclotron masses range from $1.9m_e$ to $4.3m_e$. A simple LDA+SO calculation performed for the paramagnetic ground state shows a very different Fermi surface topology, demonstrating a need for more advanced electronic structure calculations.

Specific heat $C$, thermal conductivity κ, dielectric permittivity ε, electric polarization $P$, and Raman scattering experiments are performed on $\mathrm{C}{\mathrm{u}}_3\mathrm{Bi}{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_2{\mathrm{O}}_{2}X$ ($X=\mathrm{Br},\mathrm{Cl}$) single crystals. The Cl compound undergoes a structural phase transition at $T^{*}\sim 115\mathrm{K}$ evident in $C\left(T\right)$, $\varepsilon \left(T\right)$, and $\kappa \left(T\right)$ and accompanied by the appearance of unique phonon lines in Raman scattering. No evident structural changes are detected in the Br compound. At $T<T^{*}$, a very weak polarization loop with a $P\parallel c$ axis is observed in the Cl compound. Both compounds order antiferromagnetically at comparable temperatures $T_{\mathrm{N}}\sim 25\mathrm{K}$ marked by sharp λ-type singularities. For $T<T_{\mathrm{N}}$, an intensive mode of magnetic origin appears in both compounds. At the lowest temperatures, the energy of this mode is in good agreement with both the magnon excitation observed in infrared spectroscopy and the spin gap found recently in inelastic neutron scattering of the Cl compound.