We study the isotope effect on the temperature of the proton order/disorder phase transition between ice XI and ice Ih, using the quasiharmonic approximation combined with ab initio density functional theory calculations. We show that this method is accurate enough to obtain a phase-transition temperature difference between light ice (${\mathrm{H}}_2\mathrm{O}$) and heavy ice (${\mathrm{D}}_2\mathrm{O}$) of 6 K as compared to the experimental value of 4 K. More importantly, we are able to explain the origin of the isotope effect on the much-debated large temperature difference observed in the phase transition. The source of the difference is directly linked to the physics behind the anomalous isotope effect on the volume of hexagonal ice that was recently explained in Pamuk et al. [Phys. Rev. Lett. 108, 193003 (2012)]. These results indicate that the same physics might be behind the isotope effects in transition temperatures between other ice phases.

A pressure-induced phase transition, associated with the formation of cation-cation bonding, occurs in ${\mathrm{V}}_2{\mathrm{O}}_3$ by combining synchrotron x-ray diffraction in a diamond anvil cell and ab initio evolutionary calculations. The high-pressure phase has a monoclinic structure with a $C2/c$ space group, and it is both energetically and dynamically stable at pressures above 47 GPa to at least 105 GPa. This phase transition can be viewed as a two-dimensional Peierls-like distortion, where the cation-cation dimer chains are connected along the $c$ axis of the monoclinic cell. This finding provides insights into the interplay of electron correlation and lattice distortion in ${\mathrm{V}}_2{\mathrm{O}}_3$, and it may also help to understand novel properties of other early transition-metal oxides.

Infrared spectroscopy is used to observe the orientational fine structure arising from $\text{ortho}-{\mathrm{H}}_2$ adsorbed at the primary site of the microporous framework MOF-5. The $Q_1\left(1\right)$ vibrational transition shows at least two symmetrically spaced fine structure bands on either side of the main band. These grow in relative intensity with increasing ${\mathrm{H}}_2$ concentration indicative of interacting ${\mathrm{H}}_2$ pairs. This interpretation is strongly supported by ${\mathrm{D}}_2$ addition experiments, which cause a large increase in intensity of the fine structure bands with only minimal change in the main band. The spectra are analyzed in terms of ${\mathrm{H}}_2-{\mathrm{H}}_2$ electric quadrupole-quadrupole interactions. Consistent with this approach we observe no fine structure bands for the $Q_1\left(0\right)$ vibrational transition arising from $\text{para}-{\mathrm{H}}_2$, which does not possess a quadrupole moment.

Inspired by the recent theoretical suggestion that the random-bond $S=1/2$ antiferromagnetic Heisenberg model on the triangular and the kagome lattices might exhibit a randomness-induced quantum spin liquid (QSL) behavior when the strength of the randomness exceeds a critical value, and that this “random-singlet state” might be relevant to the QSL behaviors experimentally observed in triangular organic salts $\kappa \text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3\mathrm{and}{\mathrm{EtMe}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ and in kagome herbertsmithite ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$, we further investigate the nature of the static and the dynamical spin correlations of these models. We compute the static and the dynamical spin structure factors, $S\left(\mathbf{q}\right)$ and $S(\mathbf{q},\omega )$, by means of an exact diagonalization method. In both triangular and kagome models, the computed $S(\mathbf{q},\omega )$ in the random-singlet state depends on the wave vector $\mathbf{q}$ only weakly, robustly exhibiting gapless behaviors accompanied by the broad distribution extending to higher energy $\omega$. Especially in the strongly random kagome model, $S(\mathbf{q},\omega )$ hardly depends on $\mathbf{q}$, and exhibits an almost flat distribution for a wide range of $\omega$, together with a $\omega =0$ peak. These features agree semiquantitatively with the recent neutron-scattering data on a single-crystal herbertsmithite. Furthermore, the computed magnetization curve agrees almost quantitatively with the experimental one recently measured on a single-crystal herbertsmithite. These results suggest that the QSL state observed in herbertsmithite might indeed be the randomness-induced QSL state, i.e., the random-singlet state.

An anomalous Hall effect is shown to arise from the exchange interaction of conduction electrons with dipolar spin waves in ferromagnets. This effect exists even in homogeneous ferromagnets without relativistic spin-orbit coupling. The leading contribution to the Hall conductivity is proportional to the chiral spin correlation of dynamical spin textures and is physically understood in terms of the skew scattering by dipolar magnons.

We examine the nature of the transition to the antiferromagnetically ordered state in the half-filled three-dimensional Hubbard model using the dual-fermion multiscale approach. Consistent with analytics, in the weak-coupling regime we find that spin-flip excitations across the Fermi surface are important and that the strong-coupling regime is described by Heisenberg physics. In the intermediate-interaction, strong-correlation regime we find aspects of both local and nonlocal correlations. We analyze the critical exponents of the transition in the strong-coupling regime and find them to be consistent with Heisenberg physics down to an interaction of $U/t=10$.

High field electron spin resonance (ESR) and magnetization measurements reveal the crucial role of the strong spin-lattice coupling to generate the peculiar phase transitions in the chromium spinel oxide ${\mathrm{CdCr}}_2{\mathrm{O}}_4$, which possesses a spin-driven Jahn-Teller transition and a field-induced 1/2-magnetization plateau state. From our analysis of the ESR modes and the spin wave dispersion, which was observed from the previous neutron scattering studies, these magnetic properties are shown to originate from the modifications of the exchange interactions due to the lattice distortions. The evaluated exchange constants are examined by the magnetoelastic theory proposed by Penc $\mathit{et}\mathit{al}$.

We report the observation of current-induced spin torque resonance in yttrium iron garnet/platinum bilayers. An alternating charge current at $\mathrm{GHz}$ frequencies in the platinum gives rise to dc spin pumping and spin Hall magnetoresistance rectification voltages, induced by the Oersted fields of the ac current and the spin Hall effect-mediated spin transfer torque. In ultrathin yttrium iron garnet films, we observe spin transfer torque actuated magnetization dynamics which are significantly larger than those generated by the ac Oersted field. Spin transfer torques thus efficiently couple charge currents and magnetization dynamics also in magnetic insulators, enabling charge current-based interfacing of magnetic insulators with microwave devices.

The twin issues of the nature of the “normal” state and competing order(s) in the iron arsenides are central to understanding their unconventional, high-$T_c$ superconductivity. We use a combination of transport anisotropy measurements on detwinned $\mathrm{Sr}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ single crystals and local density approximation plus dynamical mean field theory (LDA + DMFT) calculations to revisit these issues. The peculiar resistivity anisotropy and its evolution with $x$ are naturally interpreted in terms of an underlying orbital-selective Mott transition (OSMT) that gaps out the $d_{xz}$ or $d_{yz}$ states. Further, we use a Landau-Ginzburg approach using LDA + DMFT input to rationalize a wide range of anomalies seen up to optimal doping, providing strong evidence for secondary electronic nematic order. These findings suggest that strong dynamical fluctuations linked to a marginal quantum-critical point associated with this OSMT and a secondary electronic nematic order constitute an intrinsically electronic pairing mechanism for superconductivity in Fe arsenides.

Using density functional theory, we study the electronic properties of a graphene-silicene bilayer (GSB). A single layer of silicene binds to the graphene layer with adhesion energy of about 25 meV/atom. This adhesion energy between the two layers follows accurately the well-known $-1/z^2$ dispersion energy as found between two infinite parallel plates. In small flakes of GSB with hydrogenated edges, negative charge is transferred from the graphene layer to the silicene layer, producing a permanent and a switchable polar bilayer, while in an infinite GSB, the negative charge is transferred from the silicene layer to the graphene layer. The graphene-silicene bilayer is a good candidate for a nanocapacitor with piezoelectric capabilities. We found that the permanent dipole of the bilayer can be tuned by an external perpendicular electric field.

The role of charge order in the phase diagram of high-temperature cuprate superconductors has been recently emphasized with the experimental discovery of an incipient bidirectional charge density wave (CDW) phase in the underdoped regime. In a subset of the experiments, the CDW has been found to be accompanied by a $d$-wave intra-unit-cell form factor, indicating modulation of charge density on the oxygen orbitals sandwiched between neighboring Cu atoms on the CuO planes [the so-called bond density wave (BDW) phase]. Here we take a mean field $Q_1=(2\pi /3,0)$ and $Q_2=(0,2\pi /3)$ bidirectional BDW phase with a $d$-wave form factor, which closely resembles the experimentally observed charge ordered states in underdoped cuprates, and we calculate the Fermi surface topology and the resulting quasiparticle Nernst coefficient as a function of temperature and doping. We establish that, in the appropriate doping ranges where the low-temperature phase (in the absence of superconductivity) is a BDW, the Fermi surface consists of electron and hole pockets, resulting in a low-temperature negative Nernst coefficient as observed in experiments.

An observable influence of zero-point fluctuations of the vacuum electromagnetic field on bound electrons is well known in the hydrogen atom, where it produces the Lamb shift. Here, we adapt an approach used to explain the Lamb shift in terms of a slight expansion of the orbits due to interaction with the zero-point field and apply it to assemblies of $N$ electrons that are modeled as independent atomically bound two-level systems. The effect is to stabilize a collective ground-state energy, which leads to a prediction of novel effects at room temperature for quasi-two-dimensional systems over a range of parameters in the model, namely, $N$, the two-level excitation energy $\hslash \omega$ and the ionization energy $\hslash \omega +\varepsilon$. Some mesoscopic systems where these effects may be observable include water sheaths on protein or DNA, surfaces of gaseous nanobubbles, and the magnetic response of inhomogeneous, electronically dilute oxides. No such effects are envisaged for uniform three-dimensional systems.

A hydrogen-related defect in ZnO which causes two broad IR absorption bands at 3303 and $3321 {\mathrm{cm}}^{-1}$ is studied by means of infrared absorption spectroscopy and first-principles theory. In deuterated samples, the defect reveals two sharp absorption lines at 2466 and $2488 {\mathrm{cm}}^{-1}$ accompanied by weaker sidebands at 2462 and $2480 {\mathrm{cm}}^{-1}$. Isotope substitution experiments with varying concentrations of H and D together with polarization-sensitive measurements strongly suggest that these IR absorption lines are due to stretch local vibrational modes of a defect comprising three equivalent hydrogen atoms. The zinc vacancy decorated by three hydrogen atoms, $V_{\mathrm{Zn}}{\mathrm{H}}_3$, and ammonia trapped at the zinc vacancy, $\left({\mathrm{NH}}_3\right){}_{\mathrm{Zn}}$, are discussed as a possible origin for the complex.

We numerically investigate the scaling properties of a one-dimensional driven-dissipative condensate described by a stochastic complex Ginzburg-Landau equation (SCGLE). We directly extract the static and dynamical scaling exponents from the dynamics of the condensate's phase field, and find that both coincide with the ones of the one-dimensional Kardar-Parisi-Zhang (KPZ) equation. We furthermore calculate the spatial and the temporal two-point correlation functions of the condensate field itself. The decay of the temporal two-point correlator assumes a stretched-exponential form, providing further quantitative evidence for an effective KPZ description. Moreover, we confirm the observability of this nonequilibrium scaling for typical current experimental setups with exciton-polariton systems, if cavities with a reduced $Q$ factor are used.

We report on the observation of spin whirls in a radially expanding polariton condensate formed under nonresonant optical excitation. Real space imaging of polarization- and time-resolved photoluminescence reveals a spiralling polarization pattern in the plane of the microcavity. Simulations of the spatiotemporal dynamics of a spinor condensate reveal the crucial role of polariton interactions with a spinor exciton reservoir. Harnessing spin-dependent interactions between the exciton reservoir and polariton condensates allows for the manipulation of spin currents and the realization of dynamic collective spin effects in solid-state systems.

We successfully identify individual Ge and Si atoms on intermixed $\mathrm{Ge}/\mathrm{Si}\left(111\right)-(7\times 7)$ surfaces by force spectroscopy using atomic force microscopy at room temperature. Tips with high chemical reactivity show distinct peaks derived from Ge and Si in the histograms of maximum attractive forces. The ratio of the maximum attractive force on Ge to that on Si takes a constant value of 0.84 independently of the tips. We apply the present method to $\mathrm{Ge}/\mathrm{Si}\left(111\right)-(5\times 5)$ surfaces to elucidate the elemental composition of the topmost layer.

Here we present both subnanometer imaging of three-dimensional (3D) hydration structures using atomic force microscopy (AFM) and molecular dynamics simulations of the calcite-water interface. In AFM, by scanning the 3D interfacial space in pure water and recording the force on the tip, a 3D force image can be produced, which can then be directly compared to the simulated 3D water density and forces on a model tip. Analyzing in depth the resemblance between experiment and simulation as a function of the tip-sample distance allowed us to clarify the contrast mechanism in the force images and the reason for their agreement with water density distributions. This work aims to form the theoretical basis for AFM imaging of hydration structures and enables its application to future studies on important interfacial processes at the molecular scale.

We introduce time-dependent, generalized factorial cumulants $C_s^m\left(t\right)$ of the full counting statistics of electron transfer as a tool to detect interactions in nanostructures. The violation of the sign criterion ${(-1)}^{m-1}{C}_s^m\left(t\right)\ge 0$ for any time $t$, order $m$, and parameter $s$ proves the presence of interactions. For given system parameters, there is a minimal time span $t_{\text{min}}$ and a minimal order $m$ to observe the violation of the sign criterion. We demonstrate that generalized factorial cumulants are more sensitive to interactions than ordinary ones and can detect interactions even in regimes where ordinary factorial cumulants fail. We illustrate our findings with the example of a quantum dot tunnel coupled to electronic reservoirs either in or out of equilibrium.

We propose an eigenoperator scheme to study the lattice model of interacting spinless fermions at half filling and show that this model possesses a hidden form of reflection positivity in its Majorana fermion representation. Based on this observation, we prove rigourously that the ground state of this model is either unique or doubly degenerate if the lattice size $N$ is even, and is always doubly degenerate if $N$ is odd. This proof holds in all dimensions with arbitrary lattice structures.