The spin excitations in the spin-liquid phase of the anisotropic triangular lattice quantum antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ have been shown to propagate dominantly along the crystallographic $b$ axis. To test this dimensional reduction scenario, we have performed ultrasound experiments in the spin-liquid phase of ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ probing the elastic constant $c_{22}$ and the sound attenuation along the $b$ axis as a function of an external magnetic field along the $a$ axis. We show that our data can be quantitatively explained within the framework of a nearest-neighbor spin-$1/2$ Heisenberg chain, where fermions are introduced via the Jordan-Wigner transformation and the spin-phonon interaction arises from the usual exchange-striction mechanism.

Electronic transport in organic semiconductors is mediated by localized polarons. However, the dynamics on how delocalized electrons collapse into polarons through electron-nuclear interaction is not well known. In this work, we use time- and angle-resolved photoemission spectroscopy to study polaron formation in titanyl phthalocyanine deposited on $\mathrm{Au}\left(111\right)$ surfaces. Electrons are optically excited from the metal to the organic layer via the image potential state, which evolves from a dispersive to a nondispersive state after photoexcitation. The spatial size of the electrons is determined from the band structure using a tight-binding model. It is observed that the two-dimensional electron wave collapses into a wave packet of size ∼3 nm within 100 fs after photoexcitation.

We report low-temperature electronic transport results on the fractional quantum Hall effect of composite fermions at Landau level filling $\nu =4/11$ in a very high mobility and low density sample. Measurements were carried out at temperatures down to $15\mathrm{mK}$, where an activated magnetoresistance $R_{xx}$ and a quantized Hall resistance $R_{xy}$, within $1\%$ of the expected value of $h/(4/11)e^2$, were observed. The temperature dependence of the $R_{xx}$ minimum at $4/11$ yields an activation energy gap of $\sim 7\mathrm{mK}$. Developing Hall plateaus were also observed at the neighboring states at $\nu =3/8\mathrm{and}5/13$.

Valley polarization in graphene breaks inversion symmetry and therefore leads to second-harmonic generation. We present a complete theory of this effect within a single-particle approximation. It is shown that this may be a sensitive tool to measure the valley polarization created, e.g., by polarized light and, thus, can be used for the development of ultrafast valleytronics in graphene.

While the integer quantum Hall effect of composite fermions manifests as the prominent fractional quantum Hall effect (FQHE) of electrons, the FQHE of composite fermions produces further, more delicate states, arising from a weak residual interaction between composite fermions. We study the spin phase diagram of these states, motivated by the recent experimental observation by Liu and co-workers [Phys. Rev. Lett. 113, 246803 (2014) and private communication] of several spin-polarization transitions at 4/5, 5/7, 6/5, 9/7, 7/9, 8/11, and 10/13 in GaAs systems. We show that the FQHE of composite fermions is much more prevalent in multicomponent systems, and consider the feasibility of such states for systems with $\mathcal{N}$ components for an SU($\mathcal{N}$) symmetric interaction. Our results apply to GaAs quantum wells, wherein electrons have two components, to AlAs quantum wells and graphene, wherein electrons have four components (two spins and two valleys), and to an H-terminated Si(111) surface, which can have six components. The aim of this paper is to provide a fairly comprehensive list of possible incompressible fractional quantum Hall states of composite fermions, their SU($\mathcal{N}$) spin content, their energies, and their phase diagram as a function of the generalized “Zeeman” energy. We obtain results at three levels of approximation: from ground-state wave functions of the composite fermion theory, from composite fermion diagonalization, and, whenever possible, from exact diagonalization. Effects of finite quantum well thickness and Landau-level mixing are neglected in this study. We compare our theoretical results with the experiments of Liu and co-workers [Phys. Rev. Lett. 113, 246803 (2014) and private communication] as well as of Yeh et al., [Phys. Rev. Lett. 82, 592 (1999)] for a two-component system.

First-principles calculations are used to demonstrate electromechanical control of charge and spin at zigzag-edged interfaces between graphene and boron-nitride domains in hybrid monolayers. We show how, through a direct piezoelectric effect, the interfacial bound charges and associated electric fields can be tuned by application of an external mechanical force (stress) on the system. This results in mechanical control of the edge magnetization (piezomagnetic effect), and the possibility to transform a semiconducting heterostructure into a half-metal. The inverse effect (application of an external electric field to induce a mechanical deformation) goes together with a magnetoelectric response, which under ideal conditions we estimate to be comparable to that of prototypical ${\mathrm{Cr}}_2{\mathrm{O}}_3$. These effects originate from the magnetic properties of graphene's zigzag edges and the dielectric properties of the boron-nitride domain, and can also be expected in any other coplanar heterostructures with polar discontinuities.

We obtain the reflection coefficients from a graphene sheet deposited on a material substrate under a condition that graphene is described by the hydrodynamic model. Using these coefficients, the gradient of the Casimir force in the configuration of a recent experiment is calculated in the framework of the Lifshitz theory. It is shown that the hydrodynamic model is excluded by the measurement data at a 99% confidence level over a wide range of separations. From the fact that the same data are in very good agreement with theoretical predictions of the Dirac model of graphene, the low-energy character of the Casimir interaction is confirmed.

We have performed density functional calculations of the electronic structure and lattice dynamics of the binary skutterudite ${\mathrm{FeSb}}_3$. We find that the ground state of ${\mathrm{FeSb}}_3$ is a near half-metallic ferromagnet with $T_c=175$ K. Furthermore, we find that ${\mathrm{FeSb}}_3$ is softer than ${\mathrm{CoSb}}_3$ based on an analysis of the relation of the elastic constants and the shape of the phonon density of states in the two systems, which is in agreement with experimental observation. Based on these observations we find it plausible that ${\mathrm{FeSb}}_3$ will have a lower thermal conductivity than ${\mathrm{CoSb}}_3$. Additionally, our calculations indicate that ${\mathrm{FeSb}}_3$ may be stable towards decomposition into ${\mathrm{FeSb}}_2$ and Sb. Furthermore, for ferromagnetic ${\mathrm{FeSb}}_3$ we obtain real-valued phonon frequencies and also a $c_{44}$ greater than zero, indicating that the system is mechanically as well as dynamically stable.

Using first-principles calculations and reactive force field molecular dynamics simulations, we study the structural properties and dynamics of a fluorine (F) atom, either adsorbed on the surface of single layer graphene (F/GE) or between the layers of AB stacked bilayer graphene ($\mathrm{F}@$ bilayer graphene). It is found that the diffusion of the F atom is very different in those cases, and that the mobility of the F atom increases by about an order of magnitude when inserted between two graphene layers. The obtained diffusion constant for F/GE is twice larger than that experimentally found for gold adatom and theoretically found for $C_{60}$ molecule on graphene. Our study provides important physical insights into the dynamics of fluorine atoms between and on graphene layers and explains the mechanism behind the separation of graphite layers due to intercalation of F atoms.

Experimental evidence for the existence of a fluctuating charge-density wave order in the pseudogap regime of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ has renewed interest in its interplay with superconductivity. Here, we consider the problem within a nonlinear sigma model, which was recently proposed to describe the apparent competition between the two order parameters. In particular, we use a saddle-point approximation to calculate the properties of superconducting vortex excitations within such a model. In addition, we analytically calculate a collection of experimentally observable quantities, which probe both the superconducting and charge-density wave fluctuations, and identify expected signatures of thermally excited vortices. Specifically, we find that the magnetization and Nernst signal decay rapidly with temperature for a range of temperatures above the Kosterlitz-Thouless transition, while the x-ray structure factor increases in the same temperature range. At higher temperatures, the system enters a regime of Gaussian fluctuations where vortices are no longer well defined, and as a result the magnetization, Nernst signal, and x-ray structure factor all decrease moderately with temperature.

In thermal equilibrium, the fluctuation-dissipation theorem relates the linear response and correlation functions in a model and observable independent fashion. Out of equilibrium, these relations still hold if the equilibrium temperature is replaced by an observable and frequency-dependent parameter (effective temperature). When the system achieves a long-time thermal state all of these effective temperatures should be equal and constant. Following this approach we examine the long-times regime after a quantum quench in a system with bipartite entanglement in which the asymptotic values of the observable are compatible with the ones obtained in a Gibbs ensemble. We observe that when the initial entanglement is large, and for a large range of (intermediate) frequencies, the effective temperatures corresponding to the analyzed local and nonlocal operators approach an approximate constant value equal to the temperature that governs the decay of correlations. Still, the residual frequency dependence in the effective temperature, and the differences observed among observables, discards strict thermalization.

Competing Jahn-Teller distortions combined with geometrical frustration give rise to a rich phase diagram as a function of $x\left(\mathrm{Cu}\right)$ and temperature in the spinel system $\mathrm{N}{\mathrm{i}}_{1-x}\mathrm{C}{\mathrm{u}}_x\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$. The Jahn-Teller distortion of the end members acts in opposite ways, with an elongation of the $\mathrm{Ni}{\mathrm{O}}_4$ tetrahedra resulting in a structural transition at $T_{\mathrm{S}1}=317\mathrm{K}$ in $\mathrm{NiC}{\mathrm{r}}_2{\mathrm{O}}_4$, but a flattening in the $\mathrm{Cu}{\mathrm{O}}_4$ tetrahedra at $T_{\mathrm{S}1}=846\mathrm{K}$ in $\mathrm{CuC}{\mathrm{r}}_2{\mathrm{O}}_4$. In both cases the symmetry is lowered from cubic $\left(Fd\overline{3}m\right)$ to tetragonal $(I{4}_1/amd)$ on cooling. In order to follow the influence of Jahn-Teller active $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{C}{\mathrm{u}}^{2+}$ ions on the structural and magnetic properties of chromium spinels, we have investigated a series of samples of $\mathrm{N}{\mathrm{i}}_{1-x}\mathrm{C}{\mathrm{u}}_x\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$ by x-ray and neutron powder diffraction. In the critical range $0.10<x\left(\mathrm{Cu}\right)<0.20$, strong orthorhombic distortions were observed, where competing Jahn-Teller activities between the $\mathrm{C}{\mathrm{u}}^{2+}$ and $\mathrm{N}{\mathrm{i}}^{2+}$ ions result in distortions along both the $a$ and $c$ axes. For $\mathrm{N}{\mathrm{i}}_{0.85}\mathrm{C}{\mathrm{u}}_{0.15}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$, the orthorhombic structure $\left(Fddd\right)$ is stabilized up to $T_{\mathrm{S}2}=368\left(2\right)\mathrm{K}$, close to the first structural phase transition at $T_{\mathrm{S}1}=374\left(2\right)\mathrm{K}$. A ferrimagnetic spin alignment of the Ni/Cu and chromium atoms sets in at much lower temperature $T_{\mathrm{C}}=95\mathrm{K}$ in this compound. The end members $\mathrm{NiC}{\mathrm{r}}_2{\mathrm{O}}_4$ and $\mathrm{CuC}{\mathrm{r}}_2{\mathrm{O}}_4$ undergo this ferrimagnetic transition at $T_{\mathrm{C}}=74\mathrm{and}135\mathrm{K}$, respectively. These transitions are accompanied by the structural change to the orthorhombic symmetry which relieves the frustration. $\mathrm{NiC}{\mathrm{r}}_2{\mathrm{O}}_4$ and $\mathrm{N}{\mathrm{i}}_{0.85}\mathrm{C}{\mathrm{u}}_{0.15}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$ undergo a second magnetic transition at $T_{\mathrm{M}2}=24\mathrm{and}67\mathrm{K}$ due to a superimposed antiferromagnetic ordering of the Cr moments resulting in a noncollinear magnetic structure. In the system $\mathrm{N}{\mathrm{i}}_{1-x}\mathrm{C}{\mathrm{u}}_x\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_4$, the magnetic transitions $T_{\mathrm{C}}$ and $T_{\mathrm{M}2}$ merge with increasing copper content up to $x\left(\mathrm{Cu}\right)\sim 0.5$. For the Ni-rich chromites, geometrical frustration causes a strong reduction of the chromium moments, where magnetic long-range order coexists with a disordered spin-liquid-like or a reentrant-spin-glass-like state. This paper provides insight into the interplay between the Jahn-Teller effect, geometrical frustration, and long-range magnetic order in these complex systems.

We theoretically investigate the nonlinear intersubband response, in particular the intrinsic optical bistability (IOB), of quantum well (QW) structures embedded into layered systems. A semiclassical approach is employed based on the plane-wave approximation, the transfer-matrix formalism, the sheet model, and the density-matrix formalism developed to calculate the two-dimensional nonlinear intersubband electron conductivity and the IOB. A variety of presented numerical results demonstrate the importance of inhomogeneity of spatial distribution of radiation intensity induced by multiple radiation reflection for the IOB in layered structures. It is shown that knowing the radiation intensity distribution in the area occupied by QWs, in part standing-wave patterns, are crucial for understanding of IOB pictures. The IOB in systems of one, two, three, five, and ten QWs is investigated. The multistability phenomenon is addressed.

The cylindrical multishell structure is one of the prevalent atomic arrangements in nanowires. Being multishell, the well-defined atomic periodicity is hardly realized in it because the periodic units of individual shells therein generally do not match except for very few cases, posing a challenge to understanding its physical properties. Here we show that moiré patterns generated by superimposing atomic lattices of individual shells are decisive in determining its electronic structures. Double-walled carbon nanotubes, as an example, are shown to have spectacular variations in their electronic properties from metallic to semiconducting and further to insulating states depending on their moiré patterns, even when they are composed of only semiconducting nanotubes with almost similar energy gaps and diameters. Thus, aperiodic multishell nanowires can be classified into one-dimensional moiré crystals with distinct electronic structures.

It has been demonstrated numerically, mainly by considering ground state properties, that fractional quantum Hall physics can appear in lattice systems, but it is very difficult to study the anyons directly. Here, I propose to solve this problem by using conformal field theory to build semianalytical fractional quantum Hall lattice models having anyons in their ground states, and I carry out the construction explicitly for the family of bosonic and fermionic Laughlin states. This enables me to show directly that the braiding properties of the anyons are those expected from an analytical continuation of the wave functions and to compute properties such as internal structure, size, and charge of the anyons with simple Monte Carlo simulations. The models can also be used to study how the anyons behave when they approach or even pass through the edge of the sample. Finally, I compute the effective magnetic field seen by the anyons, which varies periodically due to the presence of the lattice.

We demonstrate that charge separation at donor-acceptor interfaces is a complex process that is controlled by the combined action of Coulomb binding for electron-hole pairs and partial relaxation due to quantized phonons. A joint electron-vibration quantum dynamical study reveals that high-energy vibrations sensitively tune the charge transfer probability as a function of time and injection energy, due to polaron formation. These results have bearings on the optimization of energy transfer both in organic and quantum dot photovoltaics, as well as in biological light harvesting complexes.

We develop a theoretical description of radiative thermal conductivity in hyperbolic metamaterials. We demonstrate a dramatic enhancement of the radiative thermal transport due to the supersingularity of the photonic density of states in hyperbolic media, leading to the radiative heat conductivity which can be comparable to the nonradiative contribution.

The effects of electron doping and phonon vibrations on the magnetic properties of monolayer and bilayer FeSe epitaxial films on ${\mathrm{SrTiO}}_3$ have been studied, respectively, using first-principles calculations with van der Waals correction. For monolayer FeSe epitaxial film, the combined effect of electron doping and phonon vibrations readily leads to magnetic frustration between the collinear antiferromagnetic state and the checkerboard antiferromagnetic Néel state. For bilayer FeSe epitaxial film, such magnetic frustration is much more easily induced by electron doping in its bottom layer than its top layer. The underlying physics is that the doped electrons are accumulated at the interface between the FeSe layers and the substrate. These results are consistent with existing experimental studies.

${\text{SrTiO}}_3$ undergoes a cubic-to-tetragonal phase transition at 105 K. This antiferrodistortive transition is believed to be in competition with incipient ferroelectricity. Substituting strontium by isovalent calcium induces a ferroelectric order. Introducing mobile electrons to the system by chemical nonisovalent doping, on the other hand, leads to the emergence of a dilute metal with a superconducting ground state. The link between superconductivity and the other two instabilities is a question gathering momentum in the context of a popular paradigm linking unconventional superconductors and quantum critical points. We present a set of specific-heat, neutron-scattering, dielectric-permittivity, and polarization measurements on ${\text{Sr}}_{1-x}{\text{Ca}}_x{\text{TiO}}_3$ $\left(0<x<0.009\right)$ and a study of low-temperature electric conductivity in ${\text{Sr}}_{0.9978}{\text{Ca}}_{0.0022}{\text{TiO}}_{3-\delta }$. Calcium substitution was found to enhance the transition temperature for both antiferrodistortive and ferroelectric transitions. Moreover, we find that ${\text{Sr}}_{0.9978}{\text{Ca}}_{0.0022}{\text{TiO}}_{3-\delta }$ has a superconducting ground state. The critical temperature in this rare case of a superconductor with a ferroelectric parent is slightly lower than in ${\text{SrTiO}}_{3-\delta }$ of comparable carrier concentration. A three-dimensional phase diagram for ${\text{Sr}}_{1-x}{\text{Ca}}_x{\text{TiO}}_{3-\delta }$ tracking the three transition temperatures as a function of $x$ and $\delta$ results from this study, in which ferroelectric and superconducting ground states are not immediate neighbors.