The spinel ${\text{NiCr}}_2{\text{O}}_4$ is characterized using dielectric and high-magnetic-field measurements. The trends in the magnetodielectric response fall into three clear temperature regimes corresponding to known magnetic and structural transitions. Above the Néel temperature, weak magnetic field dependence of the dielectric constant is observed with no hysteresis. Below the Néel temperature but above 30 K, a dependence of the dielectric constant on the magnetic field is observed and hysteresis develops, resulting in so-called butterfly loops. Below 30 K, magnetodielectric hysteresis is enhanced. Magnetodielectric hysteresis mirrors magnetic hysteresis, suggesting that spin-spin interactions are the mechanism for the magnetodielectric effect in ${\text{NiCr}}_2{\text{O}}_4$. We show that below 40 kOe, the field-dependent permittivity scales linearly with the squared magnetization as described by the Ginzburg-Landau theory. At high fields, however, the magnetization continues to increase while the dielectric constant saturates. Magnetodielectric measurements of ${\text{NiCr}}_2{\text{O}}_4$ suggest an additional, previously unobserved, transition at 20 K. Subtle changes in magnetism and structure at 20 K, suggest the completion of ferrimagnetic ordering and the spin-driven structural distortion. We demonstrate that magnetocapacitance is a sensitive probe of magnetostructural distortion and provide the first high-field measurements of ${\text{NiCr}}_2{\text{O}}_4$.

The exchange interactions and magnetic structure in layered system CuMnO${}_2$ (mineral crednerite) and in nonstoichiometric system Cu${}_{1.04}$Mn${}_{0.96}$O${}_2$, with triangular layers distorted due to orbital ordering of the Mn${}^{3+}$ ions, are studied by ab initio band-structure calculations, which were performed within the GGA + $U$ approximation. The exchange interaction parameters for the Heisenberg model within the Mn planes and between the Mn planes are estimated. We explain the observed in-plane magnetic structure by the dominant mechanism of the direct $d\text{−}d$ exchange between neighboring Mn ions. The superexchange via O ions, with 90${}^{\circ }$ Mn–O–Mn bonds, plays a less important role for the in-plane exchange. The interlayer coupling is largely dominated by one exchange path between the half-filled $3z^2-r^2$ orbitals of Mn${}^{3+}$. The change of interlayer coupling from antiferromagnetic in pure CuMnO${}_2$ to ferromagnetic in doped material is also explained by our calculations.

A frustrated spin-$1/2$ XXZ chain model comprising a ferromagnetic nearest-neighbor coupling with the bond alternation, $J_1(1\pm \delta )<0$, and an antiferromagnetic second-neighbor exchange coupling $J_2>0$ is studied at zero and weak magnetic fields by means of density-matrix renormalization-group calculations of order parameters, correlation functions, and the entanglement entropy, as well as an Abelian bosonization analysis. At zero magnetic field, the bond alternation $\delta >0$ suppresses the gapless phase characterized by a vector-chiral (VC) long-range order (LRO) and a quasi-LRO of an incommensurate spin spiral, whereas this phase occupies a large region in the space of $J_1/J_2$ and the easy-plane exchange anisotropy for $\delta =0$ [S. Furukawa et al., Phys. Rev. Lett. 105, 257205 (2010)]. Then, four gapped phases are found to appear as the exchange anisotropy varies from the SU(2)-symmetric case to the U(1)-symmetric case: the Haldane dimer (D${}_{+}$) phase with the same sign of the $x,y$- and $z$-component dimer order parameters, two VC dimer (VCD${}_{+}$/VCD${}_{-}$) phases with the sign of the $z$-component dimer order parameter being unaltered/reversed, and the even-parity dimer (D${}_{-}$) phase. At small magnetic fields, a field-induced ring-exchange interaction, which is proportional to a staggered scalar chirality and a magnetic flux penetrating the associated triangle, drives a transition from the D${}_{-}$ phase into a VC-Neel-dimer (VCND) phase, but not from the D${}_{+}$ phase. This VCND phase is stable up to the large magnetic field at which the Zeeman term closes the spin gap. A possible relevance to Rb${}_2$Cu${}_2$Mo${}_3$O${}_{12}$ is discussed.

We probe the real-space electronic response to a local magnetic impurity in isovalent and heterovalent doped BaFe${}_2$As${}_2$ (122) using nuclear magnetic resonance (NMR). The local moments carried by Mn impurities doped into Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ (Co-122) and BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ (P-122) at optimal doping induce a spin polarization in the vicinity of the impurity. The amplitude, shape, and extension of this polarization is given by the real part of the susceptibility ${\chi }^{\prime }\left(r\right)$ of FeAs layers and is consequently related to the nature and strength of the electronic correlations present in the system. We study this polarization using ${}^{75}$As NMR in Co-122 and both ${}^{75}$As and ${}^{31}$P NMR in P-122. The NMR spectra of Mn-doped materials are made of two essential features. First, there is a satellite line associated with nuclei located as nearest neighbors of Mn impurities. The analysis of the temperature dependence of the shift of this satellite line shows that Mn local moments behave as isolated Curie moments. The second feature is a temperature dependent broadening of the central line. We show that the broadening of the central line follows the susceptibility of Mn local moments, as expected from typical Ruderman-Kittel-Kasuya-Yosida (RKKY)-like interactions. This demonstrates that the susceptibility ${\chi }^{\prime }\left(r\right)$ of FeAs layers does not make significant contribution to the temperature dependent broadening of the central line. ${\chi }^{\prime }\left(r\right)$ is consequently only weakly temperature dependent in optimally doped Co-122 and P-122. This behavior is in contrast with that of strongly correlated materials such as underdoped cuprate high-$T_{\mathrm{c}}$ superconductors where the central line broadens faster than the impurity susceptibility grows, because of the development of strong magnetic correlations when $T$ is lowered. Moreover, the FeAs layer susceptibility is found quantitatively similar in both heterovalent doped and isolvalent doped BaFe${}_2$As${}_2$.

We report that nacre (also known as mother-of-pearl), a wondrous nanocomposite found in nature, is a rich photonic nanomaterial allowing the experimental realization of collective excitation and light amplification via coupled states. Localized modes in three-dimensional complex media are typically isolated in frequency and space. However, multiple local resonances can be hybridized in multilayered nanostructures of nacre so that the effective cavity size for efficient disordered resonators is scaled up. Localized modes in hybridized states in nacre are overlapped in frequency with similar shapes in space, thus being collectively excited and synergistically amplified. These hybridized states boost light amplification, leading to stable and regular multimode lasing at low excitation energy. The simplicity of ameliorating disordered resonators by mimicking nacre can further serve as platforms for developing cost-effective photonic systems and provide materials for fundamental research on complex media.

In this work, our experimental demonstration of the universal energy level alignment rule at oxide/organic semiconductor interfaces is reported. Photoemission spectroscopy is used to show the three different regimes of the energy level alignment: the lowest unoccupied molecular orbital (LUMO) is pinned to the substrate Fermi level at the extreme low end of work functions; the energy offset of the highest occupied molecular orbital (HOMO) follows the Schottky-Mott rule; the HOMO is pinned to the substrate Fermi level at the extreme high end of work functions. To demonstrate this, fullerene C${}_{60}$ was deposited on eight different types of transition-metal oxides, ZrO${}_2$, TiO${}_2$, NiO, Co${}_3$O${}_4$, CuO, V${}_2$O${}_5$, MoO${}_3$, and WO${}_3$, followed by in situ ultraviolet photoemission spectroscopy.

We theoretically study the phonon-mediated intersurface electron-electron interactions on the pseudo-two-dimensional metallic states at the two surfaces of a three-dimensional topological insulator. From a model of a three-dimensional topological insulator including the phonon excitation in it, we derive the effective Lagrangian which describes the two surface metallic states and the interaction between them. The intersurface electron-electron interactions can be either repulsive or attractive depending on parameters such as temperature, the speed of the phonon, and the Fermi velocity of the surface states. The attractive interaction removes the Dirac nodes from the two surface states as a result of the spontaneous symmetry breaking. On the basis of the calculated results, we also discuss how to tune the intersurface interaction.

The most common approach to dope an electronic material is to substitute a constituent atom with a suitable impurity. Here we show with first-principles calculations that this is not the best recipe for doping graphene with Al, P, Ga, or As impurities. Instead, substitution of two C atoms by one of these species is preferred (the ideal choice is phosphorus), as it shifts the Fermi level in the valence band without affecting significant changes in the electronic properties of low-energy carriers. This alternative way of doping could help optimize graphene-based devices and provide analogous doping schemes in other graphenelike materials.

We study the relationship between the circular polarization of photoluminescence and the magnetic-field-induced spin polarization of the recombining charge carriers in bulk Si and Ge/Si quantum dots. First, we quantitatively compare experimental results on the degree of circular polarization of photons resulting from phonon-assisted radiative transitions in intrinsic and doped bulk Si with calculations which we adapt from recently predicted spin-dependent phonon-assisted transition probabilities in Si. The excellent agreement of our experiments and calculations quantitatively verifies these spin-dependent transition probabilities and extends their validity to weak magnetic fields. Such magnetic fields can induce a luminescence polarization of up to 3%/T. We then investigate phononless transitions in Ge/Si quantum dots as well as in degenerately doped Si. Our experiments systematically show that the sign of the degree of circular polarization of luminescence resulting from phononless transitions is opposite to the one associated with phonon-assisted transitions in Si and with phononless transitions in direct-band-gap semiconductors. This observation implies qualitatively different spin-dependent selection rules for phononless transitions, which seem to be related to the confined character of the electron wave function.

We present unexpected structural motifs for boron-carbon nanocages of the stoichiometries ${\mathrm{B}}_{12}{\mathrm{C}}_{48}$ and ${\mathrm{B}}_{12}{\mathrm{C}}_{50}$, based on first-principles calculations. These configurations are distinct from those proposed so far because the boron atoms are not isolated and distributed over the entire surface of the cages, but rather aggregate at one location to form a patch. Our putative ground state of ${\mathrm{B}}_{12}{\mathrm{C}}_{48}$ is 1.8 eV lower in energy than the previously proposed ground state and violates all the suggested empirical rules for constructing low-energy fullerenes. The ${\mathrm{B}}_{12}{\mathrm{C}}_{50}$ configuration is energetically even more favorable than ${\mathrm{B}}_{12}{\mathrm{C}}_{48}$, showing that structures derived from the ${\mathrm{C}}_{60}$ buckminsterfullerene are not necessarily magic sizes for heterofullerenes.

We investigate the low-energy properties of (quasi) helical and fractional helical Luttinger liquids. In particular, we calculate the Drude peak of the optical conductivity, the density of states, as well as charge transport properties of the interacting system with and without attached Fermi liquid leads at small and large (compared to the gap) frequencies. For fractional wires, we find that the low-energy tunneling density of states vanishes. The conductance of a fractional helical Luttinger liquid is noninteger. It is independent of the Luttinger parameters in the wire, despite the intricate mixing of charge and spin degrees of freedom, and only depends on the relative locking of charge and spin degrees of freedom.

Pressure dependence of the Yb valence in YbNiGe${}_3$ has been investigated up to 15.6 GPa at 300 K and up to 7.7 GPa at 17 K by means of x-ray absorption spectroscopy in $L{\alpha }_1$ partial fluorescence yield mode and resonant x-ray emission spectroscopy around the Yb $L_3$ absorption edge. The Yb valence in YbNiGe${}_3$ at ambient pressure strongly fluctuates with the mean valence of $v=2.52\pm 0.01$ at 300 K. The Yb valence rapidly changes toward a trivalent state with pressure up to $\sim$5 GPa, slowly increases up to $\sim$10 GPa, and then reaches a saturated value of $v\sim 2.87$ at 15.6 GPa. The Yb valence at 17 K slightly decreases compared to that at 300 K; $v\sim 2.45$ at ambient pressure and $\sim$$2.72$ at 7.7 GPa. We found that the pressure-induced change in the intensity of a quadrupole component in the x-ray absorption spectra shows the same trend as the Yb valence in YbNiGe${}_3$. In contrast to YbNiGe${}_3$, the Yb valence in YbNiSi${}_3$ is nearly 3 at ambient pressure with almost no temperature dependence.

The organic charge-transfer salt EtMe${}_3$P[Pd(dmit)${}_2$]${}_2$ is a quasi-two-dimensional Mott insulator with localized spins $S=1/2$ residing on a distorted triangular lattice. Here we report measurements of the uniaxial thermal expansion coefficients ${\alpha }_i$ along the in-plane $i=a$ and $c$ axis as well as along the out-of-plane $b$ axis for temperatures 1.4 $\text{K}\le T\le 200$ K. Particular attention is paid to the lattice effects around the phase transition at $T_{\text{VBS}}=25$ K into a low-temperature valence-bond-solid phase and the paramagnetic regime above where effects of short-range antiferromagnetic correlations can be expected. The salient results of our study include (i) the observation of strongly anisotropic lattice distortions accompanying the formation of the valence-bond-solid phase, and (ii) a distinct anomaly in the thermal expansion coefficients in the paramagnetic regime around 40 K. Our results demonstrate that upon cooling through $T_{\text{VBS}}$ the in-plane $c$ axis, along which the valence bonds form, contracts while the second in-plane $a$ axis elongates by the same relative amount. Surprisingly, the dominant effect is observed for the out-of-plane $b$ axis which shrinks significantly upon cooling through $T_{\text{VBS}}$. The pronounced anomaly in ${\alpha }_i$ around 40 K is attributed to short-range magnetic correlations. It is argued that the position of this maximum, relative to that in the magnetic susceptibility around 70 K, speaks in favor of a more anisotropic triangular-lattice scenario for this compound than previously thought.

The long-range one-dimensional Ising spin glass with random couplings decaying as $J\left(r\right)\propto r^{-\sigma }$ presents a spin-glass phase $T_c\left(\sigma \right)>0$ for $0\le \sigma <1$ (the limit $\sigma =0$ corresponds to the mean-field Sherrington-Kirkpatrick model). We use the eigenvalue method introduced in our previous work (C. Monthus and T. Garel, J. Stat. Mech. 2009, P12017) to measure the equilibrium time $t_{eq}\left(N\right)$ at temperature $T=T_c\left(\sigma \right)/2$ as a function of the number $N$ of spins. We find the activated scaling $\overline{ln{t}_{eq}\left(N\right)}\sim N^{\psi }$ with the same barrier exponent $\psi \simeq 0.33$ in the whole region $0\le \sigma <1$.

The small-scale energy-transfer mechanism in zero-temperature superfluid turbulence of helium-4 is still a widely debated topic. Currently, the main hypothesis is that weakly nonlinear interacting Kelvin waves (KWs) transfer energy to sufficiently small scales such that energy is dissipated as heat via phonon excitations. Theoretically, there are at least two proposed theories for Kelvin-wave interactions. We perform the most comprehensive numerical simulation of weakly nonlinear interacting KWs to date and show, using a specially designed numerical algorithm incorporating the full Biot-Savart equation, that our results are consistent with the nonlocal six-wave KW interactions as proposed by L'vov and Nazarenko.

We study low-temperature electron transport in $p$-wave superconductor-insulator-normal metal junctions. In diffusive metals, the $p$-wave component of the order parameter is strongly suppressed at distances greater than the mean free path $l$. At the superconductor-normal metal boundary, due to spin-orbit interaction, there is a triplet to singlet conversion of the superconducting order parameter. The singlet component survives at distances much larger than $l$ from the boundary. It is this component that controls the low-temperature resistance of the junctions. As a result, the resistance of the system strongly depends on the angle between the insulating boundary and the $\mathbf{d}$ vector characterizing the spin structure of the triplet superconducting order parameter. We also analyze the spatial dependence of the electric potential in the presence of the current and show that the electric field is suppressed in the insulating boundary as well as in the normal metal at distances of order of the coherence length away from the boundary. This is very different from the case of the normal metal-insulator-normal metal junctions, where the voltage drop takes place predominantly at the insulator.

We infer the high-frequency flux noise spectrum in a superconducting flux qubit by studying the decay of Rabi oscillations under strong driving conditions. The large anharmonicity of the qubit and its strong inductive coupling to a microwave line enabled high-amplitude driving without causing significant additional decoherence. Rabi frequencies up to 1.7 GHz were achieved, approaching the qubit's level splitting of 4.8 GHz, a regime where the rotating-wave approximation breaks down as a model for the driven dynamics. The spectral density of flux noise observed in the wide frequency range decreases with increasing frequency up to 300 MHz, where the spectral density is not very far from the extrapolation of the $1/f$ spectrum obtained from the free-induction-decay measurements. We discuss a possible origin of the flux noise due to surface electron spins.

Optical second harmonic generation (SHG) is used to probe surface strain in 150 and 190-nm thin films of MnAs grown epitaxially on GaAs(001). The $p$-polarized SHG signal produced by $p$-polarized 775-nm, 200-fs pulses is theoretically and experimentally shown to be sensitive to the normal component of surface strain from $-20$ to 70 ${}^{\circ }$C, which includes the ferromagnetic/paramagnetic striped coexistence phase region that exists from $\sim$10 to 40 ${}^{\circ }$C. We use this dependence to time-resolve the surface strain dynamics in MnAs following pumping with 200-fs pulses of 1.0 or 2.0 mJ cm${}^{-2}$ that raise the surface temperature by tens of degrees. For a film at $-20{}^{\circ }$C the strain reaches a minimum value in $\sim$10 ps, indicative of electron-lattice thermalization, before recovering on a 500-ps time scale consistent with a one-dimensional heat diffusion model. For a film at 20 ${}^{\circ }$C the minimum strain is reached only after $\sim$200 ps and attains a value higher than predicted by the heat diffusion model; recovery, however, still occurs in $\sim$500 ps. The long strain fall time possibly reflects the influence of latent heat and stripe dynamics in the coexistence phase. The larger calculated drop in surface strain may be due to deficiencies in the one-dimensional heat diffusion model. The nonequilibrium surface strain also may not be determined by the local temperature alone but by the constraints throughout the film/substrate system, which are certainly known to govern the strain and stripe characteristics under equilibrium conditions.

We present an analytical description of the nonlocal optical response of plasmonic nanowire metamaterials that enable negative refraction, subwavelength light manipulation, and emission lifetime engineering. We show that dispersion of optical waves propagating in nanowire media results from coupling of transverse and longitudinal electromagnetic modes supported by the nanowires and derive the nonlocal effective medium approximation for this dispersion. We derive the profiles of electric field across the unit cell, and use these expressions to solve the long-standing problem of additional boundary conditions in calculations of transmission and reflection of waves by nonlocal nanowire media. We verify our analytical results with numerical solutions of Maxwell's equations and discuss generalization of the developed formalism to other uniaxial metamaterials.

We study the strong coupling limit of the extended Hubbard model in two dimensions. The model consists of hopping, on-site interaction, nearest-neighbor interaction, spin-orbit coupling, and Zeeman spin splitting. While the study of this model is motivated by a search for topological phases, and in particular, a topological superconductor, the methodology we develop may be useful for a variety of systems. We begin our treatment with a canonical transformation of the Hamiltonian to an effective model, which is appropriate when the (quartic) interaction terms are larger than the (quadratic) kinetic and spin-orbit coupling terms. We proceed by analyzing the strong coupling model variationally. Since we are mostly interested in a superconducting phase, we use a Gutzwiller projected BCS wave function as our variational state. To continue analytically, we employ the Gutzwiller approximation and compare the calculated energy with Monte Carlo calculations. Finally, we determine the topology of the ground state and map out the topology phase diagram.