A 20% substitution of Bi with La in the perovskite ${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x{\mathrm{Fe}}_{0.5}{\mathrm{Sc}}_{0.5}{\mathrm{O}}_3$ system obtained under high-pressure and high-temperature conditions has been found to induce an incommensurately modulated structural phase. The room-temperature x-ray and neutron powder diffraction patterns of this phase were successfully refined using the $Imma(0,0,\gamma )s00$ superspace group ($\gamma =0.534\left(3\right)$) with the modulation applied to Bi/La and oxygen displacements. The modulated structure is closely related to the prototype antiferroelectric structure of ${\mathrm{PbZrO}}_3$ which can be considered as the lock-in variant of the latter with $\gamma =0.5$. Below $T_{\mathrm{N}}\sim 220$ K, the neutron diffraction data provide evidence for a long-range $G$-type antiferromagnetic ordering commensurate with the average $Imma$ structure. Based on a general symmetry consideration, we show that the direction of the spins is controlled by the antisymmetric exchange imposed by the two primary structural distortions, namely oxygen octahedral tilting and incommensurate atomic displacements. The tilting is responsible for the onset of a weak ferromagnetism, observed in magnetization measurements, whereas the incommensurate displacive mode is dictated by the symmetry to couple a spin-density wave. The obtained results demonstrate that antisymmetric exchange is the dominant anisotropic interaction in $\mathrm{Fe}^{3+}$-based distorted perovskites with a nearly quenched orbital degree of freedom.

Structural and magnetic properties across the martensite-austenite phase transitions in the shape memory alloy Ni-Mn-In-Co are studied using complementary experimental techniques: ferromagnetic resonance, macroscopic magnetization measurements, and x-ray magnetic circular dichroism in the temperature range from 5 to 450 K. Ferromagnetic resonance experiments show coexisting antiferromagnetic and ferromagnetic correlations for the martensite phase and ferromagnetic and paramagnetic correlations in the austenite phase. Magnetization measurements reveal spin-glass-like behavior for $T<30$ K and Ni and Co $K$-edge x-ray magnetic circular dichroism measurements confirm an assignment of a ferromagnetic resonance line purely to Ni (and Co) for a wide temperature range from 125 to 225 K. Hence a combined analysis of ferromagnetic resonance and x-ray magnetic circular dichroism allows us to attribute particular magnetic resonance signals to individual elemental species in the alloy.

We develop a collective field theory for fractional quantum Hall (FQH) states. We show that in the leading approximation for a large number of particles, the properties of Laughlin states are captured by a Gaussian free field theory with a background charge. Gradient corrections to the Gaussian field theory arise from the covariant ultraviolet regularization of the theory, which produces the gravitational anomaly. These corrections are described by a theory closely related to the Liouville theory of quantum gravity. The field theory simplifies the computation of correlation functions in FQH states and makes manifest the effect of quantum anomalies.

We present a first-principles study of the extrinsic spin Hall effect in dilute metallic alloys and show how tailoring the magnitude of the spin Hall conductivity can be achieved by materials design concerning composition and varying the concentration of the alloy partners. An essential ingredient is the relative strength of the spin-orbit coupling of host and impurity partner. This is systematically studied by changing alloy composition and performing model calculations in which the spin-orbit coupling strength is scaled. The calculations reveal that changing the impurity concentration affects the extrinsic contributions, namely skew scattering and side jump, differently. This is put into the context of recent model calculations put forward by Fert and Levy [Phys. Rev. Lett. 106, 157208 (2011)]. A fully relativistic Kubo-Středa formalism as implemented in the multiple-scattering KKR electronic structure method is used. The calculations were carried out for Pd, Pt, and Cu hosts doped with $4d$ including Ag and Cd and $5d$ impurities including Au and Hg.

Monte Carlo simulation of a 2+1 dimensional model of voltage-biased bilayer graphene, consisting of relativistic fermions with chemical potential $\mu$ coupled to charged excitations with opposite sign on each layer, has exposed noncanonical scaling of bulk observables near a quantum critical point found at strong coupling. We present a calculation of the quasiparticle dispersion relation $E\left(k\right)$ as a function of exciton source $j$ in the same system, employing partially twisted boundary conditions to boost the number of available momentum modes. The Fermi momentum $k_F$ and superfluid gap $\mathrm{\Delta }$ are extracted in the $j\to 0$ limit for three different values of $\mu$, and support a strongly interacting scenario at the Fermi surface with $\mathrm{\Delta }\sim O\left(\mu \right)$. We propose an explanation for the observation $\mu <k_F$ in terms of a dynamical critical exponent $z<1$.

A number of strongly correlated heavy-fermion compounds, such as samarium (Sm), ytterbium (Yb), plutonium (Pu) hexaboride, are predicted to become topological insulators at low temperatures. These systems support massless Dirac fermions near certain (three) points of the surface Brillouin zone, hereafter referred to as the valleys. In strong perpendicular magnetic fields, the conical Dirac dispersions of these surface states quench onto three sets of Landau levels and we predict various possible hierarchies of incompressible quantum Hall states on the surface of hexaborides. In addition, we address the effects of strong electron-electron interaction within the surface zeroth Landau levels. Specifically, we show that depending on the relative strength of the long-range (Coulomb-type) and the finite-range (Hubbard-type) interactions, the ground state can display either a valley-polarized or a valley-coherent distribution of electronic density. We also show that the transition between two valley-polarized states is always discontinuous, while that between a valley-polarized and a valley-coherent phase is continuous. The Zeeman splitting and/or an applied uniaxial strain on the surface can drive the system through various quantum phase transitions and place it in different broken-symmetry phases. Application of uniaxial strain is also shown to considerably modify the precise sequence of quantum Hall states. We also highlight the role of topology in determining the broken-symmetry phases, disorder on the surface of topological hexaborides in strong magnetic fields.

The formation and propagation of a polariton condensate under tightly focused excitation is investigated in a ZnO microcavity both experimentally and theoretically. Two-dimensional (2D) near-field and far-field images of the condensate are measured under quasicontinuous nonresonant excitation. The corresponding spatial profiles are compared to a model based on the Gross-Pitaevskii equation under cylindrical geometry. This paper allows one to connect the experiments performed with a small excitation laser spot and the previous kinetic models of condensation in a 2D infinite microcavity and to determine the relevant parameters of both the interaction and the relaxation between the reservoir and the condensate. Two main parameters are identified: The exciton-photon detuning through the polariton effective mass and the temperature, which determines the efficiency of the relaxation from the reservoir to the condensate.

Chaotic systems exhibit rich quantum dynamical behaviors ranging from dynamical localization to normal diffusion to ballistic motion. Dynamical localization and normal diffusion simulate electron motion in an impure crystal with a vanishing and finite conductivity, i.e., an “Anderson insulator” and a “metal,” respectively. Ballistic motion simulates a perfect crystal with diverging conductivity, i.e., a “supermetal.” We analytically find and numerically confirm that, for a large class of chaotic systems, the metal-supermetal dynamics crossover occurs and is universal, determined only by the system's symmetry. Furthermore, we show that the universality of this dynamics crossover is identical to that of eigenfunction and spectral fluctuations described by the random matrix theory.

We propose a platform for interacting topological phases of fermions with time reversal symmetry $\overline{\mathrm{\Theta }}$ (such that ${\overline{\mathrm{\Theta }}}^2=1$) that can be realized in vortex lattices in the surface state of a topological insulator. The constituent particles are Majorana fermions bound to vortices and antivortices of such a lattice. We explain how the $\overline{\mathrm{\Theta }}$ symmetry arises and discuss ways in which interactions can be experimentally tuned and detected. We show how these features can be exploited to realize a class of interaction-enabled crystalline topological phases that have no analog in weakly interacting systems.

A spin chemical potential bias can induce a spin-polarized current by the exchange interaction of a ferromagnet with the spin-momentum locking surface states of the topological insulators. We carried out our ferromagnetic resonance experiment in a $\mathrm{NiFe}/\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ heterostructure. Apart from the enhanced Gilbert damping constant, we observed strong enhancement of the effective magnetic field at low temperatures. The enhanced field decreased exponentially with increasing temperature at an energy scale of 2.5 meV, representing the strength of the exchange coupling. We attribute the enhanced field to the induced spin-polarized current in the surface states of $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$.

The quasi-two-dimensional nickel chalcogenide ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$ is a newly discovered superconductor. We have performed an optical spectroscopy study on ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$ single crystals over a broad frequency range at various temperatures. The overall optical reflectance spectra are similar to those observed in its isostructure ${\mathrm{BaNi}}_2{\mathrm{As}}_2$. Both the suppression in $R\left(\omega \right)$ and the peaklike feature in ${\sigma }_1\left(\omega \right)$ suggest the progressive formation of a pseudogap feature in the mid-infrared range with decreasing temperatures, which might originate from the dynamic local fluctuation of the charge-density-wave (CDW) instability. We propose that the CDW instability in ${\mathrm{TlNi}}_2{\mathrm{Se}}_2$ is driven by the saddle-point mechanism, due to the existence of a van Hove singularity very close to the Fermi energy.

The study of high-index dielectric nanoparticles currently attracts a lot of attention. They do not suffer from absorption but promise to provide control of the properties of light comparable to plasmonic nanoparticles. To further advance the field, it is important to identify versatile dielectric nanoparticles with unconventional properties. Here, we show that breaking the symmetry of an all-dielectric nanoparticle leads to a geometrically tunable magnetoelectric coupling, i.e., an omega-type bianisotropy. The suggested nanoparticle exhibits different backscatterings and, as an interesting consequence, different optical scattering forces for opposite illumination directions. An array of such nanoparticles provides different reflection phases when illuminated from opposite directions. With a proper geometrical tuning, this bianisotropic nanoparticle is capable of providing a $2\pi$ phase change in the reflection spectrum while possessing a rather large and constant amplitude. This allows the creation of reflectarrays with near-perfect transmission out of the resonance band due to the absence of a usually employed metallic screen.

Surface optical and electronic properties are crucial for material science and have implications in fields as various as nanotechnology, nonlinear optics, and spectroscopies. In particular, the huge variation of electronic density perpendicular to the surface is expected to play a key role in absorption due to local-field effects. Numerous state-of-the-art theoretical and numerical ab initio formalisms developed for studying these properties are based on supercell approaches, in reciprocal space, due to their efficiency. In this paper, we show that the standard scheme fails for the out-of-plane optical response of the surface. This response is interpreted using the “effective-medium theory” with vacuum and also in terms of interaction between replicas, as the supercell approach implies a periodicity which is absent in the real system. We propose an alternative formulation, also based on the supercell, for computing the macroscopic dielectric function. Application to the clean Si(001) $2\times 1$ surface allows us to present the effect of the local fields for both peak positions and line shape on the bulk and surface contributions. It shows how local fields built up for the in-plane and out-of-plane dielectric responses of the surface. In addition to their conceptual impact, our results explain why the standard approach gives reliable predictions for the in-plane components, leading to correct reflectance anisotropy spectra. Our scheme can be further generalized to other low-dimensional geometries, such as clusters or nanowires, and open the way to nonlinear optics for surfaces.

We study the quantum spin waves associated to skyrmion textures. We show that the zero-point energy associated to the quantum spin fluctuations of a noncollinear spin texture produce Casimir-like magnetic fields. We study the effect of these Casimir fields on the topologically protected noncollinear spin textures known as skyrmions. In a Heisenberg model with Dzyalonshinkii-Moriya interactions, chosen so the classical ground state displays skyrmion textures, we calculate the spin-wave spectrum, using the Holstein-Primakoff approximation, and the associated zero-point energy, to the lowest order in the spin-wave expansion. Our calculations are done both for the single-skyrmion case, for which we obtain a discrete set of skyrmion bound states, as well as for the skyrmion crystal, for which the resulting spectrum gives the spin-wave bands. In both cases, our calculations show that the Casimir magnetic field contributes up to 10% of the total Zeeman energy necessary to delete the skyrmion texture with an applied field.

We study dephasing in electron transport through a large quantum dot (a Fabry-Perot interferometer) in the fractional quantum Hall regime with filling factor $2/3$. In the regime of sequential tunneling, dephasing occurs due to electron fractionalization into counterpropagating charge and neutral edge modes on the dot. In particular, when the charge mode moves much faster than the neutral mode, and at temperatures higher than the level spacing of the dot, electron fractionalization combined with the fractional statistics of the charge mode leads to the dephasing selectively suppressing $h/e$ Aharonov-Bohm oscillations but not $h/\left(2e\right)$ oscillations, resulting in oscillation-period halving.

Randomly distributed heavy adatoms on a graphene sheet can effectively enhance local spin-orbit coupling (SOC), hereafter referred to as the SOC impurity. To perform a numerically exact solution of the real-space Kubo-Bastin formula, we study the quantum transport properties of a graphene sheet in the presence of an SOC impurity. We find that the SOC impurity can establish an imperfect quantum spin Hall (QSH) plateau in the band gap region, while nonzero spin Hall conductivity occurs in bulk band regions. In the presence of a magnetic field, the QSH plateau remains in the band gap, accompanying nonzero diagonal spin conductivity and half-integer quantum Hall conductivity outside the band gap. All these features arise from the dual role of the SOC impurity. On the one hand, it acts as a driving force to the QSH state. On the other hand, it is a special scatterer to affect the electronic transport properties in graphene.

We investigate the magnetization reversal mechanism in an individual permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is affixed to the tip of a Si cantilever and positioned in order to optimally couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the NT's volume magnetization by dynamic cantilever magnetometry and its stray flux using the nanoSQUID. We observe a training effect and a temperature dependence in the magnetic hysteresis, suggesting an exchange bias. We find a low blocking temperature $T_B=18\pm 2$ K, indicating the presence of a thin antiferromagnetic native oxide, as confirmed by x-ray absorption spectroscopy on similar samples. Furthermore, we measure changes in the shape of the magnetic hysteresis as a function of temperature and increased training. These observations show that the presence of a thin exchange-coupled native oxide modifies the magnetization reversal process at low temperatures. Complementary information obtained via cantilever and nanoSQUID magnetometry allows us to conclude that, in the absence of exchange coupling, this reversal process is nucleated at the NT's ends and propagates along its length as predicted by theory.

To study the evolution of the electronic states and magnetism of the metallic and metalloid atoms across the first-order ferromagnetic transition of (Mn,${\mathrm{Fe})}_2$(P,Si,B) compounds, x-ray absorption (XAS) and magnetic circular dichroism (XMCD) spectra were recorded at the $K$ edges of P, Mn, and Fe. Surprisingly, while the Fe and Mn XAS spectra do not show pronounced changes, the $K$-edge XAS of P is strongly modified across the ferromagnetic transition and is a fingerprint of electronic structure changes at the phase transition. The evolution of the $3p$ electronic states of phosphorus is linked to changes in metal-metalloid bonding. The amplitude of this effect is correlated to the discontinuity in interatomic distances at the transition and progressively disappears with boron substitution. Moreover, it is found that the P atoms carry a finite magnetic moment in the ferromagnetic state in accordance with the results of band-structure calculations.