O. Entin-Wohlman

Magnetically driven ferroelectric order in Ni3V2O8

We show that long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition in Ni3V2O8. The temperature and magnetic-field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry using Landau theory for continuous phase transitions, which shows that the spin structure alone can break spatial inversion symmetry leading to ferroelectric order. This phenomenological theory explains our experimental observation that the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.

Three-terminal thermoelectric transport through a molecular junction

The thermoelectric transport through a molecular bridge is discussed, with an emphasis on the effects of inelastic processes of the transport electrons caused by the coupling to the vibrational modes of the molecule. In particular it is found that when the molecule is strongly coupled to a thermal bath of its own, which may be at a temperature different from those of the electronic reservoirs, a heat current between the molecule and the electrons can be converted into electric current. Expressions for the transport coefficients governing this conversion and similar ones are derived, and a possible scenario for increasing their magnitudes is outlined.

Competing magnetic phases on a kagomé staircase

We present thermodynamic and neutron data on Ni3V2O8, a spin-1 system on a kagomé staircase. The extreme degeneracy of the kagomé antiferromagnet is lifted to produce two incommensurate phases at finite T--one amplitude modulated, the other helical--plus a commensurate canted antiferromagnet for T-->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni3V2O8 thus provides an elegant example of order from subleading interactions in a highly frustrated system.

Variable-range hopping conductivity in granular materials

The temperature dependence of the conductivity of high-resistivity granular systems is analysed in terms of the Efros-Shklovskii theory (1975), which focuses on the role of the Coulomb gap. The divergence of the static dielectric constant near the metal-insulator transition is found to be manifested by experimental data. The possibility that granular samples will show a crossover between the behaviour predicted by Efros and Shklovskii and the Mott variable-range hopping law is discussed.

Long-range order in electronic transport through disordered metal films.

Ultracold atom magnetic field microscopy enables the probing of current flow patterns in planar structures with unprecedented sensitivity. In polycrystalline metal (gold) films, we observed long-range correlations forming organized patterns oriented at ±45° relative to the mean current flow, even at room temperature and at length scales larger than the diffusion length or the grain size by several orders of magnitude. The preference to form patterns at these angles is a direct consequence of universal scattering properties at defects. The observed amplitude of the current direction fluctuations scales inversely to that expected from the relative thickness variations, the grain size, and the defect concentration, all determined independently by standard methods. Ultracold atom magnetometry thus enables new insight into the interplay between disorder and transport.

Adiabatic transport in nanostructures

A confined system of noninteracting electrons, subject to the combined effect of a time-dependent potential and different external chemical potentials, is considered. The current flowing through such a system is obtained by using the adiabatic approximation in an iterative manner. A formula is derived for the charge pumped through an unbiased system (all external chemical potentials are kept at the same value); it reproduces the Brouwer formula for a two-terminal nanostructure. The formalism presented yields the effect of the chemical-potential bias on the pumped charge on one hand, and the modification of the Landauer formula (which gives the current in response to a constant chemical-potential difference) brought about by the modulating potential on the other. Corrections to the adiabatic approximation are derived and discussed.

Pair susceptibility and mode propagation in superconductors: A microscopic approach

We derive the full microscopic set of equations governing small oscillations: (1) in the magnitude of the superconducting order parameter (the Schmid mode), (2) the phase of the order parameter in a neutral superfluid (the Anderson-Bogoliubov mode), and (3) the coupled oscillations in the phase of the order parameter and in the electric field (the transverse, or Carlson-Goldman mode). The derivation is not limited by the restrictions of previous papers. No limitations are required for the magnitude of the frequency, the concentration of impurities, or the magnitude of the temperature. Special attention is given to the Carlson-Goldman (CG) mode, whose dispersion law frequency (Ω) vs. wave vector (k) and damping is calculated. The velocity of the CG mode in the propagation region % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqyYdCNaai% 4laiaacYhacaqGRbGaaeiFaaaa!3B5A!\[\omega /|{\text{k|}}\] is found to equal % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaae4yaiaab2% dacaqGBbGaaeOmaiabfs5aejaabseadaWgaaWcbaGaeq4Xdmgabeaa% kiaacIcacaaIYaGaeqiWdaNaaeivaiabes8a0jaabMcacaqGDbaaaa!448D!\[{\text{c = [2}}\Delta {\text{D}}_\chi (2\pi {\text{T}}\tau {\text{)]}}\], where D is the diffusion constant and χ is the function appearing in the theory of superconducting alloys. In the dirty (l « ξ0) and clean (l ≫ ξ0) limits, this expression reduces to those previously derived by Schmid and Schön, and by Artemenko and Volkov, respectively. At large values of k, the frequency of the CG mode approaches a limiting value of 2δ. The damping is small in this limit and tends to zero as ¦k¦ increases. p ]Our results are obtained by calculating the linear response of a superconductor to a perturbation in the magnitude and phase of the order parameter, and the electromagnetic potentials. The response of the superconductor to these perturbations is calculated by properly continuing the thermodynamic perturbation function of linear response from imaginary frequencies to the real ones, then inserting into the self-consistency BCS equation and Poissons equation. The derivation is based on the self-consistent BCS scheme. No kinetic equations are introduced at any stage of the calculation.

Towards a Microscopic Model of Magnetoelectric Interactions in Ni 3 V 2 O 8

We develop a microscopic magnetoelectric coupling in Ni

Complex magnetic order in the kagomé staircase compound Co3V2O8

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Three-terminal thermoelectric transport through a molecule placed on an Aharonov-Bohm ring

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