131 research outputs found

### What is measured when measuring a thermoelectric coefficient?

A thermal gradient generates an electric field in any solid hosting mobile
electrons. In presence of a finite magnetic field (or Berry curvature) this
electric field has a transverse component. These are known as Seebeck and
Nernst coefficients. As Callen argued, back in 1948, the Seebeck effect
quantifies the entropy carried by a flow of charged particles in absence of
thermal gradient. Similarly, the Nernst conductivity, $\alpha_{xy}$, quantifies
the entropy carried by a flow of magnetic flux in absence of thermal gradient.
The present paper summarizes a picture in which the rough amplitude of the
thermoelectric response is given by fundamental units and material-dependent
length scales. Therefore, knowledge of material-dependent length scales allows
predicting the amplitude of the signal measured by experiments. Specifically,
the Nernst conductivity scales with the square of the mean-free-path in metals.
Its anomalous component in magnets scales with the square of the fictitious
magnetic length. Ephemeral Cooper pairs in the normal state of a superconductor
generate a signal, which scales with the square of the superconducting
coherence length and smoothly evolves to the signal produced by mobile vortices
below the critical temperature.Comment: 16 pages, 6 figure

### Signatures of Electron Fractionalization in Ultraquantum Bismuth

Because of the long Fermi wavelength of itinerant electrons, the quantum
limit of elemental bismuth (unlike most metals) can be attained with a moderate
magnetic field. The quantized orbits of electrons shrink with increasing
magnetic field. Beyond the quantum limit, the circumference of these orbits
becomes shorter than the Fermi wavelength. We studied transport coefficients of
a single crystal of bismuth up to 33 tesla, which is deep in this ultraquantum
limit. The Nernst coefficient presents three unexpected maxima that are
concomitant with quasi-plateaus in the Hall coefficient. The results suggest
that this bulk element may host an exotic quantum fluid reminiscent of the one
associated with the fractional quantum Hall effect and raise the issue of
electron fractionalization in a three-dimensional metal.Comment: 9 pages, four figures and supposrting online materia

### How heat propagates in `non-Fermi liquid' $^3$He

In Landau's Fermi liquid, transport is governed by scattering between
quasi-particles. The normal liquid $^3$He conforms to this picture, but only
when T$< 0.02$ T$_F$. Here, we observe that the deviation from the standard
behavior is concomitant with the fermion-fermion scattering time falling below
the Planckian time, $\frac{\hbar}{k_BT}$. The thermal diffusivity of this
quantum liquid is bounded by a minimum set by fundamental physical constants
and earlier observed in classical liquids. This implies that collective
excitations of the liquid (a sound mode) are carrying heat. We argue that if
heat is carried by 2k$_F$ hydrodynamic sound mode, both the amplitude and the
hitherto unexplained $T^{1/2}$ temperature dependence of thermal conductivity
find an explanation with no other adjustable parameter.Comment: 7 pages, 4 figures and a supplemen

### Nernst effect and dimensionality in the quantum limit

Nernst effect, the transverse voltage generated by a longitudinal thermal
gradient in presence of magnetic field has recently emerged as a very
sensitive, yet poorly understood, probe of electron organization in solids.
Here we report on an experiment on graphite, a macroscopic stack of graphene
layers, which establishes a fundamental link between dimensionality of an
electronic system and its Nernst response. In sharp contrast with single-layer
graphene, the Nernst signal sharply peaks whenever a Landau level meets the
Fermi level. This points to the degrees of freedom provided by finite
interlayer coupling as a source of enhanced thermoelectric response in the
vicinity of the quantum limit. Since Landau quantization slices a
three-dimensional Fermi surface, each intersection of a Landau level with the
Fermi level modifies the Fermi surface topology. According to our results, the
most prominent signature of such a topological phase transition emerges in the
transverse thermoelectric response.Comment: 13 pages, 4 figures and supplementary information; To appear in
Nature Physic

### Angle dependence of the orbital magnetoresistance in bismuth

We present an extensive study of angle-dependent transverse magnetoresistance
in bismuth, with a magnetic field perpendicular to the applied electric current
and rotating in three distinct crystallographic planes. The observed angular
oscillations are confronted with the expectations of semi-classic transport
theory for a multi-valley system with anisotropic mobility and the agreement
allows us to quantify the components of the mobility tensor for both electrons
and holes. A quadratic temperature dependence is resolved. As Hartman argued
long ago, this indicates that inelastic resistivity in bismuth is dominated by
carrier-carrier scattering. At low temperature and high magnetic field, the
threefold symmetry of the lattice is suddenly lost. Specifically, a $2\pi/3$
rotation of magnetic field around the trigonal axis modifies the amplitude of
the magneto-resistance below a field-dependent temperature. By following the
evolution of this anomaly as a function of temperature and magnetic field, we
mapped the boundary in the (field, temperature) plane separating two electronic
states. In the less-symmetric state, confined to low temperature and high
magnetic field, the three Dirac valleys cease to be rotationally invariant. We
discuss the possible origins of this spontaneous valley polarization, including
a valley-nematic scenario.Comment: 15 pages, 14 figure

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