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

Multiple nodeless superconducting gaps in optimally-doped SrTi1−x_{1-x}Nbx_{x}O3_{3}

We present the first study of thermal conductivity in superconducting SrTi$_{1-x}$Nb$_{x}$O$_{3}$, sufficiently doped to be near its maximum critical temperature. The bulk critical temperature, determined by the jump in specific heat, occurs at a significantly lower temperature than the resistive T$_{c}$. Thermal conductivity, dominated by the electron contribution, deviates from its normal-state magnitude at bulk T$_{c}$, following a Bardeen-Rickayzen-Tewordt (BRT) behavior, expected for thermal transport by Bogoliubov excitations. Absence of a T-linear term at very low temperatures rules out the presence of nodal quasi-particles. On the other hand, the field dependence of thermal conductivity points to the existence of at least two distinct superconducting gaps. We conclude that optimally-doped strontium titanate is a multigap nodeless superconductor.Comment: 6 pages including a supplemen

Magnetic order in the pseudogap phase of high-TCT_C superconductors

One of the leading issues in high-$T_C$ superconductors is the origin of the pseudogap phase in underdoped cuprates. Using polarized elastic neutron diffraction, we identify a novel magnetic order in the YBa$_2$Cu$_3$O$_{6+x}$ system. The observed magnetic order preserves translational symmetry as proposed for orbital moments in the circulating current theory of the pseudogap state. To date, it is the first direct evidence of an hidden order parameter characterizing the pseudogap phase in high-$T_C$ cuprates.Comment: 3 figure

Anisotropic and strong negative magneto-resistance in the three-dimensional topological insulator Bi2Se3

We report on high-field angle-dependent magneto-transport measurements on epitaxial thin films of Bi2Se3, a three-dimensional topological insulator. At low temperature, we observe quantum oscillations that demonstrate the simultaneous presence of bulk and surface carriers. The magneto- resistance of Bi2Se3 is found to be highly anisotropic. In the presence of a parallel electric and magnetic field, we observe a strong negative longitudinal magneto-resistance that has been consid- ered as a smoking-gun for the presence of chiral fermions in a certain class of semi-metals due to the so-called axial anomaly. Its observation in a three-dimensional topological insulator implies that the axial anomaly may be in fact a far more generic phenomenon than originally thought.Comment: 6 pages, 4 figure

Dietary carotenoids and the complex role of redness in the behavior of the firemouth cichlid Thorichthys meeki.

This dissertation takes a comprehensive approach to the role of dietary carotenoids on redness and the subsequent behaviors in the firemouth cichlid, Thorichthys meeki. I start with a brief introduction into signaling, the importance of carotenoids, and mate choice. The dissertation is then divided into three data chapters which are designed to stand as independent manuscripts. Chapter II documents how altering the availability of dietary carotenoids affects redness in the integument of male and female T. meeki. I tracked how redness changed in color and distribution in individuals over the course of 12 weeks. I confirm that a dichotomy in redness can be obtained in this time period via diet alone. However, carotenoids are used by animals for more than red ornamentation including color vision. To account for this potential effect of carotenoids in my study animals, I examined if color vision was affected by the high- and low-carotenoid diet treatments (chapter III). Furthermore, I determined whether redness (a trait for which T. meeki is named) is innately attractive to this species (chapter III). The sensory bias hypothesis suggests that males and females utilize traits that are innately appealing to the opposite sex to attract a mate. My final data chapter examines whether males and females use redness and other visual displays as a signal in mate selection (chapter IV). I conclude this dissertation by summarizing my findings and proposing future directions in which I wish to examine this system further (chapter V)