85 research outputs found

### Nernst effect and diamagnetism in phase fluctuating superconductors

When a superconductor is warmed above its critical temperature $T_c$, long
range order is destroyed by fluctuations in the order parameter. These
fluctuations can be probed by measurements of conductivity, diamagnetism, and
of the Nernst effect. Here, we study a regime where superconductivity is
destroyed by phase fluctuations arising from a dilute liquid of mobile
vortices. We find that the Nernst effect and diamagnetic response differ
significantly from Gaussian fluctuations -- in particular, a much sharper decay
with temperature is obtained. We predict a rapid onset of Nernst signal at a
temperature T$_{\rm onset}$ that tracks $T_c$, rather than the pairing
temperature. We also predict a close quantitative connection with diamagnetism
-- the ratio of magnetization to transverse thermoelectric conductivity
$\alpha_{xy}$ reaches a universal value at high temperatures. We interpret
Nernst effect measurements on the underdoped cuprates in terms of a dilute
vortex liquid over a wide temperature range above $T_c$.Comment: 4 pages, 4 figures; (v2) acknowledgments adde

### Metallic quantum critical points with finite BCS couplings

We study the fate of superconductivity in the vicinity of a class of metallic
quantum critical points obtained by coupling a Fermi surface to a critical
boson. In such systems there is a competition between the enhanced pairing
tendency due to the presence of long-range attractive interactions near
criticality, and the suppression of superconductivity due to the destruction of
the Landau quasiparticles. We show that there are regimes in which these two
effects offset one another, resulting in a novel non-Fermi liquid fixed point
with finite, scale invariant, BCS coupling. While these interactions lead to
substantial superconducting fluctuations, they do not drive the system into a
superconducting ground state. The metallic quantum critical fixed points are
connected to the superconducting regime by a continuous phase transition. These
results are established using a controlled expansion in the deviation from d=3
spatial dimensions, as well as in a large number N of internal flavors. We
discuss the possible relevance of our findings to the phenomenon of
superconducting domes condensing out of a non-Fermi liquid normal state near
quantum critical points.Comment: 28 pages, 7 figure

### SEPIC Converter based-Drive for Unipolar BLDC Motor

Front-end single-ended primary inductance converter (SEPIC) and a switch in series with each phase is proposed for driving a permanent magnet brushless dc (BLDC) motor with unipolar currents. All the switches are ground-referenced, which simplifies their gate drives. The available input voltage can be boosted for better current regulation, which is an advantage for low voltage applications. The SEPIC converter is designed to operate in the discontinuous conduction mode for operation with an ac supply. In this operation mode, the line current follows the line voltage waveform to a certain extent. The reduction in low-order harmonics and improved power factor is achieved without the use of any voltage or current sensors. The simplicity and reduced parts count of the proposed topology make it an attractive low-cost choice for many variable speed drive applications. The proposed topology is simulated and verified by using MATLAB/SIMULINK.DOI:http://dx.doi.org/10.11591/ijece.v2i2.30

### Theory of the Three Dimensional Quantum Hall Effect in Graphite

We predict the existence of a three dimensional quantum Hall effect plateau
in a graphite crystal subject to a magnetic field. The plateau has a Hall
conductivity quantized at $\frac{4e^2}{\hbar} \frac{1}{c_0}$ with $c_0$ the
c-axis lattice constant. We analyze the three-dimensional Hofstadter problem of
a realistic tight-binding Hamiltonian for graphite, find the gaps in the
spectrum, and estimate the critical value of the magnetic field above which the
Hall plateau appears. When the Fermi level is in the bulk Landau gap, Hall
transport occurs through the appearance of chiral surface states. We estimate
the magnetic field necessary for the appearance of the three dimensional
quantum Hall Effect to be $15.4$T for electron carriers and $7.0$T for hole
carriers.Comment: Several new references adde

### Two-Dimensional Non-Fermi-Liquid Metals: A Solvable Large-N Limit

Significant effort has been devoted to the study of “non-Fermi-liquid” (NFL) metals: gapless conducting systems that lack a quasiparticle description. One class of NFL metals involves a finite density of fermions interacting with soft order parameter fluctuations near a quantum critical point. The problem has been extensively studied in a large-N limit (N corresponding to the number of fermion flavors) where universal behavior can be obtained by solving a set of coupled saddle-point equations. However, a remarkable study by Lee revealed the breakdown of such approximations in two spatial dimensions. We show that an alternate approach, in which the fermions belong to the fundamental representation of a global SUðNÞ flavor symmetry, while the order parameter fields transform under the adjoint representation (a “matrix large-N” theory), yields a tractable large N limit. At low energies, the system consists of an overdamped boson with dynamical exponent z ¼ 3 coupled to a non-Fermi-liquid with self-energy ΣðωÞ ∼ ω2=3 , consistent with previous studies.Fil: Aguilera Damia, Jeremías. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Kachru, Shamit. University of Stanford; Estados UnidosFil: Raghu, Srinivas. Slac National Accelerator Laboratory; Estados Unidos. University of Stanford; Estados UnidosFil: Torroba, Gonzalo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

### Theory of the low- and high-field superconducting phases of UTe$_2$

Recent nuclear magnetic resonance (NMR) and calorimetric experiments have
observed that UTe$_2$ exhibits a transition between two distinct
superconducting phases as a function of magnetic field strength for a field
applied along the crystalline $b$-axis. To determine the nature of these
phases, we employ a microscopic two-band minimal Hamiltonian with the essential
crystal symmetries and structural details. We also adopt anisotropic
ferromagnetic exchange terms. We study the resulting pairing symmetries and
properties of these low- and high-field phases in mean field theory

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