Diamagnetic vortices in Chern-Simons theory

We find a new type of topological vortex solution in the U(1)(Z) xU(1)(A) Chern-Simons gauge theory in the presence of a U(1)(A) magnetic field background. In this theory U(1)(Z) is broken spontaneously by the U(1)(A) magnetic field. These vortices exhibit long-range interactions as they are charged under the unbroken U(1)(A). They deplete the U(1)(A) magnetic field near their core and also break both charge conjugation and parity symmetries. Understanding the nature of these vortices sheds light on the ground state structure of the superconductivity studied in [1]. We also study the Berezinsky-Kosterlitz-Thouless phase transition in this class of theories and point out that superconductivity can be achieved at high temperatures by increasing the U(1)(A) magnetic field.

Effects of magnetic field on the cuprate high-T-c superconductor La2-xSrxCuO4

Henrik Moodysson Rønnow

This article discusses neutron scattering measurements on the cuprate, high transition temperature superconductor La2-xSrxCuO4 (LSCO) in an applied magnetic field. LSCO is a type-II superconductor and magnetic flux can penetrate the material via the formation of vorticies. Phase coherent superconductivity characterized by zero resistance is suppressed to the lower field-dependent irreversibility temperature (T-irr(H)) and occurs when the vortices freeze into a lattice. Because superconductivity is destroyed within the vortex cores, an investigation of the vortex state provides information about the ground state that would have appeared had superconductivity not intervened. Our measurements reveal that both optimally doped LSCO (x = 0.16, T-c = 38.5 K) and underdoped LSCO (x = 0.10, T-c = 29 K) have an enhanced antiferromagnetic response in a field. Measurements of the optimally doped system for H = 7.5 T show that inelastic sub-gap spin fluctuations first disappear with the loss of finite resistivity at T-irr, but then reappear at a lower temperature with increased lifetime and correlation length compared to the normal state. In the underdoped system elastic antiferromagnetism develops below T-c in zero field, and is significantly enhanced by application of a magnetic field; phase coherent superconductivity is then established within the anti- ferromagnetic phase at T-irr.

2004

Conventional superconductivity and hysteretic Campbell penetration depth in single crystals MgCNi3

Sergiy Katrych

Single crystals of MgCNi3, with areas sized up to 1 mm(2), were grown by the self-flux method using a cubic anvil high-pressure technique. In low applied fields, the dc magnetization exhibited a very narrow transition into the superconducting state, demonstrating good quality of the grown crystals. The first critical field H-c1, determined from a zero-temperature extrapolation, is around 18 mT. Using the tunnel-diode resonator technique, the London penetration depth was measured with no applied dc field and the Campbell penetration depth was measured with the external dc fields up to 9 T for two different sample orientations with respect to the direction of applied magnetic field. The absolute value of the London penetration depth, lambda(0) = 245 +/- 10 nm, was determined from the thermodynamic Rutgers formula. The superfluid density, rho(s) = lambda(0)/lambda(T), was found to follow the clean isotropic s-wave behavior predicted by the weak-coupling BCS theory in the whole temperature range. The low-temperature behavior of the London penetration depth fits the BCS analytic form as well and produces a value close to the weak coupling one of Delta(0)/(k(B)T(c)) = 1.71. The temperature dependence of the upper critical field H-c2 was found to be isotropic with a slope at T-c of -2.6 T/K and H-c2(0) approximate to 12.3 T at zero temperature. The Campbell penetration depth probes the vortex lattice response in the mixed state and is sensitive to the details of the pinning potential. For MgCNi3, an irreversible feature has been observed in the TDR response when the sample is field cooled and warmed versus zero-field cooled and warmed. This feature possesses a nonmonotonic field dependence and has commonly been referred to as the peak effect. It is most likely related to a field-dependent nonparabolic pinning potential. DOI: 10.1103/PhysRevB.87.094520

Amer Physical Soc2013

Electrostatically Induced Superconductivity at the Surface of WS2

Helmuth Berger

We investigate transport through ionic liquid gated field effect transistors (FETs) based on exfoliated crystals of semiconducting WS2. Upon electron accumulation, at surface densities close to, or just larger than, 10(14) cm(2), transport exhibits metallic behavior with the surface resistivity decreasing pronouncedly upon cooling. A detailed characterization as a function of temperature and magnetic field clearly shows the occurrence of a gate-induced superconducting transition below a critical temperature Tc 4 K, a finding that represents the first demonstration of superconductivity in tungsten-based semiconducting transition metal dichalcogenides. We investigate the nature of superconductivity and find significant inhomogeneity, originating from the local detaching of the frozen ionic liquid from the WS2 surface. Despite the inhomogeneity, we find that in all cases where a fully developed zero resistance state is observed, different properties of the devices exhibit a behavior characteristic of a BerezinskiiKosterlitzThouless transition, as it could be expected in view of the two-dimensional nature of the electrostatically accumulated electron system

Amer Chemical Soc2015

Conventional superconductivity and hysteretic Campbell penetration depth in single crystals MgCNi3

Sergiy Katrych

Amer Physical Soc2013