Intrinsic pinning of vortices as a direct probe of the nonuniform Larkin-Ovchinnikov-Fulde-Ferrell state in layered superconductors

Previously the search for the modulated superconducting Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state was performed by means of measurements which do not give direct information on spatial modulation of the superconducting state. We propose to measure interlayer conductivity in Josephson-coupled layered superconductors as a function of the strength and the orientation of the parallel magnetic field. We show that interlayer critical current and the conductivity have peaks when the magnetic field is perpendicular to the in-plane wave vector of the LOFF state and when the period of the Josephson vortex lattice induced by the magnetic field is commensurate with the LOFF period.     

Neutral Larkin-Ovchinnikov-Fulde-Ferrell state and chromomagnetic instability in two-flavor dense QCD

In two-flavor dense quark matter, we describe the dynamics in the single plane wave Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state satisfying the color and electric neutrality conditions. We find that because the neutral LOFF state itself suffers from a chromomagnetic instability in the whole region where it coexists with the (gapped or gapless) two-flavor superconducting phases, it cannot cure this instability in those phases. This is unlike the recently revealed gluonic phase which seems to be able to resolve this problem.     

Inhomogeneous stripe phase revisited for surface superconductivity

We consider 2D surface superconductivity in high magnetic fields parallel to the surface. We demonstrate that the spin-orbit interaction at the surface changes the properties of the inhomogeneous superconducting Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state that develops above fields given by the paramagnetic criterion. Strong spin-orbit interaction significantly broadens the range of existence of the LOFF phase, which takes the form of periodic superconducting stripes running along the field direction on the surface, leading to the anisotropy of its properties. Our results provide a tool for studying surface superconductivity as a function of doping.     

Superconductivity in an organic insulator at very high magnetic fields

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor lambda-(BETS)2FeCl4. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned Fe3+ ions. We further argue that the Fe3+ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state.     

两味LOFF态下的色超导

自从色超导理论被提出以来, 通常考虑的是参与配对的夸克的化学势不相等时的情形。 当化学势的差别达到某一合适值时, 库柏对就有非零的总动量, 这就是Larkin Ovchinnikov Fulde Ferre(LOFF)态。 这种形式的夸克凝聚自发破坏了平移不变性和旋转不变性, 导致能隙以晶格的形式周期性变化。 在中等重子数密度区的基础上, 从SU（2） NJL模型出发描述两味LOFF态, 并通过平均场近似, 引用N G基底、 傅立叶变换和频率求和等方法得到热力学势, 进而通过热力学势对序参量求偏导得到耦合的Gap方程, 并使用数值法解耦合方程找到LOFF态的窗口。 Ever since the theory of color superconductivity was issued, it is likely to involve pairing between species of quarks with differing chemical potentials. For suitable values of the differences between chemical potentials, Cooper pairs with non zero total momentum are favored, as was first realized by Larkin, Ovchinnikov, Fulde and Ferrell (LOFF). Condensates of this sort spontaneously break translational and rotational invariance, leading to gaps which vary periodically in a crystalline pattern. This article focuses on the two flavor color superconducting phase at moderate baryon density. LOFF state is described through SU(2) NJL model. By using the mean field approximation, N G basis, fourier transformation, frequency summation, the thermodynamic potential and Gap equation are obtained. Finally, the window of LOFF state is found by the numerical method.     

Vortex dynamics and the Fulde-Ferrell-Larkin-Ovchinnikov state in a magnetic-field-induced organic superconductor

Under special conditions, a superconducting state where the order parameter oscillates in real space, the so-called FFLO state, is theoretically predicted to exist near the upper critical field, as first proposed by Fulde and Ferrell, and Larkin and Ovchinnikov. We report systematic measurements of the interlayer resistance in high magnetic fields to 45 T in the two-dimensional magnetic-field-induced organic superconductor lambda-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. The resistance is found to show characteristic dip structures in the superconducting state. The results are consistent with pinning interactions between the vortices penetrating the insulating layers and the order parameter of the FFLO state. This gives strong evidence for an oscillating order parameter in real space.     

Metallic normal state of Y1Ba2Cu3O(7-delta)

Flux flow was studied over an entire temperature range down to T approximately 2% of T(c) by using intense pulsed current densities to overcome flux-vortex pinning. The resistivity at high vortex velocities is proportional to B and roughly follows rho approximately rho(n)B/H(c2), with a prefactor of order unity. Contrary to some speculation, rho(n) saturates to a finite residual value as T-->0, indicating a metallic (rho-->finite) rather than insulating (rho-->infinity) normal state, and the vortex dissipation continues to be conventional as T-->0.     

Anisotropy of Critical Fields in MgB2： Two-Band Ginzburg-Landau Theory for Layered Superconductors

The temperature dependence of the anisotropy parameter of upper critical field γHc2(T)=H_{c2}^||(T)/H_{c2}^{\bot}(T) and London penetration depth γ_λ(T)=λ_{\perp} (T)/λ_{\bot} (T) are calculated using two-band Ginzburg-Landau theory for layered superconductors. It is shown that, with decreasing temperature the anisotropy parameter γ_{H_{c2}}(T) is increased, while theLondon penetration depth anisotropy γλ(T) reveals n opposite behavior.Results of our calculations are in agreement with experimental datafor single crystal MgB₂ nd with other calculations. Results of an analysis of magnetic field H_c1 in a single vortex between superconducting layers are also presented.     

Disordered states in IPA-Cu(ClxBr1-x)3 induced by bond randomness

The mixtures of two spin-gap compounds IPA-Cu(ClxBr1-x)3 are studied by electron paramagnetic resonance and magnetization processes [M(H)]. From electron paramagnetic resonance spectra, the symmetry of the spin-gap state breaks down, even for x=0.99. From M(H) curves for x=0.95 and 0.92, however, spin gaps survive below mu0Hc1=10+/-1 T, and the M(H) slopes bend at mu0Hc3=40+/-1 T, below the saturation field Hc2. Such a curvature suggests an exotic phase transition: Bose-Einstein condensation of spin triplets occurs at Hc1 相似文献   

Quantum criticality in an organic magnet

Exchange interactions between S=1/2 sites in piperazinium hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground state. We have determined the field-temperature phase diagram by high field magnetization and neutron scattering experiments. There are two quantum critical points: Hc1=7.5 T separates a quantum paramagnet phase from a three dimensional, antiferromagnetically ordered state while Hc2=37 marks the onset of a fully polarized state. The ordered phase, which we describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum critical regime with short range correlations. A low temperature anomaly in the BEC phase boundary indicates that additional low energy features of the material become important near Hc1.     

Abrupt change of josephson plasma frequency at the phase boundary of the bragg glass in Bi(2)Sr(2)CaCu(2)O(8+delta)

We report the first detailed and quantitative study of the Josephson coupling energy in the vortex liquid, Bragg glass, and vortex glass phases of Bi(2)Sr(2)CaCu(2)O(8+delta) by the Josephson plasma resonance. The measurements revealed distinct features in the T and H dependencies of the plasma frequency omega(pl) for each of these three vortex phases. When going across either the Bragg-to-vortex glass or the Bragg-to-liquid transition line, omega(pl) shows a dramatic change. We provide a quantitative discussion on the properties of these phase transitions, including the first order nature of the Bragg-to-vortex glass transition.     

BKT phase transition in a 2D system with long-range dipole-dipole interaction

We consider phase transitions in 2D XY-like systems with long-range dipole-dipole interactions and demonstrate that BKT-type phase transition always occurs separating the ordered (ferroelectric) and the disordered (paraelectric) phases. The low-temperature phase corresponds to a thermal state with bound vortex-antivortex pairs characterized by linear attraction at large distances. Using the Maier-Schwabl topological charge model, we show that bound vortex pairs polarize and screen the vortex-antivortex interaction, leaving only the logarithmic attraction at sufficiently large separations between the vortices. At higher temperatures the pairs dissociate and the phase transition similar to BKT occurs, though at a larger temperature than in a system without the dipole-dipole interaction.     

Correlation between superfluid density and T(C) of underdoped YBa2Cu3O6+x near the superconductor-insulator transition

We report measurements of the ab-plane superfluid density n(s) (magnetic penetration depth lambda) of heavily underdoped films of YBa2Cu3O6+x, with T(C)'s from 6 to 50 K. We find the characteristic length for vortex unbinding transition equal to the film thickness, suggesting strongly coupled CuO2 layers. At the lowest dopings, T(C) is as much as 5 times larger than the upper limit set by the 2D Kosterlitz-Thouless-Berezinskii transition temperature calculated for individual CuO2 bilayers. Our main finding is that T(C) is not proportional to n(s)(0); instead, we find T(C) proportional to ns(1/2.3+/-0.4). This conflicts with a popular point of view that quasi-2D thermal phase fluctuations determine the transition temperature.     

Ni(C3H10N2)2NO2(ClO4)晶体的ESR研究

报道了Ni(C₃H₁₀N₂)₂NO₂(ClO₄)晶体在T=1.5K温度和W波段的ESR实验.建立了d⁸离子基态³A₂(F)的零场分裂参量D,E,和g因子与斜方对称晶场势参量间的关系,并应用于Ni(C₃H₁₀N₂)₂NO₂(ClO₄)晶体.计算值与实验数据符合很好,表明所给关系式是合理的.     

Instability of square vortex lattice in d-wave superconductors is due to paramagnetic depairing

The effects of paramagnetic depairing on structural transitions between vortex lattices of a quasi-two-dimensional d-wave superconductor are examined. We find that, when the Maki parameter alphaM is of order unity, a square lattice induced by a d-wave pairing is destabilized with increasing fields, and that a reentrant rhombic lattice occurs in higher fields. Further, a weak Fermi surface anisotropy competitive with the pairing symmetry induces another structural transition near Hc2. These results are consistent with the structure changes of the vortex lattice in CeCoIn5 in H parallel c determined from recent neutron scattering data.     

Phase glass is a Bose metal: a new conducting state in two dimensions

In the quantum rotor model with random exchange interactions having a nonzero mean, three phases, a (i) phase (Bose) glass, (ii) superfluid, and (iii) Mott insulator, meet at a bicritical point. We demonstrate that proximity to the bicritical point and the coupling between the energy landscape and the dissipative degrees of freedom of the phase glass lead to a metallic state at T = 0. Consequently, the phase glass is unique in that it represents a concrete example of a metallic state that is mediated by disorder, even in 2D. We propose that the experimentally observed metallic phase which intervenes between the insulator and the superconductor in a wide range of thin films is in actuality a phase glass.     

Integrative models for asymmetric fermi superfluids: emergence of a new exotic pairing phase

We introduce an exactly solvable model to study the competition between the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) and breached-pair superfluid in strongly interacting ultracold asymmetric Fermi gases. One can thus investigate homogeneous and inhomogeneous states on equal footing and establish the quantum phase diagram. For certain values of the filling and the interaction strength, the model exhibits a new stable exotic pairing phase which combines an inhomogeneous state with an interior gap to pair excitations. It is proven that this phase is the exact ground state in the strong-coupling limit, while numerical examples in finite lattices show that also at finite interaction strength it can have lower energy than the breached-pair or LOFF states.     

Correlated phases of bosons in the flat lowest band of the dice lattice

We study correlated phases occurring in the flat lowest band of the dice-lattice model at flux density one-half. We discuss how to realize this model, also referred to as the T(3) lattice, in cold atomic gases. We construct the projection of the model to the lowest dice band, which yields a Hubbard Hamiltonian with interaction-assisted hopping processes. We solve this model for bosons in two limits. In the limit of large density, we use Gross-Pitaevskii mean-field theory to reveal time-reversal symmetry breaking vortex lattice phases. At low density, we use exact diagonalization to identify three stable phases at fractional filling factors ν of the lowest band, including a classical crystal at ν = 1/3, a supersolid state at ν = 1/2, and a Mott insulator at ν = 1.     

Superfluid phase transition in two-dimensional excitonic systems

We study the superfluid phase transition in the two-dimensional (2D) excitonic system. Employing the extended Falicov–Kimball model (EFKM) and considering the local quantum correlations in the system composed of conduction band electrons and valence band holes we demonstrate the existence of the excitonic insulator (EI) state in the system. We show that at very low temperatures, the particle phase stiffness in the pure-2D excitonic system, governed by the non-local cross correlations, is responsible for the vortex–antivortex binding phase-field state, known as the Berezinskii–Kosterlitz–Thouless (BKT) superfluid state. We demonstrate that the existence of excitonic insulator phase is a necessary prerequisite, leading to quasi-long-range order in the 2D excitonic system.     

Crystalline order in superfluid (3)He films

We predict an inhomogeneous phase of superfluid (3)He films in which translational symmetry is spontaneously broken in the plane of the film. This phase is energetically favored over a range of film thicknesses, D(c2)(T) < D < D(c1)(T), separating distinct homogeneous superfluid phases. The instability at the critical film thickness, D(c2) approximately 9 xi (T), is a single-mode instability generating striped phase order in the film. Numerical calculations of the order parameter and free energy indicate a second-order instability to a periodic lattice of degenerate B-like phases separated by domain walls at D(c1) approximately 13 xi (T).