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Register a new userPhase-coherence and the boson-analogy of vortex liquids
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The statistical mechanics of the flux-line lattice in extreme type-II super- conductors is studied within the framework of the uniformly frustrated anisotropic 3D XY-model. A finite-field counterpart of an Onsager vortex-loop transition in extreme type-II superconductors renders the vortex liquid phase- incoherent when the Abrikosov vortex lattice undergoes a first order melting transition. For the filling fractions considered in this paper $f$ given by $1/f=12,14,16,20,25,32,48,64,72,84,96,112$, and 128, the vortex liquid phase is not describable as a liquid of well-defined field induced vortex lines. This is due to the proliferation of thermally induced closed vortex-loops with dia- meters of order the magnetic length in the problem, resulting in a percolation transition driven by non-field induced vortices also transverse to the direc- tion of the applied magnetic field. This immediately triggers flux-line lattice melting and loss of phase-coherence in all direction. In a non-relativistic 2D boson-analogy picture, this latter feature would correspond to a vanishing mass of the bosons. Scaling functions for the specific heat are calculated in zero and finite magnetic field. From this we conclude that the critical region is of order 10% of $T_c$ for a mass anisotropy $sqrt{M_z/M}=3$, and increases with increasing mass anisotropy. The entropy jump at the melting transition is cal- culated as a function of magnetic field for a mass-ansitropy slightly lower than that in $YBCO$, and found to be $Delta S=0.1k_B$ per vortex panncake, independent of the magnetic field for the filling fractions considered here. This is slightly lower than experimental values of $Delta S approx 0.4k_B$ found experimentally for $YBCO$. We attribute this to the slightly lower mass anisotropy used in our simulations.
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We derive a hydrodynamic model for a liquid of arbitrarily curved flux-lines and vortex loops using the mapping of the vortex liquid onto a liquid of relativistic charged quantum bosons in 2+1 dimensions recently suggested by Tesanovic and by Sudbo and collaborators. The loops in the flux-line system correspond to particle-antiparticle fluctuations in the bosons. We explicitly incorporate the externally applied magnetic field which in the boson model corresponds to a chemical potential associated with the conserved charge density of the bosons. We propose this model as a convenient and physically appealing starting point for studying the properties of the vortex liquid.
The flux pinning force density (Fp) of the single crystalline FeTe0.60Se0.40 superconductor has been calculated from the magnetization measurements. The normalized Fp versus h (=H/Hirr) curves are scaled using the Dew-Hughes formula to underline the pinning mechanism in the compound. The obtained values of pinning parameters p and q indicate the vortex pinning by the mixing of the surface and the point core pinning of the normal centers. The vortex phase diagram has also been drawn for the first time for the FeTe0.60Se0.40, which has very high values of critical current density Jc ~ 1.10(5) Amp/cm2 and the upper critical field Hc2(0) = 65T, with a reasonably high transition temperature Tc =14.5K.
The large ($10^2 - 10^5$) and strongly temperature dependent resistive anisotropy $eta = (sigma_{ab}/sigma_c)^{1/2}$ of cuprates perhaps holds the key to understanding their normal state in-plane $sigma_{ab}$ and out-of-plane $sigma_{c}$ conductivities. It can be shown that $eta$ is determined by the ratio of the phase coherence lengths $ell_i$ in the respective directions: $sigma_{ab}/sigma_c = ell_{ab}^2/ell_{c}^2$. In layered crystals in which the out-of-plane transport is incoherent, $ell_{c}$ is fixed, equal to the interlayer spacing. As a result, the T-dependence of $eta$ is determined by that of $ell_{ab}$, and vice versa, the in-plane phase coherence length can be obtained directly by measuring the resistive anisotropy. We present data for hole-doped $YBa_2Cu_3O_y$ ($6.3 < y < 6.9$) and $Y_{1-x}Pr_xBa_2Cu_3O_{7-delta }$ ($0 < x leq 0.55$) and show that $sigma_{ab}$ of crystals with different doping levels can be well described by a two parameter universal function of the in-plane phase coherence length. In the electron-doped $Nd_{2-x}Ce_{x}CuO_{4-y}$, the dependence $sigma_{ab}(eta)$ indicates a crossover from incoherent to coherent transport in the c-direction.
The phase diagram of underdoped cuprates in a magnetic field ($H$) is the key ingredient in understanding the anomalous normal state of these high-temperature superconductors. However, the upper critical field ($H_{c2}$) or the extent of superconducting phase with vortices, a type of topological excitations, and the role of charge orders that are present at high $H$, remain under debate. We address these questions by studying stripe-ordered La-214, i.e. cuprates in which charge orders are most pronounced and zero-field transition temperatures $T_{c}^{0}$ are lowest; the latter opens a much larger energy scale window to explore the vortex phases compared to previous studies. By combining linear and nonlinear transport techniques sensitive to vortex matter, we determine the $T$-$H$ phase diagram, directly detect $H_{c2}$, and reveal novel properties of the high-field ground state. Our results demonstrate that, while the vortex phase diagram of underdoped cuprates is not very sensitive to the details of the charge orders, quantum fluctuations and disorder play a key role as $Trightarrow 0$. The presence of stripes, on the other hand, seems to alter the nature of the anomalous normal state, such that the high-field ground state is a metal, as opposed to an insulator.
The large diversity of exotic electronic phases displayed by two-dimensional superconductors confronts physicists with new challenges. These include the recently discovered quantum Griffith singularity in atomic Ga films, topological phases in proximized topological insulators and unconventional Ising pairing in transition metal dichalcogenide layers. In LaAlO3/SrTiO3 heterostructures, a gate tunable superconducting electron gas is confined in a quantum well at the interface between two insulating oxides. Remarkably, the gas coexists with both magnetism and strong Rashba spin-orbit coupling and is a candidate system for the creation of Majorana fermions. However, both the origin of superconductivity and the nature of the transition to the normal state over the whole doping range remain elusive. Missing such crucial information impedes harnessing this outstanding system for future superconducting electronics and topological quantum computing. Here we show that the superconducting phase diagram of LaAlO3/SrTiO3 is controlled by the competition between electron pairing and phase coherence. Through resonant microwave experiments, we measure the superfluid stiffness and infer the gap energy as a function of carrier density. Whereas a good agreement with the Bardeen-Cooper-Schrieffer (BCS) theory is observed at high carrier doping, we find that the suppression of Tc at low doping is controlled by the loss of macroscopic phase coherence instead of electron pairing as in standard BCS theory. We find that only a very small fraction of the electrons condenses into the superconducting state and propose that this corresponds to the weak filling of a high-energy dxz/yz band, more apt to host superconductivity
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