Mottness induced superfluid phase fluctuation with increased density

Why the superfluid density tracks $T_{\rm c}$ in cuprate superconductors?

One of the first finding concerning the superconducting (SC) density $n_{\rm sc}$ in cuprates was their small magnitudes that revealed the importance of phase fluctuations. More recently, measurements in a variety of overdoped cuprates indicate that it is also much smaller than expected from BCS theories and falls smoothly to zero as doping is increased. We explain these observations by an electronic phase separation theory with a Ginzburg-Landau potential $V_{\rm GL}$ that produces alternating charge domains whose fluctuations lead to localized SC order parameters that are connected by Joseph…

Field Dependent Superfluid Density in the Optimally Doped SmFeAsO_(1-x)F_y Superconductor

The magnetic field dependence of the in-plane magnetic penetration depth for optimally doped SmFeAsO_(1-x)F_y was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density as the field is increased up to 1.4 T. This behavior is analysed within a two-band model with self-consistently derived coupled gaps, where the superfluid density related to the larger gap is field independent and the superfluid density related to the smaller gap is strongly suppressed with inc…

磁序对单晶ErNi $_{2}$ B $_{2}$ C超流响应的影响：穿透深度研究

Effect of magnetic order on the superfluid response of single-crystal ErNi$_{2}$B$_{2}$C: A penetration depth study

我们报道了使用隧道二极管为基础的自感技术在21 MHz下测量ErNi $_{2}$ B $_{2}$ C单晶中的面内磁穿透深度$Δλ$（T），直到$\sim$ 0.1 K。我们观察到四个特征：（1）在N $\acute{e}$ el温度$T_{N}$ = 6.0 K时，$Δλ$（T）略有下降，（2）在$T_{WFM}$ = 2.3 K时出现峰值，此时出现弱铁磁成分，（3）在0.45 K时出现另一个最大值，（4）最终大幅下降至0.1 K。转换到超流密度$ρ_{s}$，我们看到，在6 K的反铁磁秩序只是稍微抑制超导性。我们试图在反铁磁超导体的背景下解释上述一些特征，其中反铁磁分子场和自旋涨落散射之间的竞争决定了成对断裂的增加或减少。超流密度数据显示，在2.3 K附近的功能，对密度只有轻微的下降，从而支持在此温度范围内对体铁磁性的其他证据。

Enhancement of the critical temperature in cuprate superconductors by inhomogeneous doping

We use a renormalized mean field theory to investigate the superconducting properties of underdoped cuprates embedded with overdoped or metallic regions that carry excess dopants. The overdoped regions are considered, within two different models, first as stripes of mesoscopic size larger than the coherence length and then as point impurities. In the former case we compute the temperature dependent superfluid stiffness by solving Bogoliubov de Gennes equations within the slave boson mean field theory. We average over stripes of different orientations to obtain an isotropic result. To compute t…

不等布居数超冷气体中的BCS超流性

BCS superfluidity in ultracold gases with unequal atomic populations

我们考虑了$^{6}$ Li中两个原子数不等的超精细态的配对导致BCS超流相的存在。我们表明，这一阶段的存在域将增加到一个非常大的程度在附近的阈值崩溃。这是由于存在具有各向异性序参量的新相。这个相位是由散射的各向异性部分引起的，与原子之间通过密度涨落交换的间接相互作用有关。

Half-Quantum Vortices in Thin Film of Superfluid $^3$He

Stability of a half-quantum vortex (HQV) in superfluid $^3$He has been discussed recently by Kawakami, Tsutsumi and Machida in Phys. Rev. B {\bf 79}, 092506 (2009). We further extend this work here and consider the A$_2$ phase of superfluid $^3$He confined in thin slab geometry and analyze the HQV realized in this setting. Solutions of HQV and singly quantized singular vortex are evaluated numerically by solving the Ginzburg-Landau (GL) equation and respective first critical angular velocities are obtained by employing these solutions. We show that the HQV in the A$_2$ phase is stable near the…

超越随机相近似在电子-空穴双层超流中的应用

Beyond Random Phase Approximation in electron-hole bilayer superfluidity

本文研究了电子-空穴双层超流中一阶修正对屏蔽和有效配对的影响，发现RPA在低密度下仍为良好近似。

Locating the missing superconducting electrons in overdoped cuprates

Overdoped high-temperature cuprate superconductors have been widely believed to be described by the physics of d-wave BCS-like superconductivity. However, recent measurements indicate that as the doping is increased, the superfluid density decreases smoothly to zero rather than increasing as expected by BCS theory in the absence of disorder. Here, we combine time-domain THz spectroscopy with kHz range mutual inductance measurements on the same overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ films to determine both the superfluid and the uncondensed carrier density as a function of doping. A significant …

Possible evidence for two-gap superconductivity in TiSe_2Cu_x

We report a comprehensive TF-muSR study of TiSe_2Cu_2. The magnetic penetration depth was found to saturate at low temperature as expected in an s-wave SC. As x is increased we find that the superfluid density increases and the size of the superconducting gap, calculated from the temperature dependence of the superfluid density, is approaching the BCS value. However, for low values of x, the gap is smaller than the weak-coupling BCS prediction suggesting that two superconducting gaps are present in the sample.

Spontaneous Breakdown of Time-Reversal Symmetry Induced by Thermal Fluctuations

In systems with broken $U(1)$ symmetry, such as superfluids, superconductors or magnets, the symmetry restoration is driven by proliferation of topological defects in the form of vortex loops. Here we discuss that in certain systems the proliferation of topological defects can, by contrast, lead to the breakdown of an additional symmetry. As a particular example we demonstrate that this effect should take place in $s+is$ superconductors, which are widely discussed in connection with the Iron-based materials. In these systems a vortex excitation can create a "bubble" of fluctuating $Z_2$ order …

介观颗粒二维阵列的Phase diagram

Phase diagram of 2D array of mesoscopic granules

用格子玻色子模型研究了二维有序介观颗粒、约瑟夫森结和超流氦填充的孔隙中的有序现象。当相位和局部超流密度的量子涨落都是必不可少的时，系统的相图用解析法和量子蒙特卡罗方法进行了分析。对于强相互作用玻色子系统，发现随着玻色子密度$n_0$的增加，有序超导态的边界向{ \it较低温度}移动，在$n_0 > 8$处接近其极限位置，对应于序参量的模量的相对涨落可以忽略不计（如在“宏观”颗粒阵列中）。在相位的弱量子涨落区域中，介观现象表现为直到$n_0 \sim 10$。平均场论和泛函积分$1/n_0$展开的结果与玻色子哈伯德模型及其准经典极限量子XY模型的量子蒙特卡罗计算结果一致。

Superconducting Fluctuation investigated by THz Conductivity of La$_{2-x}$Sr$_x$CuO$_4$ Thin Films

Frequency-dependent terahertz conductivities of La$_{2-x}$Sr$_x$CuO$_4$ thin films with various carrier concentrations were investigated. The imaginary part of the complex conductivity considerably increased from far above a zero-resistance superconducting transition temperature, $T_\text{c}^\text{zero}$, because of the existence of the fluctuating superfluid density with a short lifetime. The onset temperature of the superconducting fluctuation is at most $\sim 2T_\text{c}^\text{zero}$ for underdoped samples, which is consistent with the previously reported analysis of microwave conductivity.…