Synthesis, crystal and electronic structure, and physical properties of the Mo$_6$Ga$_{31}$ endohedral cluster superconductor are reported. The compound has two crystallographic modifications, monoclinic and triclinic, in which the same {Mo$_{12}$Ga$_{62}$} building units are perpendicular or codirectional to each other, respectively. Monoclinic and triclinic structures of Mo$_6$Ga$_{31}$ possess qualitatively the same electronic density of states showing a high peak at the Fermi level. Both modifications are inherently present in the bulk specimen. Due to the proximity effect, bulk Mo$_6$Ga$_{31}$ exhibits single superconducting transition at the critical temperature of 8.2 K in zero magnetic field. The upper critical field, which is 7.8 T at zero temperature, shows clear enhancement with respect to the Werthamer-Helfand-Honenberg prediction. Accordingly, heat capacity measurements indicate strong electron-phonon coupling in the superconducting state with the large ratio of $2Δ(0)/(k_BT_c)=4.5$, where 2$Δ(0)$ is the full superconducting gap at zero temperature.
We report on synthesis and characterization of gallide cluster based Mo8Ga41 superconductor. Transport and magnetization measurements confirm the superconducting transition temperature to be 9.8 K. The upper critical field, lower critical field, Ginzburg-Landau coherence length and penetration depth are estimated to be 11.8T, 150G, 5.2nm, 148nm respectively. The electronic band structure, density of states and phonon dispersion curve calculations are obtained by using Density Functional Theory. The core level X-ray Photoelectron Spectroscopy (XPS) reveals the binding energy information of the …
In type-1.5 superconductors, vortices emerge in clusters, which grow in size with increasing magnetic field. These novel vortex clusters and their field dependence are directly visualized by scanning SQUID microscopy at very low vortex densities in MgB2 single crystals. Our observations are elucidated by simulations based on a two-gap Ginzburg-Landau theory in the type-1.5 regime.
In this work we used unsupervised machine learning methods in order to find possible clustering structures in superconducting materials data sets. We used the SuperCon database, as well as our own data sets complied from literature, in order to explore how machine learning algorithms groups superconductors. Both conventional clustering methods like k-means, hierarchical or Gaussian mixtures, as well as clustering methods based on artificial neural networks like self-organizing maps, were used. For dimensionality reduction and visualization t-SNE was found to be the best choice. Our results ind…
Visualizing Vortex Cluster Dynamics in the Weak Type-II Superconductor CaSb$_2$
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（斯坦福材料能源科学研究所，SLAC国家加速器实验室）；Geballe Laboratory for Advanced Materials, Stanford University（斯坦福大学先进材料盖巴尔实验室）；Stewart Blusson Quantum Matter Institute, University of British Columbia（不列颠哥伦比亚大学斯图尔特·布森量子物质研究所）；Department of Physics and Astronomy, University of British Columbia（不列颠哥伦比亚大学物理与天文学系）
A cluster of spins $1/2$ of a finite size can be regarded as a basic building block of a spin texture in high-temperature cuprate superconductors. If this texture has the character of a network of weakly coupled spin clusters, then spin excitation spectra of finite clusters are expected to capture the principal features of the experimental spin response. We calculate spin excitation spectra of several clusters of spins $1/2$ coupled by Heisenberg interaction. We find that the calculated spectra exhibit a high degree of variability representative of the actual phenomenology of curates, while, a…
We use scanning tunneling microscopy to map the surface structure, nanoscale electronic inhomogeneity, and vitreous vortex phase in the hole-doped superconductor Sr$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ with $T_c$=32 K. We find the low-$T$ cleaved surface is dominated by a half-Sr/K termination with $1\times 2$ ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by $σ/\barΔ$=16% on a $\sim$3 nm length scale, with average $2\barΔ/k_B T_c=3.6$ in the weak coupling limit. The vortex core size provides a measu…
We compare the phase-diagrams of an effective theory of a three-dimensional multi-band superconductor obtained within standard and cluster mean-field theories, and in large-scale Monte Carlo simulations. In three dimensions, mean field theory fails in locating correctly the positions of the phase transitions, as well as the character of the transitions between the different states. A cluster mean-field calculations taking into account order-parameter fluctuations in a local environment improves the results considerably for the case of extreme type-II superconductors where gauge-field fluctuati…
We report DC magnetization, detailed systematic linear and nonlinear AC magnetic susceptibility and transport for a single phase RuSr2Eu1.4Ce0.6Cu2O10-δ (EuRu-1222) magneto-superconductor. The studied sample is synthesized through standard solid state reaction route, which is crystallized in single phase tetragonal structure with space group I4/mmm. DC magnetic susceptibility measurements revealed that the studied EuRu-1222 is a magneto-superconductor with Ru spins ordering at around 110 K and superconductivity in the Cu-O2 planes below \approx 30 K. Temperature dependence of AC susceptibility…
We demonstrate that there is a long-range vortex-vortex attraction in ferromagnetic superconductors due to polarization of the magnetic moments. Vortex clusters are then stabilized in the ground state for low vortex densities. The motion of vortex clusters driven by the Lorentz force excites magnons. This regime becomes unstable at a threshold velocity above which domain walls are generated for slow relaxation of the magnetic moments and the vortex configuration becomes modulated. This dynamics of vortices and magnetic moments can be probed by transport measurements.
The distinctive features of equilibrium vortex structures in thin films of anisotropic superconductors in tilted magnetic fields are studied for the limits of moderate and strong anisotropy. The energetically favorable shape of isolated vortex lines is found in the framework of two particular models describing these limiting cases: London theory with an anisotropic mass tensor and London-type model for a stack of Josephson--decoupled superconducting layers. The increase of the field tilting is shown to result in qualitative changes in the vortex--vortex interaction potential: the balance betwe…
Motivated by recent experimental data on thin film superconductors and oxide interfaces we propose a random-resistor network apt to describe the occurrence of a metal-superconductor transition in a two-dimensional electron system with disorder on the mesoscopic scale. We explore the interplay between the statistical distribution of local critical temperatures and the occurrence of a lower-dimensional (e.g., fractal-like) structure of a superconducting cluster embedded in the two-dimensional network. The thermal evolution of the resistivity is determined by an exact calculation and, for compari…
We derive exact expressions for the fluctuation conductivity in two dimensional superconductors as a function of temperature and magnetic field in the whole fluctuation region above the upper critical field H_{c2}(T). Focusing on the vicinity of the quantum phase transition near zero temperature, we propose that as the magnetic field approaches the line near H_{c2}(0) from above, a peculiar dynamic state consisting of clusters of coherently rotating fluctuation Cooper-pairs forms and estimate the characteristic size and lifetime of such clusters. We find the zero temperature magnetic field dep…
We study the pseudogap present in cuprate and organic superconductors. We use the dynamical cluster approximation (DCA), treating a cluster embedded in a bath. As the Coulomb interaction is increased, cluster-bath Kondo states are destroyed and bound cluster states formed. We show that this leads to a pseudogap. Due to weaker coupling to the bath for the anti-nodal point, this happens first for this point, explaining the k-dependence of the pseudogap. The pseudogap can be understood in terms of preformed d-wave pairs, but it does not prove the existence of such pairs.
In this paper we characterize the superconductor-insulator phase transition on a network of 2d percolation clusters. Sufficiently close to the percolation threshold, this network has a broad degree distribution, and at p=p_c the degree distribution becomes scale-free. We study the Transverse Ising Model on this complex topology in order to characterize the superconductor-insulator transition in a network formed by 2d percolation clusters of a superconductor material. We show, by a mean-field treatment, that the critical temperature of superconductivity depends on the maximal eigenvalue of the …
We show that the annihilation dynamics of excess quasi-particles in superconductors may result in the spontaneous formation of large spin-polarized clusters. This presents a novel scenario for spontaneous spin polarization. We estimate the relevant scales for aluminum, finding the feasibility of clusters with total spin $S \simeq 10^4 \hbar$ that could be spread over microns. The fluctuation dynamics of such large spins may be detected by measuring the flux noise in a loop hosting a cluster.
Combined density-functional and dynamical cluster quantum Monte Carlo calculations for three-band Hubbard models for hole-doped cuprate superconductors
A recently discovered high-Tc cuprate superconductor Ba2CuO$_{4-δ}$ exhibits exceptional Jahn-Teller distortion, wherein the CuO6 octahedrons are compressed along the c axis. As a consequence, the O vacancies prefer to reside in the CuO2 plane, but the exact structure is not known. By combining first-principles total energy calculation with the automated structure inversion method, the effective cluster interactions of O vacancies are mapped out. Around $δ$=0.8, where the 73K superconductivity was observed experimentally, we predict that the ordered O vacancies slice the CuO2 plane into not on…
At low enough temperatures and high densities, the equilibrium configuration of an ensemble of ultrasoft particles is a self-assembled, ordered, cluster-crystal. In the present work we explore the out-of-equilibrium dynamics for a two-dimensional realization, which is relevant to superconducting materials with multi-scale intervortex forces. We find that for small temperatures following a quench, the suppression of the thermally-activated particle hopping hinders the ordering. This results in a glass transition for a monodispersed ensemble, for which we derive a microscopic explanation in term…
We study s-wave Cooper pairing in heavy-fermion systems. We analyze the periodic Anderson model by means of the variational cluster approach (VCA) focusing on the interorbital Cooper pairing between a conduction electron ($c$ electron) and an $f$ electron, called the "$c$-$f$ pairing." It is shown that the s-wave superconductivity appears coexisting with long-range antiferromagnetic order when electrons or holes are doped into the system at half filling. The antiferromagnetic order vanishes when the doping concentration exceeds a certain critical value, leading to a pure s-wave superconducting…
It has long been a challenge to describe the origin of unconventional superconductivity. The two known examples with high Tc, based on iron and copper, have very different electronic structures, while other materials with similar electronic structure may not show superconductivity at all. In this paper, the authors show that by using high accuracy diffusion Monte Carlo calculations, the unconventional superconductors of both high Tc types form a cluster at intermediate spin-charge coupling. The spin-charge coupling may serve as a normal state marker for unconventional superconductivity, and pr…
It is shown that local temporal correlations in addition to non-local spatial correlations are important to understand the size and the doping dependence of the d-wave superconducting order parameter of high-temperature superconductors. To this end, the hole- and electron-doped two-dimensional Hubbard model at zero temperature is considered and treated by an extension of the variational cluster approximation. Within this approach, the effects of temporal correlations can be studied systematically and in a thermodynamically consistent way by comparing results obtained from different reference c…
We report experimental evidence of formation of antiferromagnetic clusters in composites made of a cuprate superconductor La1.85Sr0.15CuO4 (LCu) and a ferromagnet La0.6Sr0.4CoO3 (LCo). From linear and non-linear ac-susceptibility measurement on this composite material it is found that the exchange field of LCo suppress the dynamic antiferromagnetic spin fluctuation of LCu and convert it into the short range ordered superparamagnetic type antiferromagnetic (AFM) clusters at the cost of superconducting volume fraction. These shrank superconducting volume fraction shows quantum size effect (QSE) …
It has been proposed that the superconducting transition temperature $T_{\mathrm{c}}$ of an unconventional superconductor with a large pairing scale but strong phase fluctuations can be enhanced by coupling it to a metal. However, the general efficacy of this approach across different parameter regimes remains an open question. Using the dynamical cluster approximation, we study this question in a system composed of an attractive Hubbard layer in the intermediate coupling regime, where the magnitude of the attractive Coulomb interaction $|U|$ is slightly larger than the bandwidth $W$, hybridiz…
We investigate theoretically the equilibrium configurations of many magnetic skyrmions interacting with many superconducting vortices in a superconductor-chiral magnet bilayer. We show that miscible mixtures of vortices and skyrmions in this system breaks down at a particular wave number for sufficiently strong coupling, giving place to remarkably diverse mesoscale patterns: gel, stripes, clusters, intercalated stripes and composite gel-cluster structures. We also demonstrate that, by appropriate choice of parameters, one can thermally tune between the homogeneous and density-modulated phases.
Two-particle (2-p) excitations such as spin and charge excitations play a key role in high-Tc cuprate superconductors (HTSC). On the basis of a parameter-free theory, which extends the Variational Cluster Approach (a recently developed embedded cluster method) to 2-p excitations, the magnetic excitations of HTSC are shown to be reproduced for a Hubbard model within the relevant strong-coupling regime. In particular, the resonance mode in the underdoped regime, its intensity and "hour-glass" dispersion are in good overall agreement with experiments.
Paired Cluster Model of High-Tc Superconductivity:Explanation for Pseudogap Critical Concentration, Vortex Core Pseudogap and Possible Stripe Phase in High-Tc Superconductors
We report structural, DC magnetization, detailed linear/non-linear AC susceptibility, (with applied frequency and amplitude) isothermal and thermoremanent magnetization (TRM) behavior for RuSr2Y1.5Ce0.5Cu2O10 (YRu-1222) magneto-superconductor to understand its complex magnetism. Studied sample is synthesized through the novel solid state High Pressure (6 GPa) High Temperature (1450oC) (HPHT) technique. The compound is crystallized in tetragonal structure with space group I4/mmm (No. 139). DC magnetic susceptibility shows that studied YRu-1222 is magneto-superconducting with Ru spins magnetic o…
Transition from Mott insulator to superconductor in GaNb$_{4}$Se$_{8}$ and GaTa$_{4}$Se$_{8}$ under high pressure
具有面心立方GaMo $_{4}$ S $_{8}$型结构的GaM $_{4}$ Se $_{8}$（M=Nb;Ta）化合物中的电子传导起源于广泛分离的四面体M $_{4}$金属团簇之间的定域未成对电子（S=1/2）的跳跃。结果表明，在13 GPa和11.5GPa下，GaNb $_{4}$ Se、$_{8}$和GaTa $_{4}$ Se $_{8}$从莫特绝缘体转变为金属态和超导态，T_（$_{C}$）分别为2.9和5.8K.超导性的发生被证明是连接与MSe $_{6}$八面体畸变的压力诱导的减少和同时软化的声子与MSe-债券。
The recent discovery of H3S and LaH10 superconductors with record high superconducting transition temperatures, Tc, at high pressure, has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we predict the existence of an unprecedented hexagonal HfH10, with an extraordinarily high Tc of around 213-234 K at 250 GPa. In HfH10, the H atoms are arranged in clusters to form a planar "penta-graphene-like" sublattice, in contrast to the covalent sixfold cubic structure in H3S and clathrate-like structure in LaH10. The Hf atom acts as a "precompressor" and ele…
Multiple correlation lengths and type-1.5 superconductivity in $U(1)$ superconductors due to hidden competition between irreducible representations of nonlocal pairing
KTH-Royal Institute of Technology（皇家理工学院）；Wallenberg Initiative Materials Science for Sustainability（瓦伦堡可持续材料科学倡议）
Phase diagrams of La2-xSrxCuO4 and YBa2Cu3O{6+/delta} as the keys to understanding the nature of high-Tc superconductors
本文提出了一个解释YBa_2Cu_3O {6+/δ}和La_（2-x）Sr_xCuO_4超导和磁相图的模型。该模型基于掺杂电荷在掺杂离子附近刚性局域化的假设。这种局部化的结果在本地变化的电子结构的母电荷转移绝缘体，这取决于当地的相互安排的掺杂电荷。结果表明，在一定条件下，该体系中的负U中心（NUC）可以形成在CuO 2平面内相邻的Cu阳离子对上。我们考虑了空穴产生的机制。计算的YBa 2Cu 3 O {6+/delta}中空穴浓度与掺杂和温度的关系与实验数据完全定量一致。在该模型框架内，考虑了YBa_2Cu_3 O {6+/delta}的相图，并对YBa_2Cu_3 O {6+/delta}中的赝隙相和60 K相进行了解释。赝能隙具有超导性质，在温度T*>Tc/inf>Tc时，由于NUC占据的大波动，在小团簇中联合了许多NUC。(Tc因此，/inf和Tc是无限和有限团簇的超导转变温度）。计算的T*（/delta）和Tc（/delta）依赖性与实验一致。T*（/delta）和Tc（/delta）之间的范围对应于小团簇由于NUC占据的波动而在超导态和正常态之间波动的波动范围。计算了La_（2-x）Sr_xCuO_4的相图。结果表明，La_（2-x）Sr_xCuO_4的超导相图特征和条纹织构特征只反映了正方晶格中存在的几何关系和不同类型掺杂有序的竞争。
A two-orbital model for Fe-pnictide superconductors is investigated using computational techniques on two-dimensional square clusters. The hopping amplitudes are derived from orbital overlap integrals, or by band structure fits, and the spin frustrating effect of the plaquette-diagonal Fe-Fe hopping is remarked. A spin 'striped' state is stable in a broad range of couplings in the undoped regime, in agreement with neutron scattering. Adding two electrons to the undoped ground state of a small cluster, the dominant pairing operators are found. Depending on parameters, two pairing operators were…
The magnetic properties attributed to the hydroxide layer of Li1-xFex(OH)Fe1-ySe have been elucidated by the study of superconducting and nonsuperconducting members of this family. Both ac magnetometry and muon spin relaxation measurements of nonsuperconductors find a magnetic state existing below approximately 10 K which exhibits slow relaxation of magnetization. This magnetic state is accompanied by a low-temperature heat capacity anomaly present in both superconducting and nonsuperconducting variants suggesting that the magnetism persists into the superconducting state. The estimated value …
