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arXiv2014

Magnetism and superconductivity in Sr$_2$VFeAsO$_3$ revealed by $^{75}$As- and $^{51}$V-NMR under elevated pressures

We report $^{75}$As- and $^{51}$V-nuclear magnetic resonance (NMR) measurements on the iron-based superconductor Sr$_2$VFeAsO$_3$ with alternating stacks structure. We find that the $^{75}$As nuclear spin-spin relaxation rate ($1/T_2$) shows a pronounced peak at $T_N$ = 165 K, below which the resonance peak shifts to a higher frequency due to the onset of an internal magnetic field. The $^{51}$V spectrum does not shift, but is broadened below $T_N$. We conclude that the Fe electrons oder antiferromagnetically below $T_N$ with a magnetic moment $m_{Fe}$ $\sim$ 0.4 $μ_B$. Application of external pressure up to 2.4 GPa reduces $T_N$ in a rate of $-$40 K/GPa, and enhances the superconducting transition temperature $T_c$ in a rate of 2 K/GPa. The pressure-temperature phase diagram for Sr$_2$VFeAsO$_3$ shows that superconductivity coexists with antiferromagnetism over a wide pressure range with an unprecedented high $T_c$ up to 36.5 K.

cond-mat.supr-con↗

arXiv2014

Paramagnetic instability of small topological superconductors

Although the diamagnetism is of the essence in the superconducting phenomena, small enough topological (or unconventional) superconductors are paramagnetic. By solving the quasiclassical Eilenberger equation and the Maxwell equation self-consistently in two-dimensional disks, we theoretically study the response of small superconductors to weak magnetic field. Because of the topologically nontrivial character of the wave function, the unconventional superconductor hosts the zero-energy surface Andreev bound states which always accompany the odd-frequency Cooper pairs. The paramagnetic property of the odd-frequency pairs explains the unusual Meissner effect at low temperature.

cond-mat.supr-con↗

arXiv2014

Spin Controlled Coexistence of 0 and π States in SFSFS Josephson Junctions

Using the Keldysh-Usadel formalism, we theoretically study the $0$-$π$ transition profiles and current-phase relations of magnetic $SFSFS$ and $SFSFFS$ Josephson nanojunctions in the diffusive regime. By allowing the magnetizations of the ferromagnetic layers to take arbitrary orientations, the strength and direction of the charge supercurrent flowing through the ferromagnetic regions can be controlled via the magnetization rotation in one of the ferromagnetic layers. Depending on the junction parameters, we find opposite current flow in the ferromagnetic layers, revealing that remarkably such configurations possess well-controlled $0$- and $π$-states simultaneously, creating a three-terminal $0$-$π$ spin switch. We demonstrate that the spin-controlled $0$-$π$ profiles trace back to the proximity induced odd-frequency superconducting correlations generated by the ferromagnetic layers. It is also shown that the spin-switching effect can be more pronounced in $SFSFFS$ structures. The current-phase relations reveal the important role of the middle $S$ electrode, where the spin controlled supercurrent depends crucially on its thickness and phase differences with the outer $S$ terminals.

cond-mat.supr-con↗

arXiv2014

Anomalous Josephson effect induced by spin-orbit interaction and Zeeman effect in semiconductor nanowires

We investigate theoretically the Josephson junction of semiconductor nanowire with strong spin-orbit (SO) interaction in the presence of magnetic field. By using a tight-binding model, the energy levels $E_n$ of Andreev bound states are numerically calculated as a function of phase difference $φ$ between two superconductors in the case of short junctions. The DC Josephson current is evaluated from the Andreev levels. In the absence of SO interaction, a $0$-$π$ transition due to the magnetic field is clearly observed. In the presence of SO interaction, the coexistence of SO interaction and Zeeman effect results in $E_n (-φ) \ne E_n (φ)$, where the anomalous Josephson current flows even at $φ=0$. In addition, the direction-dependence of critical current is observed, in accordance with experimental results.

cond-mat.mes-hall↗

arXiv2014

The pseudogap in YBa$_2$Cu$_3$O$_{6+δ}$ is not bounded by a line of phase transitions - thermodynamic evidence

We discuss a recent resonant ultrasound spectroscopy (RUS) study of YBa$_2$Cu$_3$O$_{6+δ}$, which infers a line of phase transitions bounding the pseudogap phase and argue that this scenario is not supported by thermodynamic evidence. We show that the anomalies in RUS, heat capacity and thermal expansion at the superconducting transition temperatures agree well. But there are large discrepancies between RUS and thermodynamic measurements at $T^*$ where the pseudogap phase transitions are purported to occur. Moreover, the frequency and temperature dependence of the RUS data for the crystal with $δ= 0.98$, interpreted in terms of critical slowing down near an electronic phase transition, is five orders of magnitude smaller than what is expected. For this crystal the RUS data near $T^*$ are more consistent with non-equilibrium effects such as oxygen relaxation.

cond-mat.supr-con↗

arXiv2014

Possible high temperature odd parity superconductivity in LaNi$_{7/8}$Co$_{1/8}$O$_3$ (111) bilayers

Using the functional renormalization group technique we demonstrate a route for potentially high-temperature odd-parity superconductivity in ferromagnetic materials caused by repulsive electron interactions, where the superconducting pairing is driven by charge-density wave fluctuations. Our model is directly applicable to a lightly cobalt-doped LaNiO$_3$ bilayer grown in the (111) direction. As the on-site repulsive interaction grows, a charge-density wave state with a charge pattern that respects all point-group symmetries of the bilayer is replaced by a superconducting state with an f-wave pairing.

cond-mat.str-el↗

arXiv2014

Nodal Landau Fermi-Liquid Quasiparticles in Overdoped La$_{1.77}$Sr$_{0.23}$CuO$_4$

Nodal angle resolved photoemission spectra taken on overdoped La$_{1.77}$Sr$_{0.23}$CuO$_4$ are presented and analyzed. It is proven that the low-energy excitations are true Landau Fermi-liquid quasiparticles. We show that momentum and energy distribution curves can be analyzed self-consistently without quantitative knowledge of the bare band dispersion. Finally, by imposing Kramers-Kronig consistency on the self-energy $Σ$, insight into the quasiparticle residue is gained. We conclude by comparing our results to quasiparticle properties extracted from thermodynamic, magneto-resistance, and high-field quantum oscillation experiments on overdoped Tl$_2$Ba$_2$CuO$_{6+δ}$.

cond-mat.supr-con↗

arXiv2017

Nematic and chiral superconductivity induced by odd-parity fluctuations

Recent experiments indicate that superconductivity in Bi$_2$Se$_3$ intercalated with Cu, Nb or Sr is nematic with rotational symmetry breaking. Motivated by this observation, we present a model study of nematic and chiral superconductivity induced by odd-parity fluctuations. We show that odd-parity fluctuations in the two-component $E_u$ representation of $D_{3d}$ crystal point group can generate attractive interaction in both the even-parity $s$-wave and odd-parity $E_u$ pairing channels, but repulsive interaction in other odd-parity pairing channels. Coulomb repulsion can suppress $s$-wave pairing relative to $E_u$ pairing, and thus the latter can have a higher critical temperature. $E_u$ pairing has two distinct phases: a nematic phase and a chiral phase, both of which can be realized in our model. When $s$-wave and $E_u$ pairings have similar instability temperature, we find an intermediate phase in which both types of pairing coexist.

cond-mat.supr-con↗

arXiv2017

Collapsed Tetragonal Phase Transition in LaRu$_2$P$_2$

The structural properties of LaRu$_2$P$_2$ under external pressure have been studied up to 14 GPa, employing high-energy x-ray diffraction in a diamond-anvil pressure cell. At ambient conditions, LaRu$_2$P$_2$ (I4/mmm) has a tetragonal structure with a bulk modulus of $B=105(2)$ GPa and exhibits superconductivity at $T_c= 4.1$ K. With the application of pressure, LaRu$_2$P$_2$ undergoes a phase transition to a collapsed tetragonal (cT) state with a bulk modulus of $B=175(5)$ GPa. At the transition, the c-lattice parameter exhibits a sharp decrease with a concurrent increase of the a-lattice parameter. The cT phase transition in LaRu$_2$P$_2$ is consistent with a second order transition, and was found to be temperature dependent, increasing from $P=3.9(3)$ GPa at 160 K to $P=4.6(3)$ GPa at 300 K. In total, our data are consistent with the cT transition being near, but slightly above 2 GPa at 5 K. Finally, we compare the effect of physical and chemical pressure in the RRu$_2$P$_2$ (R = Y, La-Er, Yb) isostructural series of compounds and find them to be analogous.

cond-mat.supr-con↗

arXiv2017

Impurity-induced magnetic droplet in unconventional superconductors near magnetic instability: Application to Nd doped $\mathbf{CeCoIn_5}$

Beside the spin density wave (SDW) order inside the superconducting phase in $\mathrm{CeCoIn_5}$ at high magnetic fields, recent neutron scattering measurements have found Bragg peaks in $5\%$ Nd doped $\mathrm{CeCoIn_5}$ at low fields. The intensity of Bragg peaks in low fields is suppressed by increasing field. Based on the phenomenological and microscopic modeling, we show that for the Pauli limited $d$-wave superconductors in the vicinity of SDW instability relevant for $\mathrm{CeCoIn_5}$, magnetic impurities locally induce droplets of SDW order. Because of the strong anisotropy in the momentum space in the spin fluctuations guaranteed by the $d$-wave pairing symmetry, sharp peaks in spin structure factor at $\mathbf{Q}$s are produced by the impurities, even when the droplets of SDW do not order. At zero field, the Nd impurity spins are along the $c$ axis due to the coupling to the conduction electrons with an easy $c$ axis, besides their own crystal field effect. The in-plane magnetic field cants the impurity moments toward the $ab$ plane, which suppress the droplets of SDW order. At high fields, the long-range SDW inside the superconducting phase is stabilized as a consequence of magnon condensation. Our results are consistent with the recent neutron scattering and thermal conductivity measurements.

cond-mat.supr-con↗

arXiv2018

Commensurate vs incommensurate charge ordering near the superconducting dome in Ir$_{1-x}$Pt$_x$Te$_2$ revealed by resonant x-ray scattering

The electronic-structural modulations of Ir$_{1-x}$Pt$_x$Te$_2$ (0 $\leqq x\leqq$ 0.12) have been examined by resonant elastic x-ray scattering (REXS) and resonant inelastic x-ray scattering (RIXS) techniques at both the Ir and Te edges. Charge-density-wave-like superstructure with wave vectors of $\mathbf{Q}$=(1/5 0 $-$1/5), (1/8 0 $-$1/8), and (1/6 0 $-$1/6) are observed on the same sample of IrTe$_2$ at the lowest temperature, the patterns of which are controlled by the cooling speeds. In contrast, superstructures around $\mathbf{Q}$=(1/5 0 $-$1/5) are observed for doped samples (0.02 $\leqq x\leqq$ 0.05). The superstructure reflections persist to higher Pt substitution than previously assumed, demonstrating that a charge density wave (CDW) can coexists with superconductivity. The analysis of the energy-dependent REXS and RIXS lineshape reveals the importance of the Te 5$p$ state rather than the Ir 5$d$ states in the formation of the spatial modulation of these systems. The phase diagram re-examined in this work suggests that the CDW incommensurability may correlate the emergence of superconducting states as-like Cu$_x$TiSe$_2$ and Li$_x$TaS$_2$.

cond-mat.str-el↗

arXiv2018

The role of canting and depleted-triplet minima in superconducting spin valve structures

The trilayer and pentalayer spin valve structures are revisited to determine the behavior of pair correlations and Josephson current when the magnetic layers are canted at arbitrary angle. The two systems display markedly different behaviors in the center magnetic layer. While the trilayer generates a triplet component that is weakly affected by canting, the pentalayer tunes in singlet pair correlations depending heavily on canting. We also show that a minimum with depleted $m=\pm1$ triplet components, rather than a $0-π$ transition, may be observed in the current profile $I_c(d_F)$ of a trilayer spin valve. The depleted-triplet minimum (DTM) is directly attributable to a decrease of $m=\pm1$ triplet correlations with increased thickness of the central ferromagnet, accompanied by a hidden, simultaneous sign change of the Gor'kov functions contributed from the left and right superconductors. We introduce a toy model for superconducting-magnetic proximity systems to better illuminate the behavior of individual components of the Gor'kov function and compare with a full numerical calculation.

cond-mat.supr-con↗

arXiv2018

Sequential superconductor-Bose insulator-Fermi insulator phase transitions in two-dimensional a-WSi

A zero-temperature magnetic-field-driven superconductor to insulator transition (SIT) in quasi-two-dimensional superconductors is expected to occur when the applied magnetic-field crosses a certain critical value. A fundamental question is whether this transition is due to the localization of Cooper pairs or due to the destruction of them. Here we address this question by studying the SIT in amorphous WSi. Transport measurements reveal the localization of Cooper pairs at a quantum critical field B_c^1 (Bose-insulator), with a product of the correlation length and dynamical exponents zv~4/3 near the quantum critical point (QCP). Beyond B_c^1, superconducting fluctuations still persist at finite temperatures. Above a second critical field B_c^2>B_c^1, the Cooper pairs are destroyed and the film becomes a Fermi-insulator. The different phases all merge at a tricritical point at finite temperatures with zv=2/3. Our results suggest a sequential superconductor to Bose insulator to Fermi insulator phase transition, which differs from the conventional scenario involving a single quantum critical point.

cond-mat.supr-con↗

arXiv2018

Pairing symmetry of interacting fermions on twisted bilayer graphene superlattice

The pairing symmetry of an effective Hamiltonian for interacting fermions on a twisted bilayer graphene superlattice is studied with the determinant quantum Monte Carlo method. The model has the symmetry of a triangle lattice and a nearly-flat low energy band, features which underlie the magic-angle twisted bilayer graphene superlattice. We show that the low temperature phase is insulating at half-filling, even for relatively weak interactions. The natures of the spin and pairing correlations upon doping are determined, and exhibit an electron-hole asymmetry consistent with experiment. Among the pairing symmetries allowed, we demonstrate that the dominating channels are $d$-wave, opening the possibility of condensation into an unconventional $d_{x^2-y^2}+id_{xy}$ phase, which is characterized by an integer topological invariant and gapless edge states.

cond-mat.str-el↗

arXiv2018

Lattice dynamics of the cluster chain compounds M2Mo6Se6

The lattice dynamics of members of the M2Mo6Se6 family of materials with guest ions M = K, Rb, Cs, In, and Tl has been studied using inelastic x-ray scattering and Raman spectroscopy at room temperature, as well as by ab-initio calculations. We find a good match between calculations and experiment, both for structure factors (Eigenvectors) and for the calculated phonon frequencies. The observed lattice dynamics for Tl2Mo6Se6 show no signs of anharmonicity or absence of avoided dispersion crossings, thus ruling out previously hypothesised rattling phonon modes. The reduced mode energies for In2Mo6Se6 are identified as only partially responsible for the lower superconducting transition temperature Tc in this material when compared to Tl2Mo6Se6.

cond-mat.supr-con↗

arXiv2018

Multigap Superconductivity in Chiral Noncentrosymmetric TaRh$_{2}$B$_{2}$

We report the first observation of multigap superconductivity in TaRh$_{2}$B$_{2}$. We show TaRh$_{2}$B$_{2}$ is a bulk type-II superconductor with a transition temperature, $T_{\mathrm{c}} = 6.00(5)$ K. We present transverse-field muon spin relaxation data where the superconducting gap can be fit using a two-gap $\left(s+s\right)$-wave model. We also report the zero-field electronic specific heat in the superconducting state that is best described by the same $\left(s+s\right)$ model providing further evidence of multiband behavior in this superconductor. Zero-field muon spin relaxation measurements show time-reversal symmetry is preserved in the superconducting state. We demonstrate that TaRh$_{2}$B$_{2}$ has an upper critical field of $15.2(1)$ T, which is significantly higher than previously reported and exceeds the Pauli limit.

cond-mat.supr-con↗

arXiv2018

Spatially modulated susceptibility in thin film La$_{2-x}$Ba$_x$CuO$_4$

The high critical temperature superconductor Lanthanum Barium Copper Oxide (La2-xBaxCuO4 or LBCO) exhibits a strong anomaly in critical temperature at 1/8th doping, nematicity, and other interesting properties. We report here Scanning Superconducting Quantum Interference Device (SQUID) imaging of the magnetic fields and susceptibility in a number of thin film LBCO samples with doping in the vicinity of the 1/8th anomaly. Spatially resolved measurements of the critical temperatures of these samples do not show a pronounced depression at 1/8th doping. They do, however, exhibit strong, nearly linear modulations of the susceptibility ("straie") of multiple samples with surprisingly long periods of 1-4 microns. Counterintuitively, vortices trap in positions of largest diamagnetic susceptibility in these striae. Given the rich interplay of different orders in this material system and its known sensitivity to epitaxial strain, we propose phase separation as a possible origin of these features and discuss scenarios in which that might arise.

cond-mat.supr-con↗

arXiv2018

Theory of tunneling spectroscopy of normal metal/ferromagnet/spin-triplet superconductor junctions

We study the tunneling conductance of a ballistic normal metal / ferromagnet / spin-triplet superconductor junction using the extended Blonder-Tinkham-Klapwijk formalism as a model for a $c$-axis oriented Au / SrRuO$_{3}$ / Sr$_{2}$RuO$_{4}$ junction. We compare chiral $p$-wave (CPW) and helical $p$-wave (HPW) pair potentials, combined with ferromagnet magnetization directions parallel and perpendicular to the interface. For fixed $θ_{M}$, where $θ_{M}$ is a direction of magnetization in the ferromagnet measured from the $c$-axis, the tunneling conductance of CPW and HPW clearly show different voltage dependencies. It is found that the cases where the $d$-vector is perpendicular to the magnetization direction (CPW with $θ_{M} = π/2$ and HPW with $θ_{M} = 0$) are identical. The obtained results serve as a guide to determine the pairing symmetry of the spin-triplet superconductor Sr$_{2}$RuO$_{4}$.

cond-mat.supr-con↗