arXiv subjects
Publications and source records attributed to P. K. Biswas.
We reply to the objections raised in a recent Comment (arXiv:1706.03023) regarding our observation of slow magnetic fluctuations and critical slowing down of magnetic fluctuations in the pseudogap phase of YBa$_2$Cu$_3$O$_y$ by zero-field and longitudinal-field muon spin relaxation (arXiv:1703.06799).
Evidence for intra-unit-cell (IUC) magnetic order in the pseudogap region of high-$T_c$ cuprates below a temperature $T^\ast$ is found in several studies, but NMR and $μ$SR experiments do not observe the expected static local magnetic fields. It has been noted, however, that such fields could be averaged by fluctuations. Our measurements of muon spin relaxation rates in single crystals of YBa$_2$Cu$_3$O$_y$ reveal magnetic fluctuations of the expected order of magnitude that exhibit critical slowing down at $T^\ast$. These results are strong evidence for fluctuating IUC magnetic order in the pseudogap phase.
A newly discovered family of high-Tc Fe-based superconductors, AeAFe4As4 (Ae = Ca, Sr, Eu and A = K, Rb, Cs), offers further opportunities to understand unconventional superconductivity in these materials. In this Rapid Communication, we report on the superconducting and magnetic properties of CaKFe4As4, studied using muon spectroscopy. Zero-field muon spin relaxation studies carried out on the CaKFe4As4 superconductor do not show any detectable magnetic anomaly at Tc or below, implying that time-reversal symmetry is preserved in the superconducting ground state. The temperature dependence of the superfluid density of CaKFe4As4 is found to be compatible with a two-gap s + s-wave model with gap values of 8.6(4) and 2.5(3) meV, similar to the other Fe-based superconductors. The presence of two superconducting energy gaps is consistent with theoretical and other experimental studies on this material. The value of the penetration depth at T = 0 K has been determined as 289(22) nm.
The superconducting ground state of the newly discovered superconductor Cs$_2$Cr$_3$As$_3$ with a quasi-one-dimensional crystal structure ($T_{\bf c}\sim$ 2.1(1) K) has been investigated using magnetization and muon-spin relaxation or rotation ($μ$SR), both zero-field (ZF) and transverse-field (TF), measurements. Our ZF $μ$SR measurements reveal the presence of spin fluctuations below 4 K and the ZF relaxation rate ($λ$) shows enhancement below $T_{\bf c}\sim$ 2.1 K, which might indicate that the superconducting state is unconventional. This observation suggests that the electrons are paired via unconventional channels such as spin fluctuations, as proposed on the basis of theoretical models. Our analysis of the TF $μ$SR results shows that the temperature dependence of the superfluid density is fitted better with a nodal gap structure than an isotropic s-wave model for the superconducting gap. The observation of a nodal gap in Cs$_2$Cr$_3$As$_3$ is consistent with that observed in the isostructural K$_2$Cr$_3$As$_3$ compound through TF $μ$SR measurements. Furthermore, from our TF $μ$SR study we have estimated the magnetic penetration depth $λ_{\mathrm{L}}$$(0)$ = 954 nm, superconducting carrier density $n_s = 4.98 \times 10^{26}~ $m$^{-3}$, and carrier's effective-mass enhancement $m^*$ = 1.61m$_{e}$.
We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk SmB6 , which produce local magnetic fields of about ~1.8 mT fluctuating on a time scale of ~60 ns. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of 40-90 nm, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state.
By means of magnetization, specific heat and muon-spin relaxation measurements, we investigate high-pressure oxidized \mohpo, in which overdoping is achieved up to $p \sim 0.45$ hole/Cu, well beyond the $T_c - p$ superconducting dome of cuprates, where Fermi liquid behavior is expected. Surprisingly, we find bulk superconductivity with $T_c$=84 K and superfluid density similar to those of optimally doped Y123. On the other hand, specific heat data display a large electronic contribution at low temperature, comparable to that of nonsuperconducting overdoped La214. These results point at an unusual high-$T_c$ phase with a large fraction of unpaired holes. Further experiments may assess the Fermi liquid properties of the present phase, which would put into question the paradigm that the high $T_c$ of cuprates originates from a non-Fermi liquid ground state.
The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimally-doped SmFeAsO0.88-F0.12 iron-based superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide range of manganese contents (x) by means of muon-spin spectroscopy (muSR), both in zero- and in transverse fields, respectively, to probe the magnetic and the superconducting order. The pure superconducting phase (at x < 0.03) is replaced by a crossover region at intermediate Mn values (0.03 =< x < 0.08), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for x = 0.08, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of Tc and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors [M. Gastiasoro et al., ArXiv 1606.09495], which includes correlation-enhanced RKKY-couplings between the impurity moments.
The recent discovery of the topologically protected surface states in the beta-phase of PdBi2 has reignited the research interest in this class of superconductors. Here, we show results of our muon spin relaxation and rotation (muSR) measurements carried out to investigate the superconducting and magnetic properties and the topological effect in the superconducting ground state of beta-PdBi2. Zero-field muSR data reveal that no sizeable spontaneous magnetization arises with the onset of superconductivity implying that the time reversal symmetry is preserved in the superconducting state of beta-PdBi2. Further, a strong diamagnetic shift of the applied field has been observed in the transverse-field (TF) muSR experiments, indicating that any triplet-pairing channel, if present, does not dominate the superconducting condensate. Using TF-muSR, we estimate that the magnetic penetration depth is 263(10) nm at zero temperature. Temperature dependence of the magnetic penetration depth provides evidence for the existence of a nodeless single s-wave type isotropic energy gap of 0.78(1) meV at zero temperature. Our results further suggest that the topologically protected surface states have no effect on the bulk of the superconductor.
We report muon spin rotation experiments on the magnetic penetration depth lambda and the temperature dependence of lambda^{-2} in the over-doped Fe-based high-temperature superconductor (Fe-HTS) Ba_{1-x}Rb_ xFe_2As_2 (x = 0.65) studied at ambient and under hydrostatic pressures up to p = 2.3 GPa. We find that in this system lambda^{-2}(T) is best described by d-wave scenario. This is in contrast to the case of the optimally doped x = 0.35 system which is known to be a nodeless s^{+-}-wave superconductor. This suggests that the doping induces the change of the pairing symmetry from s^{+-} to d-wave in Ba_{1-x}Rb_{x}Fe_{2}As_{2}. In addition, we find that the d-wave order parameter is robust against pressure, suggesting that d is the common and dominant pairing symmetry in over-doped Ba_{1-x}Rb_{x}Fe_{2}As_{2}. Application of pressure of p = 2.3 GPa causes a decrease of lambda(0) by less than 5 %, while at optimal doping x = 0.35 a significant decrease of lambda(0) was reported. The superconducting transition temperature T_c as well as the gap to T_c ratio 2Delta/k_BT_c show only a modest decrease with pressure. By combining the present data with those previously obtained for optimally doped system x = 0.35 and for the end member x = 1 we conclude that the SC gap symmetry as well as the pressure effects on the SC quantities strongly depend on the Rb doping level. These results are discussed in the light of the putative Lifshitz transition, i.e., a disappearance of the electron pockets in the Fermi surface of Ba_{1-x}Rb_{x}Fe_{2}As_{2} upon hole doping.
Competing electronic states are found in the large majority of unconventional superconductors, including high-Tc cuprates, iron based superconductors and many heavy fermion systems. The complex interplay is reflected in phase diagrams as a function of doping or other tuning parameters involving besides superconducting other phases (often magnetic) and quantum critical points. Superconductivity is also found in the vicinity of charge density wave (CDW) order in phase diagrams reminiscent of superconductivity mediated by magnetic fluctuations. There is however less knowledge about the interplay of superconductivity and CDW compared to the magnetic analogon. Here we report about microscopic studies by muon spin rotation as a function of pressure of the Ca_3Ir_4Sn_13 and Sr_3Ir_4Sn_13 cubic compounds, which display superconductivity and a structural phase transition associated with the formation of a CDW. We find a strong enhancement of the superfluid density and of the coupling strength above a pressure of about 1.6 GPa giving direct evidence of the presence of a quantum critical point separating a superconducting phase coexisting with CDW from a pure superconducting phase. The superconducting order parameter in both phases are found to have the same s-wave symmetry. In spite of the conventional phonon-mediated BCS character of this compound, the dependence of the effective superfluid density on the critical temperature puts this system in the "Uemura" plot close to unconventional superconductors.
Strong spin-orbit coupling (SOC) effects of heavy $d$-orbital elements have long been neglected in describing the ground states of their compounds thereby overlooking a variety of fascinating and yet unexplored magnetic and electronic states, until recently. The spin-orbit entangled electrons in such compounds can get stabilized into unusual spin-orbit multiplet $J$-states which warrants severe investigations. Here we show using detailed magnetic and thermodynamic studies and theoretical calculations the ground state of Ba$_3$ZnIr$_2$O$_9$, a 6$H$ hexagonal perovskite is a close realisation of the elusive $J$~=~0 state. However, we find that local Ir moments are spontaneously generated due to the comparable energy scales of the singlet-triplet splitting driven by SOC and the superexchange interaction mediated by strong intra-dimer hopping. While the Ir ions within the structural Ir$_2$O$_9$ dimer prefers to form a spin-orbit singlet state (SOS) with no resultant moment, substantial interdimer exchange interactions from a frustrated lattice ensure quantum fluctuations till the lowest measured temperatures and stabilize a spin-orbital liquid phase.
Muon spin relaxation and rotation (muSR) measurements have been performed to study the superconducting and magnetic properties of Nb2PdS5. Zero-field muSR data show that no sizeable spontaneous magnetization arises with the onset of superconductivity in Nb2PdS5 which indicates that the time reversal symmetry is probably preserved in the superconducting state of this system. A strong diamagnetic shift is observed in the transverse-field muSR data practically ruling out a dominant triplet-pairing superconducting state in Nb2PdS5. The temperature dependence of magnetic penetration depth evidences the existence of a single s-wave energy gap with a gap value of 1.07(4) meV at zero temperature. The gap to Tc ratio, 2.02(9), indicates that Nb2PdS5 should be considered as a moderately strong-coupling superconductor. The magnetic penetration depth at zero temperature is 785(20) nm, indicating a very low superfluid density consistent with the quasi-one-dimensional nature of this system.
The recent discovery of pressure induced superconductivity in the binary helimagnet CrAs has attracted much attention. How superconductivity emerges from the magnetic state and what is the mechanism of the superconducting pairing are two important issues which need to be resolved. In the present work, the suppression of magnetism and the occurrence of superconductivity in CrAs as a function of pressure ($p$) were studied by means of muon spin rotation. The magnetism remains bulk up to $p\simeq3.5$~kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at $p\simeq$7~kbar. At 3.5 kbar superconductivity abruptly appears with its maximum $T_c \simeq 1.2$~K which decreases upon increasing the pressure. In the intermediate pressure region ($3.5\lesssim p\lesssim 7$~kbar) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. Our results indicate that the less conductive magnetic phase provides additional carriers (doping) to the superconducting parts of the CrAs sample thus leading to an increase of the transition temperature ($T_c$) and of the superfluid density ($ρ_s$). A scaling of $ρ_s$ with $T_c^{3.2}$ as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs.
Temperature dependence of the electronic structure of SmB6 is studied by high-resolution ARPES down to 1 K. We demonstrate that there is no essential difference for the dispersions of the surface states below and above the resistivity saturating anomaly (~ 3.5 K). Quantitative analyses of the surface states indicate that the quasi-particle scattering rate increases linearly as a function of temperature and binding energy, which differs from Fermi-Liquid behavior. Most intriguingly, we observe that the hybridization between the d and f states builds gradually over a wide temperature region (30 K < T < 110 K). The surface states appear when the hybridization starts to develop. Our detailed temperature-dependence results give a complete interpretation of the exotic resistivity result of SmB6, as well as the discrepancies among experimental results concerning the temperature regions in which the topological surface states emerge and the Kondo gap opens, and give new insights into the exotic Kondo crossover and its relationship with the topological surface states in the topological Kondo insulator SmB6.
The concept of a topological Kondo insulator (TKI) has been brought forward as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to the strong spin-orbit coupling [1-3]. In contrast to other three-dimensional (3D) topological insulators (e.g. Bi2Se3), a TKI is truly insulating in the bulk [4]. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. Applying spin- and angle-resolved photoemission spectroscopy (SARPES) to the Kondo insulator SmB6, a promising TKI candidate, we reveal that the surface states of SmB6 are spin polarized, and the spin is locked to the crystal momentum. Counter-propagating states (i.e. at k and -k) have opposite spin polarizations protected by time-reversal symmetry. Together with the odd number of Fermi surfaces of surface states between the 4 time-reversal invariant momenta in the surface Brillouin zone [5], these findings prove, for the first time, that SmB6 can host non-trivial topological surface states in a full insulating gap in the bulk stemming from the Kondo effect. Hence our experimental results establish that SmB6 is the first realization of a 3D TKI. It can also serve as an ideal platform for the systematic study of the interplay between novel topological quantum states with emergent effects and competing order induced by strongly correlated electrons.
We report bulk superconductivity (SC) in Ca$_3$Ir$_4$Sn$_{13}$ by means of $^{119}$Sn nuclear magnetic resonance (NMR) experiments. Two classical signatures of BCS superconductivity in spin-lattice relaxation rate ($1/T_1$), namely the Hebel-Slichter coherence peak just below the $T_c$ and the exponential decay in the superconducting phase, are evident. The noticeable decrease of $^{119}$Sn Knight shift below $T_c$ indicates spin-singlet superconductivity. The temperature dependence of the spin-lattice relaxation rate ($1/T_1$) is convincingly described by the multigap isotropic superconducting gap. Present NMR experiments do not witness any sign of enhanced spin fluctuations in the normal state.
The field distribution in the vortex lattice of a pure niobium single crystal with an external field applied along a three-fold axis has been investigated by the transverse-field muon-spin-rotation (TF-$μ$SR) technique over a wide range of temperatures and fields. The experimental data have been analyzed with the Delrieu's solution for the form factor supplemented by phenomenological formulas for the parameters. This has enabled us to experimentally establish the temperatures and fields for the Delrieu's, Ginzburg-Landau's, and Klein's regions of the vortex lattice. Using the numerical solution of the quasiclassical Eilenberger's equation the experimental results have been reasonably understood. They should apply to all clean BCS superconductors. The analytical Delrieu's model supplemented by phenomenological formulas for its parameters is found to be reliable for analyzing TF-$μ$SR experimental data for a substantial part of the mixed phase. The Abrikosov's limit is contained in it.
The magnetic penetration depth ($λ$) as a function of applied magnetic field and temperature in SrPt$_3$P($T_c\simeq8.4$ K) was studied by means of muon-spin rotation ($μ$SR). The dependence of $λ^{-2}$ on temperature suggests the existence of a single $s-$wave energy gap with the zero-temperature value $Δ=1.58(2)$ meV. At the same time $λ$ was found to be strongly field dependent which is the characteristic feature of the nodal gap and/or multi-gap systems. The multi-gap nature of the superconduicting state is further confirmed by observation of an upward curvature of the upper critical field. This apparent contradiction would be resolved with SrPt$_3$P being a two-band superconductor with equal gaps but different coherence lengths within the two Fermi surface sheets.