Recent scanning tunneling microscopy (STM) observation of U-shaped and V-shaped spectra (and their mixture) in superconducting Nd$_{1-x}$Sr$_x$NiO$_2$ thin films has been interpreted as presence of two distinct gap symmetries in this nickelate superc
onductor [Gu et al., Nat. Comm. 11, 6027 (2020)]. Here, using a two-band model of nickelates capturing dominant contributions from Ni-$3d_{x^2-y^2}$ and rare-earth (R)-$5d_{3z^2 - r^2}$ orbitals, we show that the experimental observation can be simply explained within a pairing scenario characterized by a conventional $d_{x^2-y^2}$-wave gap structure with lowest harmonic on the Ni-band and a $d_{x^2-y^2}$-wave gap with higher-harmonics on the R-band. We perform realistic simulations of STM spectra employing first-principles Wannier functions to properly account for the tunneling processes and obtain V, U, and mixed spectral line-shapes depending on the position of the STM tip within the unit cell. The V- and U-shaped spectra are contributed from Ni and R-bands, respectively, and Wannier functions, in essence, provide position-dependent weighing factors, determining the spectral line-shape at a given intra-unit cell position. We propose a phase-sensitive experiment to distinguish between the proposed $d$-wave gap structure and time-reversal symmetry breaking $d+is$ gap which yields very similar intra-unit cell spectra.
We analyse the possibility of the appearance of spontaneous currents in proximated superconducting/normal metal (S/N) heterostructure when Cooper pairs penetrate into the normal metal from the superconductor. In particular, we calculate the free ener
gy of the S/N structure. We show that whereas the free energy of the N film $F_{N}$ in the presence of the proximity effect increases compared to the normal state, the total free energy, which includes the boundary term $F_{B}$, decreases. The condensate current decreases $F_{N}$, but increases the total free energy making the current-carrying state of the S/N system energetically unfavorable.
Machine learning is becoming widely used in analyzing the thermodynamics of many-body condensed matter systems. Restricted Boltzmann Machine (RBM) aided Monte Carlo simulations have sparked interest recently, as they manage to speed up classical Mont
e Carlo simulations. Here we employ the Convolutional Restricted Boltzmann Machine (CRBM) method and show that its use helps to reduce the number of parameters to be learned drastically by taking advantage of translation invariance. Furthermore, we show that it is possible to train the CRBM at smaller lattice sizes, and apply it to larger lattice sizes. To demonstrate the efficiency of CRBM we apply it to the paradigmatic Ising and Kitaev models in two-dimensions.
The electronic structure in unconventional superconductors holds a key to understand the momentum-dependent pairing interactions and the resulting superconducting gap function. In superconducting Fe-based chalcogenides, there have been controversial
results regarding the importance of the $k_z$ dependence of the electronic dispersion, the gap structure and the pairing mechanisms of iron-based superconductivity. Here, we present a detailed investigation of the van der Waals interaction in FeSe and its interplay with magnetic disorder and real space structural properties. Using density functional theory we show that they need to be taken into account upon investigation of the 3-dimensional effects, including non-trivial topology, of FeSe$_{1-x}$Te$_x$ and FeSe$_{1-x}$S$_x$ systems. In addition, the impact of paramagnetic (PM) disorder is considered within the spin-space average approach. Our calculations show that the PM relaxed structure supports the picture of different competing ordered magnetic states in the nematic regime, yielding magnetic frustration.
We present an extensive study of the structural, magnetic and thermodynamic properties of high-quality monocrystals of the two heterometallic oxyborates from the ludwigite family: Cu$_2$GaBO$_5$ and Cu$_2$AlBO$_5$ in the temperature range above 2 K.
The distinctive feature of the investigated structures is the selective distribution of Cu and Ga/Al cations. The unit cell of Cu$_2$GaBO$_5$ and Cu$_2$AlBO$_5$ contains four nonequivalent crystallographic sites of metal ions. Two sites in the structure from four nonequivalent crystallographic sites of metal ions of Cu$_2$GaBO$_5$ are fully occupied by Cu atoms which form the quasi one-dimensional chains along the a-axis. For Cu$_2$AlBO$_5$ all sites are partially occupied by Al and Cu atoms. The joint analysis of low-temperature data on magnetic susceptibility and magnetic contribution to the specific heat showed that Cu$_2$AlBO$_5$ and Cu$_2$GaBO$_5$ exhibit an antiferromagnetic transition at $T_{rm N} approx 3$ and 4 K, respectively. The magnetic contributions to the specific heat for both compounds were obtained after subtracting the phonon contribution. It is shown that the external magnetic field above 2.5 T leads to a broadening of the magnetic phase transition indicating suppression of the long-range antiferromagnetic order.
Robust engineering of phonon squeezed states in optically excited solids has emerged as a promising tool to control and manipulate their properties. However, in contrast to quantum optical systems, detection of phonon squeezing is subtle and elusive,
and an important question is what constitutes an unambiguous signature of it. The state of the art involves observing oscillations at twice the phonon frequency in time resolved measurements of the out of equilibrium phonon fluctuation. Using Keldysh formalism we show that such a signal is a necessary but not a sufficient signature of a squeezed phonon, since we identify several mechanisms that do not involve squeezing and yet which produce similar oscillations. We show that a reliable detection requires a time and frequency resolved measurement of the phonon spectral function.
Topological spin configurations in proximity to a superconductor have recently attracted great interest due to the potential application of the former in spintronics and also as another platform for realizing non-trivial topological superconductors.
Their application in these areas requires precise knowledge of the existing exchange fields and/or the stray-fields which are therefore essential for the study of these systems. Here, we determine the effective stray-field and the Meissner currents in a Superconductor/Ferromagnet/Superconductor (S/F/S) junction produced by various nonhomogenous magnetic textures in the F. The inhomogeneity arises either due to a periodic structure with flat domain walls (DW) or is caused by an isolated chiral magnetic skyrmion (Sk). We consider both Bloch- and N{e}el-type Sk and also analyze in detail the periodic structures of different types of DWs-- that is Bloch-type DW (BDW) and N{e}el-type DW (NDW) of finite width with in- and out-of-plane magnetization vector. The spatial dependence of the fields and Meissner currents are shown to be qualitatively different for the case of Bloch- and N{e}el-type magnetic textures. While the spatial distributions in the upper and lower S are identical for Bloch-type Sk and DWs they are asymmetric for the case of N{e}el-type magnetic textures. The depairing factor, which determines the critical temperature and which is related to vector potential of the stray-field, can have its maximum at the center of a magnetic domain but also, as we show, above the DW. For Sks the maximum is located at a finite distance within the Sk radius. Based on this, we study the nucleation of superconductivity in the presence of DWs. Because of the asymmetry for N{e}el-type structures, the critical temperature in the upper and lower S is expected to be different. The obtained results can also be applied to S/F bilayers.
The quantum-spin S = 1=2 chain system Cs$_2$CuCl$_4$ is of high interest due to competing anti-ferromagnetic intra-chain J and inter-chain exchange J interactions and represents a paramount example for Bose-Einstein condensation of magnons [R. Coldea
et al., Phys. Rev. Lett. 88, 137202 (2002)]. Substitution of chlorine by bromine allows tuning the competing exchange interactions and corresponding magnetic frustration. Here we report on electron spin resonance (ESR) in single crystals of Cs$_2$CuCl$_{4-x}$Br$_x$ with the aim to analyze the evolution of anisotropic exchange contributions. The main source of the ESR linewidth is attributed to the uniform Dzyaloshinskii-Moriya interaction. The vector components of the Dzyaloshinskii-Moriya interaction are determined from the angular dependence of the ESR spectra using a high-temperature approximation. The obtained results support the site selectivity of the Br substitution suggested from the evolution of lattice parameters and magnetic susceptibility dependent on the Br concentration.
Recent nuclear magnetic resonance studies [A. Pustogow {it et al.}, arXiv:1904.00047] have challenged the prevalent chiral triplet pairing scenario proposed for Sr$_2$RuO$_4$. To provide guidance from microscopic theory as to which other pair states
might be compatible with the new data, we perform a detailed theoretical study of spin-fluctuation mediated pairing for this compound. We map out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band-structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that even-parity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spin-orbit coupling favors a near-nodal odd-parity triplet superconducting state, which is either helical or chiral depending on the proximity of the $gamma$ band to the van Hove points. A surprising near-degeneracy of the nodal $s^prime$- and $d_{x^2-y^2}$-wave solutions leads to the possibility of a near-nodal time-reversal symmetry broken $s^prime+id_{x^2-y^2}$ pair state. Predictions for the temperature dependence of the Knight shift for fields in and out of plane are presented for all states.
We report on electron spin resonance (ESR) studies of the spin relaxation in Cs$_2$CuCl$_4$. The main source of the ESR linewidth at temperatures $T leq 150$ K is attributed to the uniform Dzyaloshinskii-Moriya interaction. The vector components of t
he Dzyaloshinskii-Moriya interaction are determined from the angular dependence of the ESR spectra using a high-temperature approximation. Both the angular and temperature dependence of the ESR linewidth have been analyzed using a self-consistent quantum-mechanical approach. In addition analytical expressions based on a quasi-classical picture for spin fluctuations are derived, which show good agreement with the quantum-approach for temperatures $T geq 2J/k_{rm B} approx 15$ K. A small modulation of the ESR linewidth observed in the $ac$-plane is attributed to the anisotropic Zeeman interaction, which reflects the two magnetically nonequivalent Cu positions.
