arXiv subjects

Saurabh Maiti

Publications and source records attributed to Saurabh Maiti.

29 records · Page 2Linked to original sources

arXiv2015

Collective modes in superconductors with competing s- and d-wave interactions

We calculate the collective mode spectrum in models of superconductors with attractive interactions in s-and d channels as a function of their relative strength, across a phase diagram that includes transition between s, s+id, and d-wave ground states. For one-band systems, we recover the largely known results for the phase, amplitude, and Bardasis-Schrieffer modes of pure s or d-states, and show how the well-defined Bardasis-Schrieffer mode softens near the s + id phase boundary and evolves in a characteristic manner through the s + id phase as a mixed symmetry mode. For two-band systems, we consider a model of hole-doped Fe-based superconductors, and find in the case of an s-wave ground state a well-defined Bardasis-Schrieffer mode below the lowest gap edge, as well as a second, damped mode of this type between the two gap energies. Both modes soften as the s + id phase is approached, and only a single "mixed-symmetry Bardasis-Schrieffer mode" below the pairbreaking continuum propagates in the s + id phase itself. These modes coexist with a damped Leggett mode with collective frequency between the two gap scales. In the pure d-state, no Bardasis-Schrieffer type s excitonic mode exists at low T. We briefly discuss Raman scattering experiments and how they can be used to identify an s + id state, and to track the evolution of competing s- and d- interactions in these systems.

cond-mat.supr-con↗

arXiv2015

Intrinsic Damping of Collective Spin Modes in a Two-Dimensional Fermi Liquid with Spin-Orbit Coupling

A Fermi liquid with spin-orbit coupling (SOC) is expected to support a new kind of collective modes: oscillations of magnetization in the absence of the magnetic field. We show that these modes are damped by the electron-electron interaction even in the limit of an infinitely long wavelength (q = 0). The linewidth of the collective mode is on the order of Δ^2=E_F , where Δ is a characteristic spin-orbit energy splitting and E_F is the Fermi energy. Such damping is in a stark contrast to known damping mechanisms of both charge and spin collective modes in the absence of SOC, all of which disappear at q = 0, and arises because none of the components of total spin is conserved in the presence of SOC.

cond-mat.str-el↗

arXiv2014

Spontaneous currents in a superconductor with s+is symmetry

We analyze $s + is$ state proposed as a candidate superconducting state for strongly hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$. Such a state breaks time-reversal symmetry (TRS) but does not break any other discrete symmetry. We address the issue whether TRS breaking alone can generate spontaneous currents near impurity sites, which could be detected in, e.g., muSR experiments. We argue that there are no spontaneous currents if only TRS is broken. However, supercurrents do emerge if the system is put under external strain and C4 lattice rotation symmetry is externally broken.

cond-mat.supr-con↗

arXiv2014

Collective modes in two- and three-dimensional electron systems with Rashba spin-orbit coupling

In addition to charge plasmons, a 2D electron system with Rashba-type spin-orbit coupling (SOC) also supports three collective modes in the spin sector: the chiral-spin modes. We study the dispersions of the charge and spin modes and their coupling to each other within a generalized Random Phase Approximation for arbitrarily strong SOC, and both in 2D and 3D systems. In both 2D and 3D, we find that the charge plasmons are coupled to only one of the three chiral-spin modes. This coupling is shown to affect the dispersions of the modes at finite but not at zero wavenumbers. In 3D, the chiral-spin modes are strongly damped by particle-hole excitations and disappear for weak electron-electron interaction. Landau damping of the chiral-spin modes in 3D is directly related to the fact that, in contrast to 2D, there is no gap for particle-hole excitations between spin-split subbands. The gapless continuum is also responsible for Landau damping of the charge plasmon in 3D - a qualitatively new feature of the SOC system. We also discuss the optical conductivity of clean 2D and 3D systems and show that SOC introduces spectral weight at finite frequency in a such way that the sum rule is satisfied. The in-plane tranverse chiral-spin mode shows up as dispersing peak in the optical conductivity at finite number which can can be measured in the presence of diffraction grating. We also discuss possible experimental manifestations of chiral-spin modes in semiconductor quantum wells such InGaAs/AlGaAs and 3D giant Rashba materials of the BiTeI family.

cond-mat.mes-hall↗

arXiv2013

How many quantum phase transitions exist inside the superconducting dome of the iron pnictides?

Recent experiments on two iron-pnictide families suggest the existence of a single quantum phase transition (QPT) inside the superconducting dome despite the fact that two separate transition lines - magnetic and nematic - cross the superconducting dome at $T_{c}$. Here we argue that these two observations are actually consistent. We show, using a microscopic model, that each order coexists with superconductivity for a wide range of parameters, and both transition lines continue into the superconducting dome below $T_{c}$. However, at some $T_{\mathrm{merge}}<T_{c}$, the two transitions merge and continue down to T=0 as a single simultaneous first-order nematic/magnetic transition. We show that superconductivity has a profound effect on the character of this first-order transition, rendering it weakly first-order and allowing strong fluctuations to exist near the QPT.

cond-mat.supr-con↗

arXiv2013

Superconductivity from repulsive interaction

The BCS theory of superconductivity named electron-phonon interaction as a glue that overcomes Coulomb repulsion and binds fermions into pairs which then condense and superconduct. We review recent and not so recent works aiming to understand whether a nominally repulsive Coulomb interaction can by itself give rise to a superconductivity. We first discuss a generic scenario of the pairing by electron-electron interaction, put forward by Kohn and Luttinger back in 1965, and then turn to modern studies of the electronic mechanism of superconductivity in the lattice models for the cuprates, the Fe-pnictides, and the doped graphene. We show that the pairing in all three classes of materials can be viewed as lattice version of Kohn-Luttinger physics, despite that the pairing symmetries are different. We discuss under what conditions the pairing occurs and rationalize the need to do parquet renormalization-group analysis. We also discuss the interplay between superconductivity and density-instabilities.

cond-mat.supr-con↗

arXiv2013

s+is State with Broken Time Reversal Symmetry in Fe-Based Superconductors

We analyze the evolution of the superconducting gap structure in strongly hole doped Ba_{1-x}K_xFe_2As_2 between x=1 and x ~ 0.4 (optimal doping). In the latter case, the pairing state is most likely s+-, with different gap signs on hole and electron pockets, but with the same signs of the gap on the two Gamma-centered hole pockets (a ++ state on hole pockets). In a pure KFe_2As_2 (x=1), which has only hole pockets, laser ARPES data suggested another s+- state, in which the gap changes sign between hole pockets (a +- state). We analyze how ++ gap transforms into a +- gap as x -> 1. We found that this transformation occurs via an intermediate s+is, state in which the gaps on the two hole pockets differ in phase by `phi', which gradually involves from `phi' = pi (the +- state) to phi =0 (the ++ state). This state breaks time-reversal symmetry and has huge potential for applications. We compute the dispersion of collective excitations and show that two different Leggett-type phase modes soften at the two end points of TRSB state.

cond-mat.supr-con↗

arXiv2012

Electronic transport in the Coulomb phase of the pyrochlore spin ice

We investigate the transport properties of itinerant electrons interacting with a background of localized spins in a correlated paramagnetic phase of the pyrochlore lattice. We find a residual resistivity at zero temperature due to the scattering of electrons by the static dipolar spin-spin correlation that characterizes the metallic Coulomb phase. As temperature increases, thermally excited topological defects, also known as magnetic monopoles, reduce the spin correlation, hence suppressing electron scattering. Combined with the usual scattering processes in metals at higher temperatures, this mechanism yields a non-monotonic resistivity, displaying a minimum at temperature scales associated with the magnetic monopole excitation energy. Our calculations agree quantitatively with resistivity measurements in Nd2Ir2O7, shedding light on the origin of the resistivity minimum observed in metallic spin-ice compounds.

cond-mat.str-el↗

arXiv2011

Relation between 2Delta/Tc and nodes in Fe-based superconductors

We analyze the interplay between the absence or presence of the nodes in the superconducting gap along electron Fermi surfaces (FSs) in Fe-pnictides, and 2Delta/T_c along hole FSs, measured by ARPES. We solve the set of coupled gap equations for 4-band and 5-band models of Fe-pnictides and relate the presence of the nodes to 2Δ/T_c being below a certain threshold. Using ARPES data for 2Δ/T_c, we find that optimally doped (BaK)Fe_2As_2 and Ba(FeCo)_2As_2 and likely nodeless, but iso-valent BaFe_2(AsP)_2 likely has nodes.

cond-mat.supr-con↗

arXiv2011

Renormalization group flow, competing phases, and the structure of superconducting gap in multi-band models of Iron based superconductors

We perform an analytical renormalization group (RG) study to address the role of Coulomb repulsion, the competition between extended s-wave superconducting order (s+-) and the spin-density wave (SDW) order and the angular dependence of the superconducting gap in multi-pocket models of Iron based superconductors. Previous analytic RG studies considered a toy model of one hole and one electron pocket. We consider more realistic models of two electron pockets and either two or three hole pockets, and also incorporate the angular dependence of interaction. We neglect for simplicity one of the two hole pockets centered at k=0, which is less nested with electron pockets, i.e., consider 3-pocket and 4-pocket models. In a toy 2-pocket model, SDW order always wins over s+- order at perfect nesting, and s+- order only appears at a finite doping, and only if RG flow extends long enough to overcome intra-pocket Coulomb repulsion. For multi-pocket models, we find two new effects. First, there always exists an attractive component of the interaction in s+- channel, such that the system necessary becomes a superconductor once it overcomes the competition from the SDW state. Second, in 3-pocket case (but not in 4-pocket case), there are situations when s+- order wins over SDW order even for perfect nesting, suggesting that SDW order is not a necessary pre-condition for the s+- order. Our results are in good agreement with recent numerical functional RG studies by Thomale et al. [arXiv:1002.3599]

cond-mat.supr-con↗

arXiv2010

Optical Integral and Sum Rule Violation

The purpose of this work is to investigate the role of the lattice in the optical Kubo sum rule in the cuprates. We compute conductivities, optical integrals W, and ΔW between superconducting and normal states for 2-D systems with lattice dispersion typical of the cuprates for four different models -- a dirty BCS model, a single Einstein boson model, a marginal Fermi liquid model, and a collective boson model with a feedback from super-conductivity on a collective boson. The goal of the paper is two-fold. First, we analyze the dependence of W on the upper cut-off w_c placed on the optical integral because in experiments W is measured up to frequencies of order bandwidth. For a BCS model, the Kubo sum rule is almost fully reproduced at w_c equal to the bandwidth. But for other models only 70%-80% of Kubo sum rule is obtained up to this scale and even less so for ΔW, implying that the Kubo sum rule has to be applied with caution. Second, we analyze the sign of ΔW. In all models we studied ΔW is positive at small w_c, then crosses zero and approaches a negative value at large w_c, i.e. the optical integral in a superconductor is smaller than in a normal state. The point of zero crossing, however, increases with the interaction strength and in a collective boson model becomes comparable to the bandwidth at strong coupling. We argue that this model exhibits the behavior consistent with that in the cuprates.

cond-mat.str-el↗