We use polarization-resolved Raman spectroscopy to demonstrate that for a three-dimensional giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long-wavelength q -> 0 limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasimomentum and thus negligibly weak in the Raman spectra, we observe a sharp in-gap plasmon mode in the Raman spectrum of BiTeI below the Rashba continuum. This coupling, in a polar system with spin-orbit coupling, occurs without assistance from phonons when the incoming photon excitation is resonant with Rashba-split intermediate states. We discuss the distinctive features of BiTeI's giant Rashba system band structure that enable the direct observation of plasmons in Raman scattering.

A combination of spin-orbit coupling and electron-electron interaction gives rise to a new type of collective spin modes, which correspond to oscillations of magnetization even in the absence of the external magnetic field. We review recent progress in theoretical understanding and experimental observation of such modes, focusing on three examples of real-life systems: a two-dimensional electron gas with Rashba and/or Dresselhaus spin-orbit coupling, graphene with proximity-induced spin-orbit coupling, and the Dirac state on the surface of a three-dimensional topological insulator. This paper is dedicated to the 95th birthday of Professor Emmanuel I. Rashba.

Electronic nematic behavior has been identified and studied in iron-based superconductors for some time, particularly in the well-known BaFe$_2$As$_2$ system, where it is well-known to compete with superconductivity. On the other hand, it has been shown recently that FeSe displays a negligible effect of nematicity on superconductivity near the superconducting transition, and actual cooperation between the two orders when the system is doped with S. Recently it has also been proposed that LiFeAs undergoes a nematic transition in the superconducting state itself. Generally, we expect superconductivity to be anisotropic when it coexists with nematic order, but it is not clear under what circumstances the two orders compete or cooperate, nor how the anisotropy of the superconducting state correlates with that in the nematic state. To address this, we study a simple mean field model of a $d$-wave Pomeranchuk instability together with a mixed $s,d$ pairing interaction, and identify when nematicity is enhanced or suppressed by superconductivity. We show that the competition or cooperation depends significantly on the distortion of the Fermi surface due to nematicity relative to the anisotropy of the superconducting gap function. Further, we discuss the implications of our results for FeSe and LiFeAs.

Using polarization-resolved resonant Raman spectroscopy, we explore collective spin excitations of the chiral surface states in a three dimensional topological insulator, Bi2Se3. We observe a sharp peak at 150 meV in the pseudovector A(2) symmetry channel of the Raman spectra. By comparing the data with calculations, we identify this peak as the transverse collective spin mode of surface Dirac fermions. This mode, unlike a Dirac plasmon or a surface plasmon in the charge sector of excitations, is analogous to a spin wave in a partially polarized Fermi liquid, with spin-orbit coupling playing the role of an effective magnetic field.

In unconventional superconductors, understanding the form of the pairing interaction is the primary goal. In this regard, Raman spectroscopy is a very useful tool, as it identifies the ground state and also the subleading pairing channels by probing collective modes. Here, we propose a general theory for a multiband Raman response and identify new features in the spectrum that can provide a robust test for a pairing theory. We identify multiple Bardasis-Schrieffer type collective modes and connect the weights of these modes to the subleading gap structures within a microscopic pairing theory. While our conclusions are completely general, we apply our approach to interpret the specific case of B_{1g} Raman scattering in hole-doped BaFe_{2}As_{2}.

Several Fe chalcogenide superconductors without hole pockets at the Fermi level display high temperature superconductivity, in apparent contradiction to naive spin fluctuation pairing arguments. Recently, scanning tunneling microscopy experiments studied the influence of impurities on some of these materials and claimed that nonmagnetic impurities do not create in-gap states, leading to the conclusion that the gap must be s(++), i.e., conventional s wave with no gap sign change. Here we critique this argument, and give various ways sign-changing gaps can be consistent with the absence of such bound states. In particular, we calculate the bound states for an s(+/-) system with a hole pocket below the Fermi level, and show that the nonmagnetic impurity bound state energy generically tracks the gap edge E-min in the system, thereby rendering it unobservable. A failure to observe a bound state in the case of a nonmagnetic impurity therefore cannot be used as an argument to exclude sign-changing pairing states.

We investigate superconductivity in a two-band system with an electronlike and a holelike band, where one of the bands is away from the Fermi level (or "incipient"). We argue that the incipient band contributes significantly to spin-fluctuation pairing in the strong coupling limit where the system is close to a magnetic instability and can lead to a large T_{c}. In this case, T_{c} is limited by a competition between the frequency range of the coupling (set by an isolated paramagnon) and the coupling strength itself, such that a domelike T_{c} dependence on the incipient band position is obtained. The coupling of electrons to phonons is found to further enhance T_{c}. The results are discussed in the context of experiments on monolayers and intercalates of FeSe.

Recent experiments on certain Fe-based superconductors have hinted at a role for paired electrons in "incipient" bands that are close to, but do not cross the Fermi level. Related theoretical works disagree on whether or not strong-coupling superconductivity is required to explain such effects, and whether a critical interaction strength exists. In this work, we consider various versions of the model problem of pairing of electrons in the presence of an incipient band, within a simple multiband weak-coupling BCS approximation. We categorize the problem into two cases: case(I) where superconductivity arises from the "incipient band pairing" alone, and case(II) where it is induced on an incipient band by pairing due to Fermi-surface based interactions. Negative conclusions regarding the importance of incipient bands have been drawn so far largely based on case(I), but we show explicitly that models under case(II) are qualitatively different, and can explain the non-exponential suppression of Tc, as well as robust large gaps on an incipient band. In the latter situation, large gaps on the incipient band do not require a critical interaction strength. We also model the interplay between phonon and spin fluctuation driven superconductivity and describe the bootstrap of electron-phonon superconductivity by spin fluctuations coupling the incipient and the regular bands. Finally, we discuss the effect of the dimensionality of the incipient band on our results. We argue that pairing on incipient bands may be significant and important in several Fe-based materials, including LiFeAs, FeSe intercalates and FeSe monolayers on strontium titanate, and indeed may contribute to high critical temperatures in some cases.

Submitted for the MAR15 Meeting of The American Physical Society Intrinsic Damping of Collective Spin Modes in a TwoDimensional Fermi Liquid with Spin-Orbit Interaction1 DMITRII MASLOV, University of Florida, Gainesville, SAURABH MAITI, University of Florida, Gainesville and National High Magnetic Field Lab., Tallahassee — We address the issue damping of spin collective modes in systems with spin orbit coupling in 2D. We show that these modes exist for arbitrary nature of spin-orbit coupling and are intrinsically damped even in the long wavelength limit. This damping is driven by electron-electron interactions and is unique to spin orbit coupled systems. Its origin is linked to an imperfect cancellation of the self energy and vertex contributions of the interaction. In the Fermi-liquid language, this is an effect arising from residual interaction between quasiparticles. This damping mechanism exists already at T=0 and without impurities and/or phonons. We also discuss the consequences of this damping for the experiment. 1This work was supported by the National Science Foundation via grant NSF DMR1308972. Saurabh Maiti University of Florida, Gainesville and National High Magnetic Field Lab., Tallahassee Date submitted: 12 Nov 2014 Electronic form version 1.4

We report measurements of Shubnikov-de Haas oscillations in the giant Rashba semiconductor BiTeI under applied pressures up to ∼2 GPa. We observe one high frequency oscillation at all pressures and one low frequency oscillation that emerges between ∼0.3-0.7 GPa indicating the appearance of a second small Fermi surface. BiTeI has a conduction band bottom that is split into two sub-bands due to the strong Rashba coupling, resulting in a 'Dirac point'. Our results suggest that the chemical potential starts below the Dirac point in the conduction band at ambient pressure and moves upward, crossing it as pressure is increased. The presence of the chemical potential above this Dirac point results in two Fermi surfaces. We present a simple model that captures this effect and can be used to understand the pressure dependence of our sample parameters. These extracted parameters are in quantitative agreement with first-principles calculations and other experiments. The parameters extracted via our model support the notion that pressure brings the system closer to the predicted topological quantum phase transition.

We revisit the issue of the gap symmetry in KFe2As2, which is an Fe-pnictide superconductor with only hole pockets. Previous theoretical studies mostly argued for a d-wave gap in KFe2As2 since transport and thermodynamic measurements point to the presence of the gap nodes. However, a d-wave gap is inconsistent with recent laser-based angle-resolved photoemission measurements. We propose the scenario for a nodal s-wave superconductivity induced by a non-magnetic intra-band and inter-band interactions between fermions near hole pockets. The superconducting gap changes sign between the hole pockets and has cos 4\theta angular dependence and accidental nodes on one or several hole pockets. We argue that strong angle dependence is the consequence of near-degeneracy between inter-pocket and intra-pocket interaction on the hole pockets. We also analyze cos 4\theta angular dependence of the gap in other Fe-pnictides and compare theoretical results with the photoemission experiments of LiFeAs.

We investigate the pairing in iron pnictides in the coexistence phase, which displays both superconducting and antiferromagnetic orders. By solving the pairing problem on the Fermi surface reconstructed by long-range magnetic order, we find that the pairing interaction necessarily becomes angle-dependent, even if it was isotropic in the paramagnetic phase, which results in an angular variation of the superconducting gap along the Fermi surfaces. We find that the gap has no nodes for a small antiferromagnetic order parameter M, but may develop accidental nodes for intermediate values of M, when one pair of the reconstructed Fermi surface pockets disappear. For even larger M, when the other pair of reconstructed Fermi pockets is gapped by long-range magnetic order, superconductivity still exists, but the quasiparticle spectrum becomes nodeless again. We also show that the application of an external magnetic field facilitates the formation of nodes. We argue that this mechanism for a nodeless-nodal-nodeless transition explains recent thermal conductivity measurements of hole-doped Ba_{1-x}K_xFe_2As_2. [J-Ph. Read et.al. arXiv:1105.2232].

We introduce an effective low-energy pairing model for Fe-based superconductors with s- and d-wave interaction components and a small number of input parameters and use it to study the doping evolution of the symmetry and the structure of the superconducting gap. We argue that the model describes the entire variety of pairing states found so far in the Fe-based superconductors and allows one to understand the mechanism of the attraction in s(±) and d(x(2)-y(2)) channels, the competition between s- and d-wave solutions, and the origin of superconductivity in heavily doped systems, when only electron or only hole pockets are present.

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba(1-x)K(x)Fe(2)As(2) over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s(±)-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations.

Santanu K. Maiti 2, ∗ Theoretical Condensed Matter Physics Division, Saha Institute of Nuclear Physics, Sector-I, Block-AF, Bidhannagar, Kolkata-700 064, India Department of Physics, Narasinha Dutt College, 129 Belilious Road, Howrah-711 101, India Abstract Electronic transport properties through some model quantum systems are re-visited. A simple tight-binding framework is given to describe the systems where all numerical calculations are made using the Green’s function formalism. First, we demonstrate electronic transport in four different polycyclic hydrocarbon molecules, namely, benzene, napthalene, anthracene and tetracene. It is observed that electron conduction through these molecular wires is highly sensitive to molecule-toelectrode coupling strength and quantum interference of electronic waves passing through different branches of the molecular ring. Our investigations predict that to design a molecular electronic device, in addition to the molecule itself, both the molecular coupling and molecule-to-electrode interface geometry are highly important. Next, we make an in-depth study to design classical logic gates with the help of simple mesoscopic rings, based on the concept of Aharonov-Bohm effect. A single mesoscopic ring or two such rings are used to establish the logical operations where the key controlling parameter is the magnetic flux threaded by the ring. The analysis might be helpful in fabricating meso-scale or nano-scale logic gates. Finally, we address multi-terminal quantum transport through a single benzene molecule using Landauer-Büttiker formalism. Quite interestingly we see that a three-terminal benzene molecule can be operated as an electronic transistor and this phenomenon is justified through current-voltage characteristics. All these essential features of electron transport may provide a basic theoretical framework to examine electron conduction through any multi-terminal quantum system.

We present a detailed study of the symmetry and structure of the pairing gap in Fe-based superconductors (FeSCs). We treat FeSCs as quasi-2D, decompose the pairing interaction in the XY plane in s-wave and d-wave channels into contributions from scattering between different Fermi surfaces, and analyze how each scattering evolves with doping and input parameters. We verify that each interaction is well approximated by the lowest angular harmonics. We use this simplification to analyze the interplay between the interaction with and without spin-fluctuation components, the origin of the attraction in the s(+/-) and d(x2-y2) channels, the competition between them, the angular dependence of the s(+/-) gaps along the electron Fermi surface, the conditions under which the s(+/-) gap develops nodes, and the origin of superconductivity in heavily electron-or hole-doped systems, when only Fermi surfaces of one type are present. We also discuss the relation between RPA and RG approaches for FeSCs.

We analyze doping dependence of the spin resonance of an s+- superconductor and its sensitivity to the ellipticity of electron pockets, to magnetic and non-magnetic impurities, and to the angle dependence of the superconducting gap along electron Fermi surfaces. We show that the maximum intensity of the resonance shifts from commensurate to incommensurate momentum above some critical doping which decreases with increasing ellipticity. Angle dependence of the gap and particularly the presence of accidental nodes lowers the overall intensity of the resonance peak and shifts its position towards the onset of the particle-hole continuum. Still, however, the resonance remains a true \delta-function in the clean limit. When non-magnetic or magnetic impurities are present, the resonance broadens and its position shifts. The shift depends on the type of impurities and on the ratio of intraband and interband scattering components. The ratio Omega_{res}/T_c increases almost linearly with the strength of the interband impurity scattering, in agreement with the experimental data. We also compare spin response of s+- and s++ superconductors. We show that there is no resonance for s++ gap, even when there is a finite mismatch between electron and hole Fermi surfaces shifted by the antiferromagnetic momentum.

We narrate electron transport properties in honeycomb lattice ribbons with zigzag edges coupled to two semi-infinite one-dimensional metallic electrodes. The calculations are based on the tight-binding model and the Green’s function method, which numerically compute the conductance-energy and current-voltage characteristics concerning the dependence on the lengths and widths of the ribbons. Our calculated results predict that for such a ribbon an energy gap always appears in the conductance spectrum across the energy E = 0. With the increase of the size of the ribbon, the gap gradually decreases but it never vanishes. This clearly manifests that a honeycomb lattice ribbon with zigzag edges always exhibits the semiconducting behavior, and it becomes much more clearly visible from our presented current-voltage characteristics. PACS No.: 73.63.-b; 73.63.Rt.

Electron transport properties of a quantum interferometer are studied based on the Green’s function formalism. The interferometer is symmetrically attached to two one-dimensional metallic electrodes, viz, source and drain, and here we adopt a simple tight-binding model to describe the bridge system. In this article we address numerically the conductance-energy and current-voltage characteristics as functions of the interferometer-to-electrodes coupling strength, magnetic fluxes threaded by the left and right subrings of the interferometer and the difference of these two fluxes. Our study provides several interesting features of electron transport across the interferometer, and these aspects may be utilized in designing nanoelectronic devices. PACS No.: 73.23.-b; 73.63.Rt.