arXiv subjects

I. Eremin

Publications and source records attributed to I. Eremin.

At least 37 records · Page 2Linked to original sources

arXiv2013

Doping asymmetry of superconductivity coexisting with antiferromagnetism in spin fluctuation theory

We generalize the theory of Cooper pairing by spin excitations in the metallic antiferromagnetic state to include situations with electron and/or hole pockets. We show that Cooper pairing arises from transverse spin waves and from gapped longitudinal spin fluctuations of comparable strength. However, each of these interactions, projected on a particular symmetry of the superconducting gap, acts primarily within one type of pocket. We find a nodeless $d_{x^2-y^2}$-wave state is supported primarily by the longitudinal fluctuations on the electron pockets, and both transverse and longitudinal fluctuations support nodeless odd-parity spin singlet $p-$wave symmetry on the hole pockets. Our results may be relevant to the asymmetry of the AF/SC coexistence state in the cuprate phase diagram, as well as for the "nodal gap" observed recently for strongly underdoped cuprates.

cond-mat.supr-con↗

arXiv2013

Ultrafast modulation of the chemical potential in BaFe$_2$As$_2$ by coherent phonons

Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in BaFe$_2$As$_2$ around the high-symmetry points $Γ$ and $M$. A global oscillation of the Fermi level at the frequency of the $A_{1g}$(As) phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the $A_{1g}$ phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling.

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arXiv2012

Incommensurate magnetic fluctuations and Fermi surface topology in LiFeAs

Using the angle-resolved photoemission spectroscopy (ARPES) data accumulated over the whole Brillouin zone (BZ) in LiFeAs we analyze the itinerant component of the dynamic spin susceptibility in this system in the normal and superconducting state. We identify the origin of the incommensurate magnetic inelastic neutron scattering (INS) intensity as scattering between the electron pockets, centered around the $(π,π)$ point of the BZ and the large two-dimensional hole pocket, centered around the $Γ$-point of the BZ. As the magnitude of the superconducting gap within the large hole pocket is relatively small and angle dependent, we interpret the INS data in the superconducting state as a renormalization of the particle-hole continuum rather than a true spin exciton. Our comparison indicates that the INS data can be reasonably well described by both the sign changing symmetry of the superconducting gap between electron and hole pockets as well as sign preserving gap, depending on the assumptions made for the fermionic damping.

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arXiv2012

Spin excitations in layered antiferromagnetic metals and superconductors

The proximity of antiferromagnetic order in high-temperature superconducting materials is considered a possible clue to the electronic excitations which form superconducting pairs. Here we study the transverse and longitudinal spin excitation spectrum in a one-band model in the pure spin density wave (SDW) state and in the coexistence state of SDW and the superconductivity. We start from a Stoner insulator and study the evolution of the spectrum with doping, including distinct situations with only hole pockets, with only electron pockets and with pockets of both types. In addition to the usual spin-wave modes, in the partially gapped cases we find significant weight of low-energy particle-hole excitations. We discuss the implications of our findings for neutron scattering experiments and for theories of Cooper-pairing in the metallic SDW state.

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arXiv2012

Pair breaking by nonmagnetic impurities in the noncentrosymmetric superconductor CePt3Si

We have studied the effect of Ge substitution and pressure on the heavy-fermion superconductor CePt3Si. Ge substitution on the Si site acts as negative chemical pressure leading to an increase in the unit-cell volume but also introduces chemical disorder. We carried out electrical resistivity and ac heat-capacity experiments under hydrostatic pressure on CePt3Si1-xGex (x=0, 0.06). Our experiments show that the suppression of superconductivity in CePt3Si1-xGex is mainly caused by the scattering potential, rather than volume expansion, introduced by the Ge dopants. The antiferromagnetic order is essentially not affected by the chemical disorder.

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arXiv2012

Preemptive nematic order, pseudogap, and orbital order in the iron pnictides

Starting from a microscopic itinerant model, we derive and analyze the effective low-energy model for collective magnetic excitations in the iron pnictides. We show that the stripe magnetic order is generally preempted by an Ising-nematic order which breaks $C_{4}$ lattice symmetry but preserves O(3) spin-rotational symmetry. This leads to a rich phase diagram as function of doping, pressure, and elastic moduli, displaying split magnetic and nematic tri-critical points. The nematic transition may instantly bring the system to the verge of a magnetic transition, or it may occur first, being followed by a magnetic transition at a lower temperature. In the latter case, the preemptive nematic transition is accompanied by either a jump or a rapid increase of the magnetic correlation length, triggering a pseudogap behavior associated with magnetic precursors. Furthermore, due to the distinct orbital character of each Fermi pocket, the nematic transition also induces orbital order. We compare our results to various experiments, showing that they correctly address the changes in the character of the magneto-structural transition across the phase diagrams of different compounds, as well as the relationship between the orthorhombic and magnetic order parameters.

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arXiv2011

Magnetic resonance from the interplay of frustration and superconductivity

Motivated by the iron-based superconductors, we develop a self-consistent electronic theory for the itinerant spin excitations in the regime of coexistence of the antiferromagnetic stripe order with wavevector ${\bf Q}_{1} = (π,0)$ and $s^{+-}$ superconductivity. The onset of superconductivity leads to the appearance of a {\em magnetic} resonance near the wavevector ${\bf Q}_{2} = (0,π)$ where magnetic order is absent. This resonance is isotropic in spin space, unlike the excitations near ${\bf Q}_{1}$ where the magnetic Goldstone mode resides. We discuss several features which can be observed experimentally.

cond-mat.supr-con↗

arXiv2011

Effect of nodes, ellipticity and impurities on the spin resonance in Iron-based superconductors

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 δ-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.

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arXiv2011

Effect of Fermi Surface Nesting on Resonant Spin Excitations in Ba1-xKxFe2As2

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1-xKxFe2As2 over a broad range of electron band filling. The fall in the superconducting transi- tion temperature with hole doping coincides with the magnetic excitations splitting into two incom- mensurate 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.

cond-mat.supr-con↗

arXiv2011

Multiorbital Spin Susceptibility in a Magnetically Ordered State - Orbital versus Excitonic Spin Density Wave Scenario

We present a general theory of multiorbital spin waves in magnetically ordered metallic systems. Motivated by the itinerant magnetism of iron-based superconductors, we compare the magnetic excitations for two different scenarios: when the magnetic order either sets in on the on-site orbital level; or when it appears as an electron-hole pairing between different bands of electron and hole character. As an example we treat the two-orbital model for iron-based superconductors. For small magnetic moments the spin excitations look similar in both scenarios. Going to larger interactions and larger magnetic moments, the difference between both scenarios becomes striking. While in the excitonic scenario the spin waves form a closed structure over the entire Brillouin zone and the particle-hole continuum is gapped, the spin excitations in the orbital scenario can be treated as spin waves only in a close vicinity to the ordering momenta. The origin of this is a gapless electronic structure with Dirac cones which is a source of large damping. We analyze our results in connection with recent neutron scattering measurements and show that certain features of the orbital scenario with multiple order parameters can be observed experimentally.

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arXiv2010

Quasiparticle interference in iron-based superconductors

We systematically calculate quasiparticle interference (QPI) signatures for the whole phase diagram of iron-based superconductors. Impurities inherent in the sample together with ordered phases lead to distinct features in the QPI images that are believed to be measured in spectroscopic imaging-scanning tunneling microscopy (SI-STM). In the spin-density wave phase the rotational symmetry of the electronic structure is broken, signatures of which are also seen in the coexistence regime with both superconducting and magnetic order. In the superconducting regime we show how the different scattering behavior for magnetic and non-magnetic impurities allows to verify the $s^{+-}$ symmetry of the order parameter. The effect of possible gap minima or nodes is discussed.

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arXiv2010

Angular resolved specific heat in iron-based superconductors: the case for nodeless extended $s$-wave gap

We consider the variation of the field-induced component of the specific heat $C({\bf H})$ with the direction of the applied field in $Fe-$pnictides within quasi-classical Doppler-shift approximation, with special emphasis to recent experiments on FeSe$_{0.4}$Te$_{0.6}$ [Zheng et al., arXiv:1004.2236]. We show that for extended $s-$wave gap with no nodes, $C({\bf H})$ has $\cos 4 ϕ$ component, where $ϕ$ is the angle between ${\bf H}$ and the direction between hole and electron Fermi surfaces. The maxima of $C({\bf H})$ are at $π/4$, $3π/4$, etc. if the applied field is smaller than $H_0 \leq 1T$, and at $ϕ=0, π/2$, etc. if the applied field is larger than $H_0$. The angle-dependence of $C({\bf H})$, the positions of the maxima, and the relative magnitude of the oscillating component are consistent with the experiments performed in the field of $9T >> H_0$. We show that the observed $\cos 4 ϕ$ variation does not hold if the $s-$wave gap has accidental nodes along the two electron Fermi surfaces.

cond-mat.supr-con↗

arXiv2010

Interplay of magnetic and structural transitions in Fe-based pnictide superconductors

The interplay between the structural and magnetic phase transitions occurring in the Fe-based pnictide superconductors is studied within a Ginzburg-Landau approach. We show that the magnetoelastic coupling between the corresponding order parameters is behind the salient features observed in the phase diagram of these systems. This naturally explains the coincidence of transition temperatures observed in some cases as well as the character (first or second-order) of the transitions. We also show that magnetoelastic coupling is the key ingredient determining the collinearity of the magnetic ordering, and we propose an experimental criterion to distinguish between a pure elastic from a spin-nematic-driven structural transition.

cond-mat.str-el↗

arXiv2010

Quasiparticle interference in the spin-density wave phase of iron-based superconductors

We propose an explanation for the electronic nematic state observed recently in parent iron-based superconductors [T.-M. Chuang et al., Science 327, 181 (2010)]. We argue that the quasi-one-dimensional nanostructure identified in the quasiparticle interference (QPI) is a consequence of the interplay of the magnetic ($π$, 0) spin-density wave (SDW) order with the underlying electronic structure. We show that the evolution of the QPI peaks largely reflects quasiparticle scattering between electronic bands involved in the SDW formation. Because of the ellipticity of the electron pocket and the fact that only one of the electron pockets is involved in the SDW, the resulting QPI has a pronounced one-dimensional structure. We further predict that the QPI crosses over to two-dimensionality on an energy scale, set by the SDW gap, which we estimate from neutron scattering data to be around 90 meV.

cond-mat.supr-con↗

arXiv2010

Unusual disorder effects in superconducting LaFeAs_(1-d)O_0.9F_0.1 as revealed by NMR spectroscopy

We report 75As NMR measurements of the spin-lattice relaxation in the superconducting state of LaFeAsO_(0.9)F_(0.1) and As-deficient LaFeAs_(1-d)O_0.9F_0.1. The temperature behavior of 1/T_1 below T_c changes drastically from a T^3-dependence for LaFeAsO_(0.9)F_(0.1) to a T^5-dependence for the As-deficient sample. These results, together with the previously reported unexpected increase of T_c and the slope of the upper critical field near T_c for the As-deficient sample, are discussed in terms of non-universal SC gaps in Fe-pnictides and the effect of As deficiency as an exotic case where nonmagnetic 'smart' impurities even stabilize an s+- wave superconductor or within a scenario of a disorder-driven change to s++ superconductivity.

cond-mat.supr-con↗

arXiv2010

Theory of itinerant magnetic excitations in the SDW phase of iron-based superconductors

We argue that salient experimental features of the magnetic excitations in the SDW phase of iron-based superconductors can be understood within an itinerant model. We identify a minimal itinerant model and use a multi-band RPA treatment of the dynamical spin susceptibility. Weakly-damped spin-waves are found near the ordering momentum and it is shown how they dissolve into the particle-hole continuum. We show that ellipticity of the electron bands accounts for the anisotropy of the spin waves along different crystallographic directions and the gap at the momentum conjugated to the ordering one. We argue that our theory agrees well with the neutron scattering data.

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arXiv2009

Incommensurate itinerant antiferromagnetic excitations and spin resonance in the FeTe$_{0.6}$Se$_{0.4}$ superconductor

We report on inelastic neutron scattering measurements that find incommensurate itinerant like magnetic excitations in the normal state of superconducting FeTe$_{0.6}$Se$_{0.4}$ (\Tc=14K) at wave-vector $\mathbf{Q}_{inc}=(1/2\pmε,1/2\mpε)$ with $ε$=0.09(1). In the superconducting state only the lower energy part of the spectrum shows significant changes by the formation of a gap and a magnetic resonance that follows the dispersion of the normal state excitations. We use a four band model to describe the Fermi surface topology of iron-based superconductors with the extended $s(\pm)$ symmetry and find that it qualitatively captures the salient features of these data.

cond-mat.supr-con↗

arXiv2009

Self-energy effects and electron-phonon coupling in Fe-As superconductors

We report on Raman scattering experiments of the undoped SrFe2As2 and superconducting Sr0.85K0.15Fe2As2 (Tc=28K) and Ba0.72K0.28Fe2As2 (Tc=32K) single crystals. The frequency and linewidth of the B1g mode at 210 cm-1 exhibits an appreciable temperature dependence induced by the superconducting and spin density wave transitions. We give estimates of the electron-phonon coupling related to this renormalization. In addition, we observe a pronounced quasi-elastic Raman response for the undoped compound, suggesting persisting magnetic fluctuations to low temperatures. In the superconducting state the renormalization of an electronic continuum is observed with a threshold energy of 61cm-1.

cond-mat.supr-con↗