arXiv subjects

A. V. Chubukov

Publications and source records attributed to A. V. Chubukov.

At least 19 recordsLinked to original sources

arXiv2026

Avoided Stoner instability at a single ordinary Van Hove point

When the Fermi surface and the Brillouin zone boundary touch at a Van Hove point, mean-field analysis predicts a ferromagnetic (Stoner) instability at finite $T_{MF}$ for any coupling strength due to the divergent density of states. However, the predicted effect has not been observed experimentally. Several qualitative theoretical proposals have been put forward to explain why the mean-field prediction fails. Based on numerically exact results for the two-dimensional Hubbard model with an ordinary Van Hove singularity, we uncover the mechanisms behind the suppression of the ferromagnetic instability. We employ two diagrammatic Monte Carlo approaches: (i) the four-channel self-consistent approximation and (ii) numerically exact method of combinatorial summation of diagrams with controlled resummation of the truncated expansion. We find that the system avoids the Stoner instability down to temperatures an order of magnitude below $T_{MF}$ due to the combination of the downward renormalization of the effective coupling and the suppression of the density of states by the loss of the quasiparticle residue.

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arXiv2024

Quantum-criticality and superconductivity in twisted transition metal di-chalcogenides

We analyze a model for electronic structure and interactions in twisted transition metal chalcogenide WSe$_2$ for superconductivity. In this material, spin-orbit scattering locks the z-components of spins of low-energy fermions near the Dirac ${\bf K}$ and ${\bf K}'$ points of the hexagonal Brillouin zone, reducing the symmetry of spin-spin interactions to that of an xy model. We show that a nominally repulsive 4-fermion interaction gives rise to an attraction for pairing in a two-component $E^{-}$ channel, which is a hexagonal lattice representation of the $\ell =1$ channel. The gap function is inversion-odd and a linear combination of spin singlet and spin triplet. At weak coupling superconductivity emerges via the Kohn-Luttinger mechanism; we compute $T_c$ for the Fermi-level lying close to the van Hove singularity. At strong coupling, the pairing is mediated by XY magnetic fluctuations peaked at momenta ${\bf K}-{\bf K}' = 2 {\bf K}$ and we estimate $T_c$ using the form of the quantum-critical XY fluctuations, displaying $ω/T$ scaling.

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arXiv2018

Superconductivity-induced nematicity

The role of nematic order for the mechanism of high-temperature superconductivity is highly debated. In most iron-based superconductors (IBS) the tetragonal symmetry is broken already in the normal state, resulting in orthorhombic lattice distortions, static stripe magnetic order, or both. Superconductivity then emerges, at least at weak doping, already from the state with broken $C_4$ rotational symmetry. One of the few stoichiometric IBS, lithium iron arsenide, superconducts below 18 K and does not display either structural or magnetic transition in the normal state. Here we demonstrate, using angle-resolved photoemission, that even superconducting state in LiFeAs is also a nematic one. We observe spontaneous breaking of the rotational symmetry in the gap amplitude on all Fermi surfaces, as well as unidirectional distortion of the Fermi pockets. Remarkably, these deformations disappear above superconducting $T_c$. Our results demonstrate the realization of a novel phenomenon of superconductivity-induced nematicity in IBS, emphasizing the intimate relation between them. We suggest a theoretical explanation based on the emergence of a secondary instability inside the superconducting state, which leads to the nematic order and s-d mixing in the gap function.

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arXiv2016

Orbital order from the on-site orbital attraction

We study the model of Fe-based superconductors with intraorbital attraction, designed to favor a spontaneous orbital polarization. Previous studies of this model within the two-orbital approximation indicated that the leading instability is toward s-wave superconductivity and the subleading one is toward anti-ferro-orbital order, which breaks the translational symmetry of the crystal. The two-orbital approximation is, however, not consistent with the Fermi surface geometry of Fe superconductors, as it yields the wrong position of one of the hole pockets. Here we analyze the model with the same interaction but with realistic Fermi surface geometry (two hole pockets at the center of the Brillouin zone and two electron pockets at its boundary). We apply the parquet renormalization-group (pRG) technique to detect the leading instability upon the lowering of the temperature. We argue that the pRG analysis strongly favors a q = 0 orbital order, which in the band basis is a d-wave Pomeranchuk order.

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arXiv2014

Interplay between tetragonal magnetic order, stripe magnetism, and superconductivity in iron-based materials

Motivated by recent experiments in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ {[}A. E. Böhmer \textit{et al}, to be published{]}, we analyze the type of spin-density wave (SDW) order in doped iron-pnictides and the discontinuities of the superconducting transition temperature $T_{c}$ in the coexistence phase with SDW magnetism. By tracking the magnetic transition line $T_{N}(x)$ towards optimal doping within an itinerant fermionic model, we find a sequence of transitions from the stripe-orthorhombic ($C_{2}$) SDW order to the tetragonal ($C_{4}$) order and then back to the $C_{2}$ order. We argue that the superconducting $T_{c}$ has two discontinuities -- it jumps to a smaller value upon entering the coexistence region with the $C_{4}$ magnetic phase, and then jumps to a larger value inside the SDW state when it crosses the boundary between the $C_{4}$ and $C_{2}$ SDW orders. The full agreement with the experimental phase diagram provides a strong indication that the itinerant approach is adequate to describe the physics of weakly/moderately doped iron-pnictides.

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arXiv2014

Composite charge order in the pseudogap region of the cuprates

We study the Ginzburg-Landau free energy functional for two coupled U(1) charge order parameters describing two non-equivalent charge orders with wave vector ${\bf Q}$ detected in X-ray and STM measurements of underdoped cuprates. We do not rely on a mean-field analysis, but rather utilize a field-theoretical technique suitable to study the interplay between vortex physics and discrete symmetry breaking in two-dimensional systems with U(1) symmetry. Our calculations support the idea that in the clean systems there are two transitions: from a high temperature disordered state into a state with a composite charge order which breaks time-reversal symmetry, but leaves U(1) fields disordered, and then into a state with quasi long range order in the U(1) fields.

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arXiv2014

Collective modes in multiband superconductors: Raman scattering in iron selenides

We study Raman scattering in the superconducting state of alkali-intercalated iron selenide materials AxFe2-ySe2 (A=K,Rb,Cs) in which Fermi surface has only electron pockets. Theory predicts that both s-wave and d-wave pairing channels are attractive in this material, and the gap can have either s-wave or d-wave symmetry, depending on the system parameters. ARPES data favor $s-$wave superconductivity. We present the theory of Raman scattering in AxFe2-ySe2 assuming that the ground state has s-wave symmetry but $d-$ wave is a close second. We argue that Raman profile in d-wave B2g channel displays two collective modes. One is a particle-hole exciton, another is a Bardasis-Schrieffer-type mode associated with superconducting fluctuations in d-wave channel. At a finite damping, the two modes merge into one broad peak. We present Raman data for AxFe2-ySe2 and compare them with theoretical Raman profile.

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arXiv2014

On infrared pseudogap in cuprate and pnictide high-temperature superconductors

We investigate infrared manifestations of the pseudogap in the prototypical cuprate and pnictide superconductors: YBa2Cu3Oy and BaFe2As2 (Ba122) systems. We find remarkable similarities between the spectroscopic features attributable to the pseudogap in these two classes of superconductors. The hallmarks of the pseudogap state in both systems include a weak absorption feature at about 500 cm-1 followed by a featureless continuum between 500 and 1500 cm-1 in the conductivity data and a significant suppression in the scattering rate below 700 - 900 cm-1. The latter result allows us to identify the energy scale associated with the pseudogap $Δ_{PG}$. We find that in the Ba122-based materials the superconductivity-induced changes of the infrared spectra occur in the frequency region below 100 - 200 cm-1, which is much lower than the energy scale of the pseudogap. We performed theoretical analysis of the scattering rate data of the two compounds using the same model which accounts for the effects of the pseudogap and electron-boson coupling. We find that the scattering rate suppression in Ba122-based compounds below $Δ_{PG}$ is solely due to the pseudogap formation whereas the impact of the electron-boson coupling effects is limited to lower frequencies. The magnetic resonance modes used as inputs in our modeling are found to evolve with the development of the pseudogap, suggesting an intimate correlation between the pseudogap and magnetism.

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arXiv2014

Superconductivity from repulsion in LiFeAs: novel s-wave symmetry and potential time-reversal symmetry breaking

We analyze the structure of the pairing interaction and superconducting gap in LiFeAs by decomposing the pairing interaction for various kz cuts into s- and d-wave components and by studying the leading superconducting instabilities. We use the ten orbital tight-binding model, derived from ab-initio LDA calculations with hopping parameters extracted from the fit to ARPES experiments. We find that the pairing interaction almost decouples between two subsets, one consists of the outer hole pocket and two electron pockets, which are quasi-2D and are made largely out of dxy orbital, and the other consists of the two inner hole pockets, which are quasi-3D and are made mostly out of dxz and dyz orbitals. Furthermore, the bare inter-pocket and intra-pocket interactions within each subset are nearly equal. In this situation, small changes in the intra-pocket and inter-pocket interactions due to renormalizations by high-energy fermions give rise to a variety of different gap structures. We find four different configurations of the s-wave gap immediately below Tc: the one in which superconducting gap changes sign between two inner hole pockets and between the outer hole pocket and two electron pockets, the one in which the gap changes sign between two electron pockets and three hole pockets, the one in which the gap on the outer hole pocket differs in sign from the gaps on the other four pockets, and the one in which the gaps on two inner hole pockets have one sign, and the gaps on the outer hole pockets and on electron pockets have different sign. Different s-wave gap configurations emerge depending on whether the renormalized interactions increase attraction within each subset or increase the coupling between particular components of the two subsets. We argue that the state with opposite sign of the gaps on the two inner hole pockets has the best overlap with ARPES data.

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arXiv2013

Spin resonance in AFe2Se2 with s-wave pairing symmetry

We study spin resonance in the superconducting state of recently discovered alkali-intercalated iron selenide materials A_xFe_(2-y)Se_2 (A=K,Rb,Cs) in which the Fermi surface has only electron pockets. Recent angle-resolved photoemission spectroscopy (ARPES) studies [M. Xu et al., Phys. Rev. B 85, 220504(R) (2012)] were interpreted as strong evidence for s-wave gap in these materials, while the observation of the resonance peak in neutron scattering measurements [G. Friemel et al., Phys. Rev. B 85, 140511 (2012)] suggests that the gap must have different signs at Fermi surface points connected by the momentum at which the resonance has been observed. We consider recently proposed unconventional s+- superconducting state of A_xFe_(2-y)Se_2 with superconducting gap changing sign between the hybridized electron pockets. We argue that such a state supports a spin resonance. We compute the dynamical structure factor and show that it is consistent with the results of inelastic neutron scattering.

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arXiv2013

Nematic order in iron superconductors - who is in the driver's seat?

Although the existence of nematic order in iron-based superconductors is now a well-established experimental fact, its origin remains controversial. Nematic order breaks the discrete lattice rotational symmetry by making the $x$ and $y$ directions in the Fe plane non-equivalent. This can happen because of (i) a tetragonal to orthorhombic structural transition, (ii) a spontaneous breaking of an orbital symmetry, or (iii) a spontaneous development of an Ising-type spin-nematic order - a magnetic state that breaks rotational symmetry but preserves time-reversal symmetry. The Landau theory of phase transitions dictates that the development of one of these orders should immediately induce the other two, making the origin of nematicity a physics realization of a "chicken and egg problem". The three scenarios are, however, quite different from a microscopic perspective. While in the structural scenario lattice vibrations (phonons) play the dominant role, in the other two scenarios electronic correlations are responsible for the nematic order. In this review, we argue that experimental and theoretical evidence strongly points to the electronic rather than phononic mechanism, placing the nematic order in the class of correlation-driven electronic instabilities, like superconductivity and density-wave transitions. We discuss different microscopic models for nematicity in the iron pnictides, and link nematicity to other ordered states of the global phase diagram of these materials -- magnetism and superconductivity. In the magnetic model nematic order pre-empts stripe-type magnetic order, and the same interaction which favors nematicity also gives rise to an unconventional $s^{+-}$ superconductivity. In the charge/orbital model magnetism appears as a secondary effect of ferro-orbital order, and the interaction which favors nematicity gives rise to a conventional $s^{++}$ superconductivity.

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arXiv2013

Enhancement of the London penetration depth in pnictides at the onset of SDW order under superconducting dome

Recent measurements of the doping dependence of the London penetration depth λ(x) at low temperatures in clean samples of isovalent BaFe_2[As_(1-x)P_x]_2 at T<<Tc [Hashimoto et al., Science 336, 1554 (2012)] revealed a peak in λ(x) near optimal doping x=0.3. The observation of the peak at T<<Tc, points to the existence of the quantum critical point (QCP) beneath the superconducting dome. We associate such a QCP with the onset of a spin- density-wave order and show that the renormalization of λ(x) by critical magnetic fluctuations, gives rise to the observed feature. We argue that the case of pnictides is conceptually different from a one-component Galilean invariant Fermi liquid, for which correlation effects do not cause the renormalization of the London penetration depth at T=0.

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arXiv2012

Vertical loop nodes in iron-based superconductors

We consider Fe-based superconductors with s^{+-} gap with accidental nodes on electron pockets. We analyze how the gap structure changes if we include into the consideration the hybridization between the two electron pockets (the inter-pocket hopping term with momentum (pi,pi,pi). We derive the hybridization term and relate it to the absence of inversion symmetry in the Fe-plane because of two non-equivalent locations of pnictogen (chalcogen) above and below the plane. We find that the hybridization tends to eliminate the nodes -- as it increases, the pairs of neighboring nodes approach each other, merge and disappear once the hybridization exceeds a certain threshold. The nodes disappear first around k_z =pi/2, and vertical line nodes split into two vertical loops centered at k_z =0 and k_z = pi. We also show that the hybridization moves the nodes along the loops away from the normal state Fermi surfaces. This creates a subset of k-points at which the peak in the spectral function does not shift as the system enters into a superconducting state ("no-shift" lines). These "no-shift" lines evolve with increasing hybridization in highly non-trivial manner and eventually form horizontal loops in (k_x, k_y) plane, surrounding the nodes. Both vertical line nodes and horizontal "no-shift" loops surrounding them should be detectable in photoemission experiments.

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arXiv2012

First-Matsubara-frequency rule in a Fermi liquid. Part II: Optical conductivity and comparison to experiment

Motivated by recent optical measurements on a number of strongly correlated electron systems, we revisit the dependence of the conductivity of a Fermi liquid, σ(Ω,T), on the frequency Ωand temperature T. Using the Kubo formalism and taking full account of vertex corrections, we show that the Fermi liquid form Reσ^{-1}(Ω,T)\propto Ω^2+4π^2T^2 holds under very general conditions, namely in any dimensionality above one, for a Fermi surface of an arbitrary shape (but away from nesting and van Hove singularities), and to any order in the electron-electron interaction. We also show that the scaling form of Reσ^{-1}(Ω,T) is determined by the analytic properties of the conductivity along the Matsubara axis. If a system contains not only itinerant electrons but also localized degrees of freedom which scatter electrons elastically, e.g., magnetic moments or resonant levels, the scaling form changes to Reσ^{-1}(Ω,T)\propto Ω^2+bπ^2T^2, with 1\leq b<\infty. For purely elastic scattering, b =1. Our analysis implies that the value of b\approx 1, reported for URu_2Si_2 and some rare-earth based doped Mott insulators, indicates that the optical conductivity in these materials is controlled by an elastic scattering mechanism, whereas the values of b\approx 2.3 and b\approx 5.6, reported for underdoped cuprates and organics, correspondingly, imply that both elastic and inelastic mechanisms contribute to the optical conductivity.

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arXiv2012

Enhancement of $T_{c}$ by disorder in underdoped iron pnictides

We analyze how disorder affects the transition temperature $T_{c}$ of the $s^{+-}$superconducting state in the iron pnictides. The conventional wisdom is that $T_{c}$ should rapidly decrease with increasing inter-band non-magnetic impurity scattering, but we show that this behavior holds only in the overdoped region of the phase diagram. In the underdoped regime, where superconductivity emerges from a pre-existing magnetic state, disorder gives rise to two competing effects: breaking of the Cooper pairs, which tends to reduce $T_{c}$, and suppression of the itinerant magnetic order, which tends to bring $T_{c}$ up. We show that for a wide range of parameters the second effect wins, leading to an increase of $T_{c}$ with disorder in the coexistence state. Our results explain several recent experimental findings and provide another evidence for $s^{+-}$-pairing in the iron pnictides.

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arXiv2012

Gap nodes induced by coexistence with antiferromagnetism in iron-based superconductors

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].

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arXiv2012

Inter-pocket pairing and gap symmetry in Fe-based superconductors with only electron pockets

Pairing symmetry in recently discovered Fe-based metallic superconductors AFe$_2$Se$_2$ (A = K, Rb, Cs) with high transition temperature $T_c \sim 40$ K is currently a subject of intensive debates. These systems contain only electron pockets, according to photoemission, and differ from the majority of Fe-based superconductors in which both electron and hole pockets are present. Both d-wave and s-wave pairing symmetries have been proposed for AFe$_2$Se$_2$, but a d-wave gap generally has nodes, while experiments clearly point to no-nodal behavior, and a conventional s-wave gap is inconsistent with the observation of the neutron resonance below $T_c$. We argue that current theories of pairing in such systems are incomplete and must include not only intra-pocket pairing condensate but also inter-pocket condensate made of fermions belonging to different electron pockets. We analyze the interplay between intra-pocket and inter-pocket pairing depending on the ellipticity of electron pockets and the strength of their hybridization and show that hybridization brings the system into a new $s^{+-}$ state, in which the gap changes sign between hybridized pockets. This state has the full gap and at the same time supports spin resonance, in agreement with the data. Near the boundary of $s^{+-}$ state we found a long-thought $s+id$ state which breaks time-reversal symmetry.

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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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