arXiv subjects

A. V. Chubukov

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

At least 55 records · Page 3Linked to original sources

arXiv2009

Renormalization group analysis of competing orders and the pairing symmetry in Fe-based superconductors

We analyze antiferromagnetism and superconductivity in novel Fe-based superconductors within the weak-coupling, itinerant model of electron and hole pockets near (0,0) and (π,π) in the folded Brillouin zone. We discuss the interaction Hamiltonian, the nesting, the RG flow of the couplings at energies above and below the Fermi energy, and the interplay between SDW magnetism, superconductivity and charge orbital order. We argue that SDW antiferromagnetism wins at zero doping but looses to superconductivity upon doping. We show that the most likely symmetry of the superconducting gap is A_{1g} in the folded zone. This gap has no nodes on the Fermi surface but changes sign between hole and electron pockets. We also argue that at weak coupling, this pairing predominantly comes not from a spin fluctuation exchange but from a direct pair hopping between hole and electron pockets.

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arXiv2009

Non-analytic spin susceptibility of a nested Fermi liquid: the case of Fe-based pnictides

We propose an explanation of the peculiar linear temperature dependence of the uniform spin susceptibility $χ(T)$ in ferropnictides. We argue that the linear in $T$ term appears due to non-analytic temperature dependence of $χ(T)$ in a two-dimensional Fermi liquid. We show that the prefactor of the $T$ term is expressed via the square of the spin-density-wave (SDW) amplitude connecting nested hole and electron pockets. Due to an incipient SDW instability, this amplitude is large, which, along with a small value of the Fermi energy, makes the $T$ dependence of $χ(T)$ strong. We demonstrate that this mechanism is in quantitative agreement with the experiment.

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arXiv2008

Interplay between magnetism and superconductivity in Fe-pnictides

We consider phase transitions and potential co-existence of spin-density-wave (SDW) magnetic order and extended s-wave ($s^+$) superconducting order within a two-band itinerant model of iron pnictides, in which SDW magnetism and $s^+$ superconductivity are competing orders. We show that depending on parameters, the transition between these two states is either first order, or involves an intermediate phase in which the two orders co-exist. We demonstrate that such co-existence is possible when SDW order is incommensurate.

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arXiv2008

Magnetism, superconductivity, and pairing symmetry in Fe-based superconductors

We analyze antiferromagnetism and superconductivity in novel $Fe-$based superconductors within the itinerant model of small electron and hole pockets near $(0,0)$ and $(π,π)$. We argue that the effective interactions in both channels logarithmically flow towards the same values at low energies, {\it i.e.}, antiferromagnetism and superconductivity must be treated on equal footings. The magnetic instability comes first for equal sizes of the two pockets, but looses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces (extended s-wave symmetry). We argue that the $T$ dependencies of the spin susceptibility and NMR relaxation rate for such state are exponential only at very low $T$, and can be well fitted by power-laws over a wide $T$ range below $T_c$.

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arXiv2008

Gap anisotropy and universal pairing scale in a spin fluctuation model for cuprates

We consider the evolution of d-wave pairing, mediated by nearly critical spin fluctuations, with the coupling strength. We show that the onset temperature for pairing, T*, smoothly evolves between weak and strong coupling, passing through a broad maximum at intermediate coupling. At strong coupling, T* is of order the magnetic exchange energy J. We argue that for all couplings, pairing is confined to the vicinity of the Fermi surface. We also find that thermal spin fluctuations only modestly reduce T*, even at criticality, but they substantially smooth the gap anisotropy. The latter evolves with coupling, being the largest at weak coupling.

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arXiv2008

On the observation of the spin resonance in superconducting CeCoIn_5

Recent observation of a resonance spin excitation at (1/2,1/2,1/2) in the superconducting state of CeCoIn_5 [C. Stock et al., Phys. Rev. Lett. {\bf 100} 087001 (2008)] was interpreted as an evidence for d_{x^2-y^2} gap symmetry, by analogy with the cuprates. This is true if the resonance is a spin exciton. We argue that such description is undermined by the three-dimensionality of CeCoIn_5. We show that in 3D systems the excitonic resonance only emerges at strong coupling, and is weak. We argue in favor of the alternative, magnon scenario, which does not require a d_{x^2-y^2} gap.

cond-mat.str-el↗

arXiv2008

Raman scattering in a d-wave superconductor - a one gap scenario

Recent Raman scattering data in B_{1g} and B_{2g} geometries in the superconducting state of underdoped cuprates were interpreted as evidence for two distinct energy gaps. We argue that these data can be equally well explained within a one gap scenario if final state interactions are taken into account. In particular, we show that they can account for the differing doping dependences of the Raman peaks in these two geometries.

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arXiv2007

Optical Integral in the Cuprates and the Question of Sum Rule Violation

Much attention has been given to a possible violation of the optical sum rule in the cuprates, and the connection this might have to kinetic energy lowering. The optical integral is composed of a cut-off independent term (whose temperature dependence is a measure of the sum rule violation), plus a cut-off dependent term that accounts for the extension of the Drude peak beyond the upper bound of the integral. We find that the temperature dependence of the optical integral in the normal state of the cuprates can be accounted for solely by the latter term, implying that the dominant contribution to the observed sum rule `violation' in the normal state is due to the finite cut-off. This cut-off dependent term is well modeled by a theory of electrons interacting with a broad spectrum of bosons.

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arXiv2007

Gapless superconductivity and the Fermi arc in the cuprates

We argue that the Fermi arc observed in angle resolved photoemission measurements in underdoped cuprates can be understood as a consequence of inelastic scattering in a d-wave superconductor. We analyze this phenomenon in the context of strong coupling Eliashberg theory, deriving a `single lifetime' model for describing the temperature evolution of the spectral gap as measured by single particle probes such as photoemission and tunneling.

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arXiv2007

Spin-wave spectra of a kagome stripe

We study ground state degeneracy and spin-wave excitations in a 1D version of a Kagome antiferromagnet -- a Heisenberg antiferromagnet on a Kagome stripe. We show that for nearest-neighbor interaction, the classical ground state is infinitely degenerate. For any spin configuration from the degenerate set, the classical spin-wave spectrum contains, in addition to Goldstone modes, a branch of zero energy excitations, and a zero mode in another branch. We demonstrate that the interactions beyond nearest neighbors lift the degeneracy, eliminate a zero mode, and give a finite dispersion to formerly zero-energy branch, leaving only Goldstone modes as zero-energy excitations.

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arXiv2007

Spin-liquid model of the sharp resistivity drop in $La_{1.85}Ba_{0.125}CuO_4$

We use the phenomenological model proposed in our previous paper [Phys. Rev. Lett. {\bf 98}, 237001 (2007)] to analyse the magnetic field dependence of the onset temperature for two-dimensional fluctuating superconductivity $T^{**} (H)$. We demonstrate that the slope of $T^{**} (H)$ progressively goes down as $H$ increases, such that the upper critical field progressively increases as $T$ decreases. The quantitative agreement with the recent measurements of $T^{**} (H)$ in $La_{1.85}Ba_{0.125}CuO_4$ is achieved for the same parameter value as was derived in our previous publication from the analysis of the electron self energy.

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arXiv2007

Phenomenological theory of the underdoped phase of a high-T$_c$ superconductor

We model the Fermi surface of the cuprates by one-dimensional nested parts near $(0,π)$ and $(π,0)$ and unnested parts near the zone diagonals. Fermions in the nested regions form 1D spin liquids, and develop spectral gaps below some $\sim T^*$, but superconducting order is prevented by 1D phase fluctuations. We show that the Josephson coupling between order parameters at $(0,π)$ and $(π,0)$ locks their relative phase at a crossover scale $T^{**}< T^*$. Below $T^{**}$, the system response becomes two-dimensional, and the system displays Nernst effect. The remaining total phase gets locked at $T_c < T^{**}$, at which the system develops a (quasi-) long-range superconducting order.

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arXiv2006

Renormalization of the electron-spin-fluctuation interaction in the t-t'-U Hubbard model

We study the renormalization of the electron-spin-fluctuation (el-sp) vertex in a two-dimensional Hubbard model with nearest-neighbor (t) and next-nearest-neighbor (t') hopping by a Quantum-Monte-Carlo calculation. Our results show that for t'=0, the renormalized el-sp vertex decreases quite generally with decreasing temperature at all spin-fluctuation momentum transfers. The suppression of the el-sp vertex results in a substantial reduction of the effective pairing interaction mediated by antiferromagnetic spin fluctuations in both the intermediate- and strong-correlation regimes. The inclusion of a finite t'/t<0, increases the Landau damping rate of spin fluctuations, especially in the overdoped region. The increased damping rate leads to smaller vertex corrections, in agreement with earlier diagrammatic calculations. Still, the vertex correction reduces the spin-fermion vertex, as at t'=0.

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arXiv2006

The effect of Fermi surface curvature on low-energy properties of fermions with singular interactions

We discuss the effect of Fermi surface curvature on long-distance/time asymptotic behaviors of two-dimensional fermions interacting via a gapless mode described by an effective gauge field-like propagator. By comparing the predictions based on the idea of multi-dimensional bosonization with those of the strong- coupling Eliashberg approach, we demonstrate that an agreement between the two requires a further extension of the former technique.

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arXiv2006

Momentum-dependent light scattering in a 2D Heisenberg antiferromagnet

Motivated by the achievements of the $X$-ray scattering technique, we analyzed the profile of the light scattering intensity $R(q, ω)$ at a finite $q$ in a 2D Heisenberg antiferromagnet. Previous Raman scattering studies at $q=0$ identified the two-magnon peak in $B_{1g}$ scattering geometry. We found that the $B_{1g}$ peak disperses downwards at a finite $q$, and its intensity increases, reaching its maximum at $q= q_0 = (0, π)$ and symmetry related points. In addition, the intensity in the $A_{1g}$ geometry becomes non-zero at a finite $q$, and also displays a two-magnon peak which gains strength and disperses to larger frequencies with increasing $q$, and reaches its highest intensity at $q_0$. We found that the profile of $R(q_0, ω)$ is equivalent in $A_{1g}$ and $B_{1g}$ geometries.

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arXiv2006

Quantum critical behavior in itinerant electron systems -- Eliashberg theory and instability of a ferromagnetic quantum-critical point

We consider the problem of fermions interacting with gapless long-wavelength collective bosonic modes. The theory describes, among other cases, a ferromagnetic quantum-critical point (QCP) and a QCP towards nematic ordering. We construct a controllable expansion at the QCP in two steps: we first create a new, non Fermi-liquid ``zero-order'' Eliashberg-type theory, and then demonstrate that the residual interaction effects are small. We prove that this approach is justified under two conditions: the interaction should be smaller than the fermionic bandwidth, and either the band mass $m_B$ should be much smaller than $m = p_F/v_F$, or the number of fermionic flavors $N$ should be large. For an SU(2) symmetric ferromagnetic QCP, we find that the Eliashberg theory itself includes a set of singular renormalizations which can be understood as a consequence of an effective long-range dynamic interaction between quasi-particles, generated by the Landau damping term. These singular renormalizations give rise to a negative non-analytic $q^{3/2}$ correction to the static spin susceptibility, and destroy a ferromagnetic QCP. We demonstrate that this effect can be understood in the framework of the $ϕ^4$ theory of quantum criticality. We also show that the non-analytic $q^{3/2}$ correction to the bosonic propagator is specific to the SU(2) symmetric case. For systems with a scalar order parameter, the $q^{3/2}$ contributions from individual diagrams cancel out in the full expression of the susceptibility, and the QCP remains stable.

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arXiv2005

Self-energy of a nodal fermion in a d-wave superconductor

We re-consider the self-energy of a nodal (Dirac) fermion in a 2D d-wave superconductor. A conventional belief is that Im Σ(ω, T) \sim max (ω^3, T^3). We show that Σ(ω, k, T) for k along the nodal direction is actually a complex function of ω, T, and the deviation from the mass shell. In particular, the second-order self-energy diverges at a finite T when either ωor k-k_F vanish. We show that the full summation of infinite diagrammatic series recovers a finite result for Σ, but the full ARPES spectral function is non-monotonic and has a kink whose location compared to the mass shell differs qualitatively for spin-and charge-mediated interactions.

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arXiv2005

High Frequency Behavior of the Infrared Conductivity of Cuprates

We analyze recent infrared conductivity data in the normal state of the cuprates. We find that the high frequency behavior, which has been suggested as evidence for quantum critical scaling, is generally characteristic of electrons interacting with a broad spectrum of bosons. From explicit calculations, we find a frequency exponent for the modulus of the conductivity, and a phase angle, in good agreement with experiment. The data indicate an upper cut-off of the boson spectrum of order 300 meV. This implies that the bosons are of electronic origin rather than phonons.

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