arXiv subjects

Andrey V. Chubukov

Publications and source records attributed to Andrey V. Chubukov.

At least 163 records · Page 9Linked to original sources

arXiv2006

Flat spin wave dispersion in a triangular antiferromagnet

The excitation spectrum of a S=1/2 2D triangular quantum antiferromagnet is studied using 1/S expansion. Due to the non-collinearity of the classical ground state significant and non-trivial corrections to the spin wave spectrum appear already in the first order in 1/S in contrast to the square lattice antiferromagnet. The resulting magnon dispersion is almost flat in a substantial portion of the Brillouin zone. Our results are in quantitative agreement with recent series expansion studies by Zheng, Fjaerestad, Singh, McKenzie, and Coldea [PRL 96, 057201 (2006) and cond-mat/0608008].

cond-mat.str-el↗

arXiv2006

Spin susceptibility in bilayered cuprates: resonant magnetic excitations

We study the momentum and frequency dependence of the dynamical spin susceptibility in the superconducting state of bilayer cuprate superconductors. We show that there exists a resonance mode in the odd as well as the even channel of the spin susceptibility, with the even mode being located at higher energies than the odd mode. We demonstrate that this energy splitting between the two modes arises not only from a difference in the interaction, but also from a difference in the free-fermion susceptibilities of the even and odd channels. Moreover, we show that the even resonance mode disperses downwards at deviations from ${\bf Q}=(π,π)$. In addition, we demonstrate that there exists a second branch of the even resonance, similar to the recently observed second branch (the $Q^*$-mode) of the odd resonance. Finally, we identify the origin of the qualitatively different doping dependence of the even and odd resonance. Our results suggest further experimental test that may finally resolve the long-standing question regarding the origin of the resonance peak.

cond-mat.supr-con↗

arXiv2006

Nonanalytic Magnetic Response of Fermi- and non-Fermi Liquids

We study the nonanalytic behavior of the static spin susceptibility of 2D fermions as a function of temperature and magnetic field. For a generic Fermi liquid, χ_s (T, H)= const+c_1 max (T,μ_B|H|), where c_1 is shown to be expressed via complicated combinations of the Landau parameters, rather than via the backscattering amplitude, contrary to the case of the specific heat. Near a ferromagnetic quantum critical point, the field dependence acquires a universal form χ^{-1}_s(H)= const- c_2|H|^{3/2}$, with $c_2>0$. This behavior implies a first-order transition into a ferromagnetic state. We establish a criterion for such a transition to win over the transition into an incommensurate phase.

cond-mat.str-el↗

arXiv2006

Theory of non-Fermi liquid and pairing in electron-doped cuprates

We apply the spin-fermion model to study the normal state and pairing instability in electron-doped cuprates near the antiferromagnetic QCP. Peculiar frequency dependencies of the normal state properties are shown to emerge from the self-consistent equations on the fermionic and bosonic self-energies, and are in agreement with experimentally observed ones. We argue that the pairing instability is in the $d_{x^{2}-y^{2}}$ channel, as in hole-doped cuprates, but theoretical $T_{c}$ is much lower than in the hole-doped case. For the same hopping integrals and the interaction strength as in hole-doped materials, we obtain $T_{c}\sim10$K at the end point of the antiferromagnetic phase. We argue that a strong reduction of $T_{c}$ in electron-doped cuprates compared to hole-doped ones is due to critical role of the Fermi surface curvature for electron-doped materials. The $d_{x^{2}-y^{2}}$-pairing gap $Δ(\mathbf{k},ω)$ is strongly non-monotonic along the Fermi surface. The position of the gap maxima, however, does not coincide with the hot spots, as the non-monotonic $d_{x^{2}-y^{2}}$ gap persists even at doping when the hot spots merge on the Brillouin zone diagonals.

cond-mat.supr-con↗

arXiv2006

Nonanalytic corrections to the specific heat and susceptibility of a non-Galilean-Invariant Two-Dimensional Fermi Liquid

We consider the leading non-analytic temperature dependence of the specific heat and temperature and momentum dependence of the spin susceptibility for two dimensional fermionic systems with non-circular Fermi surfaces. We demonstrate the crucial role played by Fermi surface curvature. For a Fermi surface with inflection points, we demonstrate that thermal corrections to the uniform susceptibility in D=2 change from $χ_{s} \propto T$ to $χ_{s} \propto T^{2/3}$ for generic inflection points, and to $χ_{s} \propto T^{1/2}$ for special inflection points along symmetry directions. Errors in previous work are corrected. Application of the results to $Sr_2RuO_4$ is given.

cond-mat.str-el↗

arXiv2005

Non-Fermi liquid and pairing in electron-doped cuprates

We study the normal state and pairing instability in electron-doped cuprates near optimal doping. We show that the fermionic self-energy has a non-Fermi liquid form leading to peculiar frequency dependencies of the conductivity and the Raman response. We solve the pairing problem and demonstrate that T_c is determined by the curvature of the Fermi surface, and the pairing gap Δ(k,ω) is strongly non-monotonic along the Fermi surface. The normal state frequency dependencies, the value of T_c\sim10K and the k-dependence of the gap agree with the experiment.

cond-mat.supr-con↗

arXiv2005

Condensation energy in Eliashberg theory -- from weak to strong coupling

We consider two issues related to the condensation energy in superconductors described by the Eliashberg theory for various forms of the pairing interaction, associated either with phonon or electronic mechanisms of superconductivity. First, we derive a leading correction to the BCS formula for the condensation energy to first order in the coupling $λ$. Second, we show that at a given $λ$, the value of the condensation energy strongly depends on the functional form of the effective pairing interaction $Γ(ω)$.

cond-mat.supr-con↗

arXiv2005

Strong coupling superconductivity due to massless boson exchange

We solve the problem of fermionic pairing mediated by a massless boson in the limit of large coupling constant. At weak coupling, the transition temperature is exponentially small and superconductivity is robust against phase fluctuation. In the strong coupling limit, the pair formation occurs at a temperature of the order of the Fermi energy, however, the actual transition temperature is much smaller due to phase and amplitude fluctuations of the pairing gap. Our model calculations describe superconductivity due to color magnetic interactions in quark matter and in systems close to a ferromagnetic quantum critical point with Ising symmetry. Our strong coupling results are, however, more general and can be applied to other systems as well, including the antiferromagnetic exchange in 2D used for description of the cuprates.

cond-mat.str-el↗

arXiv2005

Thermodynamics of a Fermi liquid beyond the low-energy limit

We consider the non-analytic temperature dependences of the specific heat coefficient, C(T)/T, and spin susceptibility, χ_{s} (T), of 2D interacting fermions beyond the weak-coupling limit. We demonstrate within the Luttinger-Ward formalism that the leading temperature dependences of C(T)/T and χ_s (T) are linear in T, and are described by the Fermi liquid theory. We show that these temperature dependences are universally determined by the states near the Fermi level and, for a generic interaction, are expressed via the spin and charge components of the exact backscattering amplitude of quasi-particles. We compare our theory to recent experiments on monolayers of 3He.

cond-mat.str-el↗

arXiv2005

Self-generated locality near a ferromagnetic quantum-critical point

We analyze the behavior of interacting fermions near a ferromagnetic Stoner instability. We show that the Landau damping of the spin susceptibility is a relevant perturbation near a ferromagnetic quantum-critical point (FQCP). We argue that, as the system approaches a FQCP, the fermionic self-energy crosses over from predominantly momentum dependent away from the transition to predominantly frequency dependent in the immediate vicinity of the transition. We argue that due to this self-generated locality, the quasiparticle effective mass does not diverge before a FQCP is reached.

cond-mat.str-el↗

arXiv2005

Quantum critical behavior near a density-wave instability in an isotropic Fermi liquid

We study the quantum critical behavior in an isotropic Fermi liquid in the vicinity of a zero-temperature density-wave transition at a finite wave vector q_c. We show that, near the transition, the Landau damping of the soft bosonic mode yields a crossover in the fermionic self-energy from Sigma(k,omega) ~ Sigma(k) to Sigma(k,omega) ~ Sigma(omega), where k and omega are momentum and frequency. Because of this self-generated locality, the fermionic effective mass diverges right at the quantum critical point, not before, i.e., the Fermi liquid survives up to the critical point.

cond-mat.str-el↗

arXiv2004

Ward identities for strongly coupled Eliashberg theories

We discuss Ward identities for strongly interacting fermion systems described by Eliashberg-type theories. We show that Ward identities are not in conflict with Migdal theorem. We derive diagrammatically Ward identity for a charge vertex in a Fermi liquid, and when a Fermi liquid is destroyed at quantum criticality. We argue that Ward identity for a spin vertex cannot be obtained within Eliashberg theory.

cond-mat.str-el↗

arXiv2004

Novel neutron resonance mode in $d_{x^2-y^2}$ superconductors

We show that a new resonant magnetic excitation at incommensurate momenta, observed recently by inelastic neutron scattering experiments on YBa$_2$Cu$_3$O$_{6.85}$ and YBa$_2$Cu$_3$O$_{6.6}$, is a {\it spin exciton}. Its location in the magnetic Brillouin zone and its frequency are determined by the momentum dependence of the particle-hole continuum. We identify several features that distinguish this novel mode from the previous resonance mode observed near ${\bf Q}=(π,π)$, such as its intensity maximum which occurs in a different part of the magnetic Brillouin zone.

cond-mat.supr-con↗

arXiv2004

The spin resonance and high frequency optical properties of the cuprates

We argue that recently observed superconductivity-induced blue shift of the plasma frequency $δω_{pl}$ in $Bi_2Sr_2CaCu_2O_{8+δ}$ is related to the change in the integrated dynamical structure factor associated with the development of the spin resonance below $T_c$. We show that the magnitude of $δω_{pl}$ is consistent with the small integrated spectral weight of the resonance, and its temperature dependences closely follow that of the spin resonance peak.

cond-mat.supr-con↗

arXiv2003

Quantum-critical behavior of itinerant ferromagnets

We study the stability of the Quantum Critical Point (QCP) for itinerant ferromagnets commonly described by the Hertz-Millis-Moriya (HMM) theory. We argue that in $D \leq 3$, long-range spatial correlations associated with the Landau damping of the order parameter field generate a universal {\it negative}, non-analytic $|q|^{(D+1)/2}$ contribution to the static magnetic susceptibility $χ_s (q, 0)$, which makes HMM theory unstable. We argue that the actual transition is either towards incommensurate ordering, or first order. We also show that singular corrections are specific to the spin problem, while charge susceptibility remains analytic at criticality.

cond-mat.str-el↗

arXiv2003

The differential sum rule for the relaxation rate in dirty superconductors

We consider the differential sum rule for the effective scattering rate $% 1/τ(ω)$ and optical conductivity $σ_{1}(ω) $ in a dirty BCS superconductor, for arbitrary ratio of the superconducting gap $% Δ$ and the normal state constant damping rate $1/τ$. We show that if $τ$ is independent of $T$, the area under $1/τ(ω)$ does not change between the normal and the superconducting states, i.e., there exists an exact differential sum rule for the scattering rate. For \textit{any} value of the dimensionless parameter $Δτ$, the sum rule is exhausted at frequencies controlled by $Δ$. %but the numerical convergence is weak. We show that in the dirty limit the convergence of the differential sum rule for the scattering rate is much faster then the convergence of the $f-$sum rule, but slower then the convergence of the differential sum rule for conductivity.

cond-mat.supr-con↗

arXiv2002

Condensation energy in strongly coupled superconductors

We consider the condensation energy in superconductors where the pairing is electronic in origin and is mediated by a collective bosonic mode. We use magnetically-mediated superconductivity as an example, and show that for large spin-fermion couplings, the physics is qualitatively different from the BCS theory as the condensation energy results from the feedback on spin excitations, while the electronic contribution to the condensation energy is positive due to an ``undressing'' feedback on the fermions. The same feedback effect accounts for the gain of the kinetic energy at strong couplings.

cond-mat.str-el↗

arXiv2002

A novel spin wave expansion, finite temperature corrections and order from disorder effects in the double exchange model

The magnetic excitations of the double exchange (DE) model are usually discussed in terms of an equivalent ferromagnetic Heisenberg model. We argue that this equivalence is valid only at a quasi--classical level -- both quantum and thermal corrections to the magnetic properties of DE model differ from any effective Heisenberg model because its spin excitations interact only indirectly, through the exchange of charge fluctuations. To demonstrate this, we perform a novel large S expansion for the coupled spin and charge degrees of freedom of the DE model, aimed at projecting out all electrons not locally aligned with core spins. We generalized the Holstein--Primakoff transformation to the case when the length of the spin is by itself an operator, and explicitly constructed new fermionic and bosonic operators to fourth order in 1/\sqrt{S}. This procedure removes all spin variables from the Hund coupling term, and yields an effective Hamiltonian with an overall scale of electron hopping, for which we evaluate corrections to the magnetic and electronic properties in 1/S expansion to order O(1/S^2). We also consider the effect of a direct superexchange antiferromagnetic interaction between core spins. We find that the competition between ferromagnetic double exchange and an antiferromagnetic superexchange provides a new example of an "order from disorder" phenomenon -- when the two interactions are of comparable strength, an intermediate spin configuration (either a canted or a spiral state) is selected by quantum and/or thermal fluctuations.

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