arXiv subjects

Ar. Abanov

Publications and source records attributed to Ar. Abanov.

48 records · Page 3Linked to original sources

arXiv2001

Singularities in the optical response of cuprates

We argue that the detailed analysis of the optical response in cuprate superconductors allows one to verify the magnetic scenario of superconductivity in cuprates, as for strong coupling charge carriers to antiferromagnetic spin fluctuations, the second derivative of optical conductivity should contain detectable singularities at $2Δ+Δ_{\rm spin}$, $4Δ$, and $2Δ+2Δ_{\rm spin}$, where $Δ$ is the amplitude of the superconducting gap, and $Δ_{s}$ is the resonance energy of spin fluctuations measured in neutron scattering. We argue that there is a good chance that these singularities have already been detected in the experiments on optimally doped $YBCO$.

cond-mat.supr-con↗

arXiv2001

Quantum-critical superconductivity in underdoped cuprates

We argue that the pseudogap phase may be an attribute of the non-BCS pairing of quantum-critical, diffusive fermions near the antiferromagnetic quantum critical point. We derive and solve a set of three coupled Eliashberg-type equations for spin-mediated pairing and show that in some $T$ range below the pairing instability, there is no feedback from superconductivity on fermionic excitations, and fermions remain diffusive despite of the pairing. We conject that in this regime, fluctuations of the pairing gap destroy the superconducting condensate but preserve the leading edge gap in the fermionic spectral function.

cond-mat.supr-con↗

arXiv2000

Fingerprints of spin-fermion pairing in cuprates

We demonstrate that the feedback effect from bosonic excitations on fermions, which in the past allowed one to verify the phononic mechanism of a conventional, $s-$wave superconductivity, may also allow one to experimentally detect the ``fingerprints'' of the pairing mechanism in cuprates. We argue that for spin-mediated $d-$wave superconductivity, the fermionic spectral function, the density of states, the tunneling conductance through an insulating junction, and the optical conductivity are affected by the interaction with collective spin excitations, which below $T_c$ are propagating, magnon-like quasiparticles with gap $Δ_s$. We show that the interaction with a propagating spin excitation gives rise to singularities at frequencies $Δ+ Δ_s$ for the spectral function and the density of states, and at $2Δ+ Δ_s$ for tunneling and optical conductivities, where $Δ$ is the maximum value of the $d-$wave gap. We further argue that recent optical measurements also allow one to detect subleading singularities at $4Δ$ and $2Δ+ 2Δ_s$. We consider the experimental detection of these singularities as a strong evidence in favor of the magnetic scenario for superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2000

Spectral function and conductivity in the normal state of the cuprates: a spin fluctuation study

We study the spectral function $A_k (ω)$ and the optical conductivity $σ_1 (ω)$ for a system of fermions interacting with their collective spin fluctuations (the spin-fermion model), and apply the results to optimally doped cuprates in the normal state. We show that there is no qualitative distinction between hot and cold regions in the Brillouin zone - in both cases, there exists a wide range of frequencies where the width of the peak in $A_k (ω)$ scales linearly with $ω$. We demonstrate that $σ_1 (ω)$ is inversely linear in $ω$ up to very high frequencies. We argue that these results agree {\it quantitatively} with the photoemission and optical data for $Bi2212$.

cond-mat.supr-con↗

arXiv2000

Spin-fermion model near the quantum critical point: one-loop renormalization group results

We consider spin and electronic properties of itinerant electron systems, described by the spin-fermion model, near the antiferromagnetic critical point. We expand in the inverse number of hot spots in the Brillouin zone, N and present the results beyond previously studied $N = \infty $ limit. We found two new effects: (i) Fermi surface becomes nested at hot spots, and (ii) vertex corrections give rise to anomalous spin dynamics and change the dynamical critical exponent from z=2 to z>2. To first order in 1/N we found $z = 2N/(N-2)$ which for a physical N=8 yields $z\approx 2.67$.

cond-mat.supr-con↗

arXiv2000

SIN and SIS tunneling in cuprates

We calculate the SIN and SIS tunneling conductances for the spin-fermion model. We argue that at strong spin-fermion coupling, relevant to cuprates, both conductances have dip features near the threshold frequencies when a tunneling electron begin emitting propagating spin excitation. We argue that the resonance spin frequency measured in neutron scattering can be inferred from the tunneling data by analyzing the derivatives of SIN and SIS conductances.

cond-mat.supr-con↗

arXiv1999

On the relative positions of the $2Δ$ peaks in Raman and tunneling spectra of d-wave superconductors

We study $B_{1g}$ Raman intensity $R(Ω)$ and the density of states $N(ω)$ in isotropic 2D d-wave superconductors. For an ideal gas, $R(Ω)$ and $N(ω)$ have sharp peaks at $Ω=2Δ$ and $ω=Δ$, respectively, where $Δ$ is the maximum value of the gap. We study how the peak positions are affected by the fermionic damping due to impurity scattering. We show that while the damping generally shifts the peak positions to larger frequencies, the peak in $R(Ω)$ still occurs at almost twice the peak position in $N(ω)$ and therefore cannot account for the experimentally observed downturn shift of the peak frequency in $R(Ω)$ in underdoped cuprates compared to twice that in $N(ω)$. We also discuss how the fermionic damping affects the dynamical spin susceptibility.

cond-mat.supr-con↗

arXiv1999

Condensation energy in the spin-fermion model for cuprates

We use the Scalapino-White relation between the condensation energy and the difference between the dynamical structure factor in the normal and the superconducting states to compute the condensation energy in the spin-fermion model. We show that at strong coupling, the extra low-frequency spectral weight associated with the resonance peak in the dynamical structure factor in a superconductor is compensated only at energies $\sim J$ which are much larger than the superconducting gap $Δ$. We argue that in this situation, the condensation energy is large and well accounts for the data for cuprates.

cond-mat.supr-con↗

arXiv1999

A relation between the resonance neutron peak and ARPES data in cuprates

We argue that the resonant peak observed in neutron scattering experiments on superconducting cuprates and the peak/dip/hump features observed in ARPES measurements are byproducts of the same physical phenomenon. We argue that both are due to feedback effects on the damping of spin fluctuations in a $d-$wave superconductor. We consider the spin-fermion model at strong coupling, solve a set of coupled integral equations for fermionic and bosonic propagators and show that the dynamical spin susceptibility below $T_c$ possesses the resonance peak at $Ω_{res} \propto ξ^{-1}$. The scattering of these magnetic excitations by electrons gives rise to a peak/dip/hump behavior of the electronic spectral function, the peak-dip separation is exactly $Ω_{res}$.

cond-mat.supr-con↗

arXiv1998

Skyrmion in a real magnetic film

Skyrmions are the magnetic defects in an ultrathin magnetic film, similar to the bubble domains in the thicker films. Even weak uniaxial anisotropy determines its radius unambiguously. We consider the dynamics of the skyrmion decay. We show that the discreteness of the lattice in an isotropic 2D magnet leads to a slow rotation of the local magnetization in the skyrmion and, provided a small dissipation, to decay of the skyrmion. The radius of such a skyrmion as a function of time is calculated. We prove that uniaxial anisotropy stabilizes the skyrmion and study the relaxation process.

cond-mat↗

arXiv1996

Long Wavelength Anomalous Diffusion Mode in the 2D XY Dipole Magnet

In 2D XY ferromagnet the dipole force induces a strong interaction between spin-waves in the long-wavelength limit. The major effect of this interaction is the transformation of a propagating spin-wave into a diffusion mode. We study the anomalous dynamics of such diffusion modes. We find that the Janssen-De Dominics functional, which governs this dynamics, approaches the non-Gaussian fixed-point. A spin-wave propagates by an anomalous anisotropic diffusion with the dispersion relation: $iω{\sim}k_{y}^{Δ_y}$ and $iω{\sim}k_{x}^{Δ_x}$, where ${Δ_y}=47/27$ and ${Δ_x}=47/36$. The low-frequency response to the external magnetic field is found.

cond-mat↗

arXiv1996

Spin Diffusion in 2D XY Ferromagnet with Dipolar Interaction

In the ordered phase of 2D XY ferromagnet the dipolar interaction between spins induces a strong, relevant interaction between spin-waves. We study quasi-excitations of the interacting spin-wave 'liquid' in the long wavelength limit. We employ the Janssen-De-Dominicis method for classical Langevin equation to find the transformation of the spin-wave excitation into a new soft-mode excitation in the intermediate range of wavelengths; and into an anomalous anisotropic diffusion mode excitation at long wavelengths. The dissipation of a spin-wave at short wavelengths is found to be highly anisotropic.

cond-mat↗