arXiv subjects

Joerg Schmalian

Publications and source records attributed to Joerg Schmalian.

63 records · Page 4Linked to original sources

arXiv2000

Andreev interferometry as a probe of superconducting phase correlations in the pseudogap regime of the cuprates

Andreev interferometry - the sensitivity of the tunneling current to spatial variations in the local superconducting order at an interface - is proposed as a probe of the spatial structure of the phase correlations in the pseudogap state of the cuprate superconductors. To demonstrate this idea theoretically, a simple tunneling model is considered, via which the tunneling current is related to the equilibrium phase-phase correlator in the pseudogap state. These considerations suggest that measurement of the low-voltage conductance through mesoscopic contacts of varying areas provides a scheme for accessing phase-phase correlation information. For illustrative purposes, quantitative predictions are made for a model of the pseudogap state in which the phase (but not the amplitude) of the superconducting order varies randomly, and does so with correlations consistent with certain proposed pictures of the pseudogap state.

cond-mat.supr-con↗

arXiv1999

Magnetic Roughness and Domain Correlations in Antiferromagnetically Coupled Multilayers

The in-plane correlation lengths and magnetic disorder of magnetic domains in a transition metal multilayer have been studied using neutron scattering techniques. A new theoretical framework is presented connecting the observed scattering to the in-plane correlation length and the dispersion of the local magnetization vector about the mean macroscopic direction. The results unambiguously show the highly correlated nature of the antiferromagnetically coupled domain structure vertically throughout the multilayer. We are easily able to relate the neutron determined magnetic dispersion and domain correlations to magnetization and magnetotransport experiments.

cond-mat.mtrl-sci↗

arXiv1998

Quantum critical point with competing propagating and diffusive spin excitations

Feedback effects due to spin fluctuation induced precursors in the fermionic quasiparticle spectrum are taken into account in the description of a quantum critical point of itinerant spin systems. A correlation length dependent spin damping occurs, leading to a dynamical scaling with z\approx 1 which non-trivially competes with the conventional spin wave behavior. We obtain, within a one loop renormalization group approach, a quantitative explanation for the scaling behavior seen in underdoped cuprate superconductors.

cond-mat.str-el↗

arXiv1998

Pairing due to Spin Fluctuations in Layered Organic Superconductors

I show that for a κ-type organic (BEDT-TTF)_2-X molecular crystal, a superconducting state with T_c ~ 10 K and gap nodes on the Fermi surface can be caused by short-ranged antiferromagnetic spin fluctuations. Using a two-band description for the anti-bonding orbitals on a BEDT-TTF dimer of the κ-type salt, and an intermediate local Coulomb repulsion between two holes on one dimer, the magnetic interaction and the superconducting gap-function are determined self consistently within the fluctuation exchange approximation. The pairing interaction is predominantly caused by inter-band coupling and additionally affected by spin excitations of the quasi one-dimensional band.

cond-mat.str-el↗

arXiv1998

Microscopic theory of weak pseudogap behavior in the underdoped cuprate superconductors I: General theory and quasiparticle properties

We derive in detail a novel solution of the spin fermion model which is valid in the quasi-static limit pi T<<omega_sf, found in the intermediate (pseudoscaling) regime of the magnetic phase diagram of cuprate superconductors, and use it to obtain results for the temperature and doping dependence of the single particle spectral density, the electron-spin fluctuation vertex function, and the low frequency dynamical spin susceptibility. The resulting strong anisotropy of the spectral density and the vertex function lead to the qualitatively different behavior of_hot_ (around k=(pi,0)) and_cold_ (around k=(pi/2,pi/2)) quasiparticles seen in ARPES experiments. We find that the broad high energy features found in ARPES measurements of the spectral density of the underdoped cuprate superconductors are determined by strong antiferromagnetic (AF) correlations and incoherent precursor effects of an SDW state, with reduced renormalized effective coupling constant. The electron spin-fluctuation vertex function, i.e. the effective interaction of low energy quasiparticles and spin degrees of freedom, is found to be strongly anisotropic and enhanced for hot quasiparticles; the corresponding charge-fluctuation vertex is considerably diminished. We thus demonstrate that, once established, strong AF correlations act to reduce substantially the effective electron-phonon coupling constant in cuprate superconductors.

cond-mat↗

arXiv1998

Probing the susceptibility $χ'(q)$ in cuprates using Ni impurities

Recent NMR experiments on $YBa_2(Cu_{1-x}Ni_x)_3O_{6+δ}$ (Bobroff et al. Phys. Rev. Lett. 79, 2117 (1997)) have shown that Ni impurities provide an important probe for the momentum dependence of the static spin susceptibility $χ'(q)$. Combining the measurements of the $^{17}$O line width $Δν$ with those of the Gaussian relaxation time $T_{2G}$, we find that the magnetic correlation length $ξ$ must have a substantial temperature dependence. Using numerical simulations we present a detailed analysis of $Δν$ as a function of temperature and impurity concentration. For a Lorentzian shape of $χ'(q)$ $Δν$ strongly depends on $ξ$, in contrast to the Gaussian form. This result together with the experimental finding that $ξ$ is temperature dependent rules out the Gaussian form of $χ'(q)$.

cond-mat.str-el↗

arXiv1997

Temperature variation of the pseudogap in underdoped cuprates

We consider thermal evolution of the spectral function A(Omega, T) in underdoped cuprates at T >Tc. We find that in the strong coupling limit, the fermionic Green's function near (0,π) behaves as G^{-1} (Omega) ~ \sqrt{Omega} in the whole frequency range relevant to photoemission experiments. The analysis of the pairing problem with this form of G yields a leading edge gap in A(Ω) and a broad maximum at larger frequencies. We find that classical fluctuations predominantly add a frequency independent term to G^{-1} (Omega). This term destroys the leading edge gap at T =T* (k)$which has a maximum value of about 200K near (0,π).

cond-mat.str-el↗

arXiv1997

Comment on: ``Using Ni Substitution and $^{17}$O NMR to Probe the Susceptibility $χ'(q)$ in Cuprates''

In a recent letter (PRL 79, 2117 (1997)), Bobroff et al. presented novel $^{17}$O NMR measurements for YBa_2(Cu_{1-x}Ni_x)_3O_{6+y} which offer a new probe of the momentum dependence of the static spin susceptibility $χ'(q)$. They analyzed the $^{17}$O line width by assuming a Gaussian form for $χ'$, and came to the conclusion that the correlation length $ξ$ is independent of temperature $T$. Furthermore, they claim that a Lorentzian form of $χ'$ yields the same result. We will demonstrate that the Lorentzian and Gaussian form of $χ'$ actually yield qualitatively different results for the $^{17}$O line width and its dependence on $ξ$. We show that their data when combined with $T_{2G}$ data from Takigawa require that $ξ$ be T-dependent. Furthermore, assuming a Gaussian form of $χ'$ leads to the unphysical result that $ξ$ increases with $T$. In contrast, the Lorentzian form of $χ'$ yields that $ξ$ decreases with increasing $T$ and is thus fully compatible with the experimental results.

cond-mat.str-el↗

arXiv1997

Weak Pseudogap Behavior in the Underdoped Cuprate Superconductors

We report on an exact solution of the nearly antiferromagnetic Fermi liquid spin fermion model in the limit πT << ω_{sf}, which demonstrates that the broad high energy features found in ARPES measurements of the spectral density of the underdoped cuprate superconductors are determined by strong antiferromagnetic (AF) correlations and precursor effects of an SDW state. We show that the onset temperature, T^{cr}, of weak pseudo-gap (pseudoscaling) behavior is determined by the strength, ξ, of the AF correlations, and obtain the generic changes in low frequency magnetic behavior seen in NMR experiments with ξ(T^{cr}) \approx 2, confirming the Barzykin and Pines crossover criterion.

cond-mat.supr-con↗