arXiv subjects

A. Knigavko

Publications and source records attributed to A. Knigavko.

11 recordsLinked to original sources

arXiv2014

Small-Angle Neutron Scattering and Magnetization Study of HoNi2B2C

The superconducting and magnetic properties of HoNi2B2C single crystals are investigated through transport, magnetometry and small-angle neutron scattering measurements. In the magnetic phases that enter below the superconducting critical temperature, the small-angle neutron scattering data uncover networks of magnetic surfaces. These likely originate from uncompensated moments e.g. at domain walls pinned to crystallographic grain boundaries. The field and temperature dependent behaviour appears consistent with the metamagnetic transitions reported in earlier works.

cond-mat.supr-con↗

arXiv2007

Divergence of the orbital nuclear magnetic relaxation rate in metals

We analyze the nuclear magnetic relaxation rate $(1/T_1)_{orb}$ due to the coupling of nuclear spin to the orbital moment of itinerant electrons in metals. In the clean non--interacting case, contributions from large--distance current fluctuations add up to cause a divergence of $(1/T_1)_{orb}$. When impurity scattering is present, the elastic mean free time $τ$ cuts off the divergence, and the magnitude of the effect at low temperatures is controlled by the parameter $\ln(μτ)$, where $μ$ is the chemical potential. The spin--dipolar hyperfine coupling, while has the same spatial variation $1/r^3$ as the orbital hyperfine coupling, does not produce a divergence in the nuclear magnetic relaxation rate.

cond-mat.supr-con↗

arXiv2006

High energy scales in electronic self-energy imaged by optics

We use a new technique to directly extract an estimate of the quasiparticle self-energy from the optical conductivity which can be easily related to both theory and angle-resolved photoemission spectroscopy (ARPES) experiments. In the high Tc cuprate Bi-2212 we find evidence for a new high energy scale at 900 meV in addition to the two previously well known ones at roughly 50 and 400 meV. The intermediate scale at 400 meV has been recently seen in ARPES as a large kink which optics finds to be weaker and shifted. In YBCO, the three energy scales are shifted to lower energy relative to Bi-2212 and we observe the emergence of a possible fourth high energy feature at 600 meV.

cond-mat.supr-con↗

arXiv2006

Observation of phonon structure in electron density of states of normal metal

Within standard treatments of the interacting electron--phonon system the electron density of states (EDOS) shows no sign of the phonons when the system is in the normal state. On the other hand, emergence of a fine energy scale in the superconducting state makes the phonons readily observable in the EDOS and other spectroscopic quantities. Here we wish to re-examine the conditions under which phonon structure might be present in the normal state. Accounting for a finite, i.e. non-infinite, electronic bandwidth results in easily distinguishable phonon structure in the EDOS. We argue that for narrow band metals, the resolution of photoemission spectroscopy is sufficient for resolving these structures. Manifestations of finite band effects in $K_3C_{60}$ are discussed.

cond-mat.mtrl-sci↗

arXiv2004

Elastic anomalies in HoNi2B2C single crystals

We have measured temperature and magnetic field dependencies of the sound velocities and the sound attenuation in HoNi2B2C single crystals. The main result is a huge softening the velocity of C66 mode due to a cooperative Jahn-Teller effect, resulting in a tetragonal-orthorhombic structural phase transition. Anomalies in the behavior of the C66 mode through various magnetic phase transitions permit us to revise the low temperature H-T phase diagrams of this compound.

cond-mat.supr-con↗

arXiv2004

Optical sum increase due to electron undressing

For a system with a fixed number of electrons, the total optical sum is a constant, independent of many-body interactions, of impurity scattering and of temperature. For a single band in a metal, such a sum rule is no longer independent of the interactions or temperature, when the dispersion and/or finite bandwidth is accounted for. We adopt such a model, with electrons coupled to a single Einstein oscillator of frequency $ω_{E}$, and study the optical spectral weight. The optical sum depends on both the strength of the coupling and on the characteristic phonon frequency, $ω_{E}$. A hardening of $ω_{E}$, due, for example, to a phase transition, leads to electron undressing and translates into a decrease in the electron kinetic energy and an increase in the total optical sum, as observed in recent experiments in the cuprate superconductors.

cond-mat.supr-con↗

arXiv2003

Inversion of ARPES measurements in high Tc cuprates

Recent energy dispersion measurements in several families of the hole-doped copper oxides have revealed a kink in the energy vs. momentum relation. These have tentatively been identified as due to electron phonon coupling. We invert this data directly to determine the bosonic spectral function; the kink gives rise to a singular function in the phonon energy region.

cond-mat.supr-con↗

arXiv2001

Constraints from $T_c$ and the isotope effect for MgB$_2$

With the constraint that $T_c = 39$ K, as observed for MgB$_2$, we use the Eliashberg equations to compute possible allowed values of the isotope coefficient, $β$. We find that while the observed value $β= 0.32$ can be obtained in principle, it is difficult to reconcile a recently calculated spectral function with such a low observed value.

cond-mat.supr-con↗

arXiv2000

Nature of 45 degree vortex lattice reorientation in tetragonal superconductors

The transformation of the vortex lattice in a tetragonal superconductor which consists of its 45 degree reorientation relative to the crystal axes is studied using the nonlocal London model. It is shown that the reorientation occurs as two successive second order (continuous) phase transitions. The transition magnetic fields are calculated for a range of parameters relevant for borocarbide superconductors in which the reorientation has been observed.

cond-mat.supr-con↗

arXiv1999

Magnetic skyrmions and their lattices in triplet superconductors

Complete topological classification of solutions in SO(3) symmetric Ginzburg-Landau free energy has been performed and a new class of solutions in weak external magnetic field carrying two units of magnetic flux has been identified. These solutions, magnetic skyrmions, do not have singular core like Abrikosov vortices and at low magnetic field become lighter for strongly type II superconductors. As a consequence, the lower critical magnetic field Hc1 is reduced by a factor of log(kappa). Magnetic skyrmions repel each other as 1/r at distances much larger then magnetic penetration depth forming relatively robust triangular lattice. Magnetic induction near Hc1 increases gradually as (H-Hc1)^2. This agrees very well with experiments on heavy fermion superconductor UPt3. Newly discovered Ru based compounds Sr2RuO4 and Sr2YRu(1-x)Cu(x)O6 are other possible candidates to possess skyrmion lattices. Deviations from exact SO(3) symmetry are also studied.

cond-mat.supr-con↗

arXiv1999

Magnetic skyrmion lattices in heavy fermion superconductor UPt3

Topological analysis of nearly SO(3)_{spin} symmetric Ginzburg--Landau theory, proposed for UPt$_{3}$ by Machida et al, shows that there exists a new class of solutions carrying two units of magnetic flux: the magnetic skyrmion. These solutions do not have singular core like Abrikosov vortices and at low magnetic fields they become lighter for strongly type II superconductors. Magnetic skyrmions repel each other as $1/r$ at distances much larger then the magnetic penetration depth $λ$, forming a relatively robust triangular lattice. The magnetic induction near $H_{c1}$ is found to increase as $(H-H_{c1})^{2}$. This behavior agrees well with experiments.

cond-mat.supr-con↗