arXiv subjects

P. D. Grigoriev

Publications and source records attributed to P. D. Grigoriev.

At least 37 records · Page 2Linked to original sources

arXiv2018

Inelastic neutron scattering as a confirmation of a new type of gapped surface excitations in liquid helium

We analyze the experimental data on inelastic neutron scattering by a thin ~5-atomic-layer film of liquid helium at three different temperatures: T=0.4K, 0.98K and 1.3K. These data were partially published previously, but here we present them in a better quality and at various temperatures. The neutron scattering intensity plots, in addition to the previously know dispersion of phonons and ripplons, suggest a branch of gapped surface excitations with activation energy $\sim 4.5$K and the dispersion similar to that expected for surfons - the bound quantum states of helium atoms above liquid helium surface, proposed and investigated theoretically. These data, probably, provide the first direct experimental confirmation of surfons. Before these surface excitations received only indirect experimental substantiation, based on the temperature dependence of surface tension coefficient and on their interaction with surface electrons. The existence of surfons as an additional type of surface excitations, although being debated yet, is very important for various physical properties of He surface. We also analyze previous numerical results on excitations in liquid helium and argue that surface excitations similar to surfons have been previously obtained by numerical calculations and called resonance interface states.

cond-mat.other↗

arXiv2018

Conductivity of anisotropic inhomogeneous superconductors above critical temperature

We propose a model and derive analytical expressions for conductivity in heterogeneous fully anisotropic conductors with ellipsoid superconducting inclusions. This model and calculations are useful to analyze the observed temperature dependence of conductivity anisotropy in various anisotropic superconductors, where superconductivity onset happens inhomogeneously in the form of isolated superconducting islands. The results are applied to explain the experimental data on resistivity above the transition temperature $T_c$ in the high-temperature superconductor $\mathrm{YBa_2Cu_4O_8}$ and in the organic superconductor $β$-(BEDT-TTF)$_{2}$I$_{3}$. The comparison of resistivity data and diamagnetic response in $β$-(BEDT-TTF)$_{2}$I$_{3}$ allows us to estimate the size of superconducting inclusions as $d\sim 1μm$.

cond-mat.supr-con↗

arXiv2016

Gossamer bulk high-temperature superconductivity in FeSe

The cuprates and iron-based high-temperature superconductors share many common features: layered strongly anisotropic crystal structure, strong electronic correlations, interplay between different types of electronic ordering, the intrinsic spatial inhomogeneity due to doping. The understanding of complex interplay between these factors is crucial for a directed search of new high-temperature superconductors. Here we show the appearance of inhomogeneous gossamer superconductivity in bulk FeSe compound at ambient pressure and at temperature 5 times higher than its zero-resistance $T_c$. This discovery helps to understand numerous remarkable superconducting properties of FeSe. We also find and prove a general property: if inhomogeneous superconductivity in a anisotropic conductor first appears in the form of isolated superconducting islands, it reduces electric resistivity anisotropically with maximal effect along the least conducting axis. This gives a simple and very general tool to detect inhomogeneous superconductivity in anisotropic compounds, which is critically important to study the onset of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2015

Slow in-plane magnetoresistance oscillations in multiband quasi-two-dimensional metals

Slow oscillations (SlO) of magnetoresistance is a convenient tool to measure electronic structure parameters in quasi-two-dimensional metals. We study the possibility to apply this method to multi-band conductors, e.g. to iron-based high-temperature superconducting materials. We show that SlO can be used to measure the interlayer transfer integral in multi-band conductors similar to single-band metals. In addition, the SlO allow to measure and compare the effective masses or the electron scattering rates in various bands.

cond-mat.str-el↗

arXiv2015

Neutrons on a surface of liquid helium

We investigate the possibility of ultracold neutron (UCN) storage in quantum states defined by the combined potentials of the Earth's gravity and the neutron optical repulsion by a horizontal surface of liquid helium. We analyse the stability of the lowest quantum state, which is most susceptible to perturbations due to surface excitations, against scattering by helium atoms in the vapor and by excitations of the liquid, comprised of ripplons, phonons and surfons. This is an unusual scattering problem since the kinetic energy of the neutron parallel to the surface may be much greater than the binding energies perpendicular. The total scattering time constant of these UCNs at 0.7 K is found to exceed one hour, and rapidly increasing with decreasing temperature. Such low scattering rates should enable high-precision measurements of the scheme of discrete energy levels, thus providing improved access to short-range gravity. The system might also be useful for neutron beta-decay experiments. We also sketch new experimental concepts for level population and trapping of UCNs above a flat horizontal mirror.

nucl-ex↗

arXiv2015

Slow oscillations of in-plane magnetoresistance in strongly anisotropic quasi-two-dimensional rare-earth tritellurides

Slow oscillations of the in-plane magnetoresistance are observed in the rare-earth tritellurides and proposed as an effective tool to determine the parameters of electronic structure in various strongly anisotropic quasi-two-dimensional compounds. These oscillations do not originate from the small Fermi surface pockets, as revealed usually by the Shubnikov-de-Haas oscillations, but from the entanglement of close frequencies due to a finite interlayer transfer integral $t_z$, which allows to estimate its value. For TbTe$_3$ and GdTe$_3$ we obtain the estimate $t_z\approx 1$ meV.

cond-mat.str-el↗

arXiv2014

Conductivity anisotropy helps to reveal the microscopic structure of a density wave at imperfect nesting

Superconductivity or metallic state may coexist with density wave ordering at imperfect nesting of the Fermi surface. In addition to the macroscopic spatial phase separation, there are, at least, two possible microscopic structures of such coexistence: (i) the soliton-wall phase and (ii) the ungapped Fermi-surface pockets. We show that the conductivity anisotropy allows to distinguish between these two microscopic density-wave structures. The results obtained may help to analyze the experimental observations in layered organic metals (TMTSF)$_{2}$PF$_{6}$, (TMTSF)$_{2}$ClO$_{4}$, $α$-(BEDT-TTF)$_{2}$KHg(SCN)$_{4}$ and in other compounds.

cond-mat.str-el↗

arXiv2013

Magnetic quantum oscillations in the charge-density-wave state of the organic metals $α$-(BEDT-TTF)$_2$MHg(SCN)$_4$ with M = K and Tl

The low-temperature charge-density-wave (CDW) state in the layered organic metals $α$-(BEDT-TTF)$_2$MHg(SCN)$_4$ has been studied by means of the Shubnikov -- de Haas and de Haas -- van Alphen effects. In addition to the dominant alpha-frequency, which is also observed in the normal state, both the magnetoresistance and magnetic torque possess a slowly oscillating component. These slow oscillations provide a firm evidence for the CDW-induced reconstruction of the original cylindrical Fermi surface. The alpha-oscillations of the interlayer magnetoresistance exhibit an anomalous phase inversion in the CDW state, whereas the de Haas -- van Alphen signal maintains the normal phase. We argue that the anomaly may be attributed to the magnetic-breakdown origin of the alpha-oscillations in the CDW state. A theoretical model illustrating the possibility of a phase inversion in the oscillating interlayer conductivity in the presence of a spatially fluctuating magnetic breakdown gap is proposed.

cond-mat.str-el↗

arXiv2013

Spontaneous breaking of isotropy observed in the electronic transport of rare-earth tritellurides

We show that the isotropic conductivity in the normal state of rare-earth tritelluride RTe_3 compounds is broken by the occurrence of the unidirectional charge density wave (CDW) in the (a,c) plane below the Peierls transition temperature. In contrast with quasi-one-dimensional systems, the resistivity anomaly associated with the CDW transition is strong in the direction perpendicular to the CDW wave vector Q (a-axis) and very weak in the CDW wave vector Q direction (c-axis). We qualitatively explain this result by calculating the electrical conductivity for the electron dispersion with momentum-dependent CDW gap as determined by angle-resolved photoemission spectroscopy (ARPES).

cond-mat.str-el↗

arXiv2012

Longitudinal interlayer magnetoresistance in quasi-2D metals

The longitudinal interlayer magnetoresistance $R_{zz}(B_{z})$ is calculated in strongly anisotropic layered metals, when the interlayer band width $4t_{z}$ is less than the Landau level separation $\hbar ω_{c}$. The impurity scattering has much stronger effect in this regime than in 3D metals and leads to a linear longitudinal interlayer magnetoresistance $R_{zz}\propto B_{z}$ in the interval $\hbar ω_{c}>4t_{z}>>\sqrt{Γ_{0}\hbar ω_{c}}$ changing to a square-root dependence $R_{zz}\propto B_{z}^{1/2}$ at higher field or smaller $t_{z}$. The crossover field allows to estimate the interlayer transfer integral as $t_{z}\sim \sqrt{Γ_{0}\hbar ω_{c}}$. Longitudinal interlayer magnetoresistance, being robust to the increase of temperature or long-range disorder, is easy for measurements and provides a useful tool to investigate the electronic structure of quasi-two-dimensional compounds.

cond-mat.str-el↗

arXiv2012

Magnetic-field-induced dimensional crossover in the organic metal $α$-(BEDT-TTF)$_{2}$KHg(SCN)$_{4}$

The field dependence of interlayer magnetoresistance of the pressurized (to the normal state) layered organic metal $α$-(BEDT-TTF)$_{2}$KHg(SCN)$_{4}$ is investigated. The high quasi-two-dimensional anisotropy, when the interlayer hopping time is longer than the electron mean-free time and than the cyclotron period, leads to a dimensional crossover and to strong violations of the conventional three-dimensional theory of magnetoresistance. The monotonic field dependence is found to change from the conventional behavior at low magnetic fields to an anomalous one at high fields. The shape of Landau levels, determined from the damping of magnetic quantum oscillations, changes from Lorentzian to Gaussian. This indicates the change of electron dynamics in the disorder potential from the usual coherent three-dimensional regime to a new regime, which can be referred to as weakly coherent.

cond-mat.mes-hall↗

arXiv2011

Monotonic growth of interlayer magnetoresistance in strong magnetic field in very anisotropic layered metals

It is shown, that the monotonic part of interlayer electronic conductivity strongly decreases in high magnetic field perpendicular to the conducting layers. We consider only the coherent interlayer tunnelling, and the obtained result strongly contradicts the standard theory. This effect appears in very anisotropic layered quasi-two-dimensional metals, when the interlayer transfer integral is less than the Landau level separation.

cond-mat.mes-hall↗

arXiv2010

Angular dependence of magnetoresistance and Fermi-surface shape in quasi-2D metals

The analytical and numerical study of the angular dependence of magnetoresistance in layered quasi-two-dimensional (Q2D) metals is performed. The harmonic expansion analytical formulas for the angular dependence of Fermi-surface cross-section area in external magnetic field are obtained for various typical crystal symmetries. The simple azimuth-angle dependence of the Yamaji angles is derived for the elliptic in-plane Fermi surface. These formulas correct some previous results and allow the simple and effective interpretation of the magnetic quantum oscillations data in cuprate high-temperature superconducting materials, in organic metals and other Q2D metals. The relation between the angular dependence of magnetoresistance and of Fermi-surface cross-section area is derived. The applicability region of all results obtained and of some previous widely used analytical results is investigated using the numerical calculations.

cond-mat.mes-hall↗

arXiv2009

Surface state atoms and their contribution to the surface tension of quantum liquids

We investigate the new type of excitations on the surface of liquid helium. These excitations, called surfons, appear because helium atoms have discrete energy level at the liquid surface, being attracted to the surface by the van der Waals force and repulsed at a hard-core interatomic distance. The concentration of the surfons increases with temperature. The surfons propagate along the surface and form a two-dimensional gas. Basing on the simple model of the surfon microscopic structure, we estimate the surfon activation energy and effective mass for both helium isotopes. We also calculate the contribution of the surfons to the temperature dependence of the surface tension. This contribution explains the great and long-standing discrepancy between theory and experiment on this temperature dependence in both helium isotopes. The achieved agreement between our theory and experiment is extremely high. The comparison with experiment allows to extract the surfon activation energy and effective mass. The values of these surfon microscopic parameters are in a reasonable agreement with the calculated from the proposed simple model of surfon structure.

cond-mat.mes-hall↗

arXiv2008

Superconductivity on the density wave background with soliton-wall structure

Superconductivity (SC) may microscopically coexist with density wave (DW) when the nesting of the Fermi surface (FS) is not perfect. There are, at least, two possible microscopic structures of a DW state with quasi-particle states remaining on the Fermi level and leading to the Cooper instability: (i) the soliton-wall phase and (ii) the small ungapped Fermi-surface pockets. The dispersion of such quasi-particle states strongly differs from that without DW, and so do the properties of SC on the DW background. The upper critical field $H_{c2}$ in such a SC state strongly increases as the system approaches the critical pressure, where superconductivity first appears. $H_{c2}$ may considerably exceed its typical value without DW and has unusual upward curvature as function of temperature. The results obtained explain the experimental observations in layered organic superconductors (TMTSF)$_{2}$PF$_{6}$ and $α$-(BEDT TTF)$_{2}$KHg(SCN)$_{4}$.

cond-mat.supr-con↗

arXiv2008

Magnetic Field Induced Coherence-Incoherence Crossover in the Interlayer Conductivity of a Layered Organic Metal

The angle-dependent interlayer magnetoresistance of the layered organic metal $α$-(BEDT-TTF)$_2$KHg(SCN)$_4$ is found to undergo a dramatic change from the classical conventional behavior at low magnetic fields to an anomalous one at high fields. This field-induced crossover and its dependence on the sample purity and temperature imply the existence of two parallel channels in the interlayer transport: a classical Boltzmann conductivity $σ_{c}$ and an incoherent channel $σ_{i}$. We propose a simple model for $σ_{i}$ explaining its metallic temperature dependence and low sensitivity to the inplane field component.

cond-mat.mes-hall↗

arXiv2008

Properties of superconductivity on the density wave background with small ungapped Fermi surface pockets

We investigate the properties and the microscopic structure of superconductivity (SC), coexisting and sharing the common conducting band with density wave (DW). Such coexistence may take place when the nesting of the Fermi surface (FS) is not perfect, and in the DW state some quasi-particle states remain on the Fermi level and lead to the Cooper instability. The dispersion of such quasi-particle states is, in general, very different from that without DW. Therefore, the properties of SC on the DW background may strongly differ from those without DW. The upper critical field $H_{c2}$ in such a SC state increases as the system approaches the critical pressure, where the ungapped quasi-particles and superconductivity just appear, and it may considerably exceed the usual $H_{c2}$ value without DW. The SDW background strongly suppresses the singlet SC pairing, while it does not affect so much the triplet SC transition temperature. The results obtained explain the experimental observations in layered organic metals (TMTSF)$_{2}$PF$_{6}$ and $α$-(BEDT-TTF)$_{2}$KHg(SCN)$_{4}$, where SC appears in the DW states under pressure and shows many unusual properties.

cond-mat.supr-con↗

arXiv2007

Electron mobility on a surface of dielectric media: influence of surface level atoms

We calculate the contribution to the electron scattering rate from the surface level atoms (SLA), proposed in [A.M. Dyugaev, P.D. Grigoriev, JETP Lett. 78, 466 (2003)]. The inclusion of these states into account was sufficient to explain the long-standing puzzles in the temperature dependence of the surface tension of both He isotopes and to reach a very good agreement between theory and experiment. We calculate the contribution from these SLA to the surface electron scattering rate and explain some features in the temperature dependence of the surface electron mobility. This contribution is essential at low temperature $T<0.5$ when the He vapor concentration is exponentially small. For an accurate calculation of the electron mobility one also needs to consider the influence of the clamping electric field on the surface electron wave function and the temperature dependence of the He3 chemical potential.

cond-mat.mes-hall↗