arXiv subjects

Andrei G. Lebed

Publications and source records attributed to Andrei G. Lebed.

At least 19 recordsLinked to original sources

arXiv2025

Non Fermi-Liquid Magnetoresistance Oscillations in Quasi-One-Dimensional Conductors

We theoretically demonstrate that strong non Fermi-liquid magnetic oscillations of electron-electron scattering time can exist in quasi-one-dimensional (Q1D) conductors under condition of the magnetic breakdown between two open electron orbits. They are shown to be due to electron-electron interactions in a metallic phase under condition of the magnetic breakdown and they are beyond the Fermi-liquid theory. In particular, we consider as example the organic conductor (TMTSF)$_2$ClO$_4$ and perform both analytical and numerical calculations for its known electron spectrum. We also argue that similar oscillations of resistivity can exist in a metallic phase of another Q1D organic conductor - (Per)$_2$Au(mnt)$_2$.

cond-mat.str-el↗

arXiv2025

Strongly Forbidden Thermodynamic Oscillations in Quasi-One-Dimensional Conductors

We theoretically show that strongly forbidden oscillations of a specific heat have to exist in metallic phases of some quasi-one-dimensional (Q1D) conductors. They appear due to electron-electron interactions under condition of the magnetic breakdown phenomenon between the so-called open interference electron orbits. We argue that such forbidden thermodynamic oscillations can exist in Q1D conductors (TMTSF)$_2$ClO$_4$ and (Per)$_2$Au(mnt)$_2$, where TMTSF stands for tetramethyltetraselenafulvalene, Per is polycyclic aromatic hydrocarbon and mnt is mononitrotoluene, and suggest to discover them.

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arXiv2024

Theory of Lee-Naughton-Lebed's Oscillations in Moderately Strong Electric Fields in Layered Quasi-One-Dimensional Conductors

In framework of some extension of the quasi-classical Boltzmann kinetic equation, we show that a moderately strong electric field splits the so-called Lee-Naughton-Lebed's magnetoconductivity maxima in a layered quasi-one-dimensional conductor, if we use some reasonable approximation to the equation. By means of the above mentioned approximation, we obtain analytical formula for conductivity in high magnetic and moderately high electric fields and show that it coincides with the hypothetical formula as well as adequately describes the pioneering experimental data by Kobayashi et al. [K. Kobayashi, M. Saito, E. Omichi, and T. Osada, Phys. Rev. Lett. \textbf{96}, 126601 (2006)].

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arXiv2024

Quantum Theory of Lee-Naughton-Lebed's Angular Effect in Strong Electric Fields

Some time ago, Kobayashi et al. experimentally studied the so-called Lee-Naughton-Lebed's (LNL) angular effect in strong electric fields [K. Kobayashi, M. Saito, E. Omichi, and T. Osada, Phys. Rev. Lett. \textbf{96}, 126601 (2006)]. They found that strong electric fields split the LNL conductivity maxima in $α$-(ET)$_2$-based organic conductor and hypothetically introduced the corresponding equation for conductivity. In this Letter, for the first time we suggest quantum mechanical theory of the LNL angular oscillations in moderately strong electric fields. In particular, we demonstrate that the obtained by us approximate theoretical formula coincides with the hypothetical one and well describes the above mentioned experiments.

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arXiv2023

Violation of the Einstein's Equivalence Principle for a Composite Quantum Body

Recently, we have started to investigate behavior of a composite quantum body in an external gravitational field in the framework of General Relativity [see, for a review, A. G. Lebed, Mod. Phys. Lett. A, {\bf 35}, 2030010 (2020)]. As the simplest example, we have considered a hydrogen atom in a weak gravitational field. Our results are the following. The Einstein's Equivalence Principle survives for the most of macroscopic ensembles of the atoms, containing the stationary quantum states. On the other hand, we have demonstrated that this principle is sometimes broken. In particular, it is broken for the so-called Gravitational demons, which are the coherent macroscopic ensembles of two or more stationary quantum states in the hydrogen atoms. In the above cited paper we have considered the Gedanken experiment, where the gravitational field is suddenly switched on in a free from gravitation space. In the current paper we consider the much more realistic from experimental point of view Gedanken experiment and come to the same conclusion about violations of the Einstein's Equivalence Principle for the Gravitational demons.

gr-qc↗

arXiv2022

A Chiral Triplet Quasi-Two-Dimensional Superconductor in a Parallel Magnetic Field

We calculate the parallel upper critical magnetic field $H_{\parallel}(0)$ for an in-plane isotropic quasi-two-dimensional (Q2D) chiral triplet superconductor at zero temperature, $T=0$. In particular, the ratio $H_{\parallel}(0)/(|dH^{GL}_{\parallel}/dT|_{T=T_c}T_c) = 0.815$ is defined, where $|dH^{GL}_{\parallel}/dT|_{T=T_c}$ is the so-called Ginzburg-Landau slope of the upper critical magnetic field, $T_c$ is a superconducting transition temperature at $H=0$. We show that the theoretically obtained above mentioned value strongly contradicts to the experimentally measured ones in a candidate for a chiral triplet superconductivity Sr$_2$RuO$_4$, which provides one more argument against the chiral triplet scenario of superconductivity in this compound. Our results may be useful for establishing chiral triplet superconductivity in other Q2D candidates for this phenomenon.

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arXiv2021

Reentrant orbital effect against superconductivity in the quasi-two-dimensional superconductor NbS$_2$

We derive integral equation for superconducting gap, which takes into account the quantum nature of electron motion in a parallel magnetic field in a quasi-two-dimensional (Q2D) superconductor in the presence of a non-zero perpendicular field component. By comparison of our theoretical results with the recent experimental data obtained on the NbS$_2$, we show that the orbital effect against superconductivity partially destroys superconductivity in the so-called Ginzburg-Landau area of this Q2D conductor, as expected. Nevertheless, at relatively high magnetic fields, $H \simeq 15 \ T$, the orbital effect starts to improve the Fulde-Ferrell-Larkin-Ovchinnikov phase in the NbS$_2$, due to the quantum nature of electron motion in a parallel magnetic field. In our opinion, this is the most clear demonstration that the orbital effect against superconductivity in a parallel magnetic field has a reversible nature.

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arXiv2020

Restoration of superconductivity in high magnetic fields in UTe$_2$

It was theoretically predicted more than 20 years ago [A.G. Lebed and K. Yamaji, {\it Phys. Rev. Lett.} {\bf80}, 2697 (1998)] that a triplet quasi-two-dimensional (Q2D) superconductor could restore its superconducting state in parallel magnetic fields, which are higher than its upper critical magnetic field, $H > H_{c2}(0)$. It is very likely that recently such phenomenon has been experimentally discovered in the Q2D superconductor UTe$_2$ by Nicholas Butch, Sheng Ran and their colleagues and has been confirmed by Japanese-French team. We review our previous theoretical results, using such a general method that it describes the reentrant superconductivity in the above mentioned compound as well as will hopefully describe the similar phenomena, which can be discovered in other Q2D superconductors.

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arXiv2020

Four-fold anisotropy of the parallel upper critical magnetic field in a pure layered d-wave superconductor at $T = 0$

It is well known that a four-fold symmetry of the parallel upper critical magnetic field disappears in the Ginzburg-Landau (GL) region in quasi-two-dimensional (Q2D) $d$-wave superconductors. Therefore, it has been accurately calculated so far as a correction to the GL results, which is valid close to superconducting transition temperature and is expected to be stronger at low temperatures. As to the case $T=0$, some approximated methods have been used, which are good only for closed electron orbits and unappropriate for the open orbits which exist in a parallel magnetic field in Q2D superconductors. For the first time, we accurately calculate the four-fold anisotropy of the parallel upper critical magnetic field in a pure Q2D $d$-wave superconductor at $T=0$, where it has the highest possible value. Our results are applicable to Q2D $d$-wave high-Tc and organic superconductors.

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arXiv2020

Breakdown of the Einstein's Equivalence Principle for a quantum body

We review our recent theoretical results about inequivalence between passive gravitational mass and energy for a composite quantum body at a macroscopic level. In particular, we consider macroscopic ensembles of the simplest composite quantum bodies - hydrogen atoms. Our results are as follows. For the most ensembles, the Einstein's Equivalence Principle is valid. On the other hand, we discuss that for some special quantum ensembles - ensembles of the coherent superpositions of the stationary quantum states in the hydrogen atoms (which we call Gravitational demons) - the Equivalence Principle between passive gravitational mass and energy is broken. We show that, for such superpositions, the expectation values of passive gravitational masses are not related to the expectation values of energies by the famous Einstein's equation, i.e, $m_g \neq \frac{E}{c^2}$. Possible experiments at the Earth's laboratories are briefly discussed, in contrast to the numerous attempts and projects to discover the possible breakdown of the Einstein's Equivalence Principle during the space missions.

gr-qc↗

arXiv2020

Layered superconductor in a magnetic field: breakdown of the effective masses model

We theoretically study the upper critical magnetic fields at zero temperature in a quasi-two-dimensional (Q2D) superconductor in the parallel and perpendicular fields, $H_{c2}^{\parallel}(0)$ and $H_{c2}^{\perp}$(0), respectively. We find that $H_{c2}^{\parallel}(0) \approx 0.75 \ | d H_{c2}^{\parallel}/ dT|_{T_c} T_c $ and that $H_{c2}^{\perp}(0) \approx 0.59 \ | dH_{c2}^{\perp}/ d T|_{T_c} T_c$, where $| d H_{c2}^{\parallel}/ dT|_{T_c}$ and $| d H_{c2}^{\perp}/ d T|_{T_c}$ are the corresponding Ginzburg-Landau slopes of the upper critical magnetic fields. Our results demonstrate the breakdown of the so-called effective mass model in Q2D case and may be partially responsible for the experimentally observed deviations from the effective mass model in a number of layered superconductors, including $MgB_2$.

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arXiv2019

Breakdown of the equivalence between gravitational mass and energy due to quantum effects

We review our recent theoretical results about inequivalence between passive and active gravitational masses and energy in semiclassical variant of general relativity, where gravitational field is not quantized but matter is quantized. To this end, we consider the simplest quantum body with internal degrees of freedom - a hydrogen atom. We concentrate our attention on the following physical effects, related to electron mass. The first one is inequivalence between passive gravitational mass and energy at microscopic level. Indeed, quantum measurement of gravitational mass can give result, which is different from the expected, $m \neq m_e + \frac{E_1}{c^2}$, where electron is initially in its ground state; $m_e$ is the bare electron mass. The second effect is that the expectation values of both passive and active gravitational masses of stationary quantum states are equivalent to the expectation value of energy. The most spectacular effects are inequivalence of passive and active gravitational masses and energy at macroscopic level for ensemble of coherent superpositions of stationary quantum states. We show that, for such superpositions, the expectation values of passive and active gravitational masses are not related to the expectation value of energy by the famous Einstein's equation, $m \neq \frac{E}{c^2}$. In this review, we also improve several drawbacks of the original pioneering works.

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arXiv2019

Inequivalence between gravitational mass and energy of a composite quantum body in general relativity

We consider the so-called semiclassical variant of general relativity, where gravitational field is not quantized but matter is quantized, for the simplest composite quantum body - a hydrogen atom. We create a stationary electron quantum state in the atom in the absence of gravitational field and study its time evolution in the presence of the field, using the local Lorentz invariance property of spacetime. It is shown that this state with a definite energy in the absence of gravitational field is not anymore a stationary state in the field. Therefore, quantum measurements of passive gravitational mass of electron in a hydrogen atom can give the following quantized values, $m_n = m_e + E_n/c^2$, where $m_e$ is the bare electron mass and $E_n$ is its energy level in the atom. We discuss some difficulties in the possible experimental observations of this mass quantization phenomenon.

gr-qc↗

arXiv2019

Unconventional Field-Induced Spin-Density-Wave Phases in Quasi-One-Dimensional Conductors in High Magnetic Fields

It is known that the Field-Induced Spin-Density-Wave (FISDW) phases are experimentally observed in the quasi-one-dimensional (Q1D) organic conductors with chemical formula (TMTSF)$_2$X (X=PF$_6$, ClO$_4$, etc.) and some others in moderate magnetic fields. From a theoretical point of view, they appear as a result of "one-dimensionalization" of the Q1D electron spectra due to the orbital electron effect in a magnetic field. We predict that the novel FISDW phases with different physical meaning have to appear in inclined high magnetic fields in Q1D conductors as a result of combination of the spin-splitting and orbital electron effects. We suggest performing the corresponding experiments in the (TMTSF)$_2$X materials.

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arXiv2018

Can a Magnetic Field Destroy a Spin-Density-Wave Phase in a Quasi-One-Dimensional Conductor?

It is known that, in a pure one-dimensional case, Charge-Density-Wave (CDW) phase is destroyed by a magnetic field, whereas Spin-Density-Wave (SDW) one does not feel the field. In reality, SDW phase is often observed in quasi-one-dimensional (Q1D) conductors due to the so-called "nesting" property of their electron spectra. We show that, in the latter case, a high magnetic field generates some "anti-nesting" term in a Q1D electron spectrum, which destroys SDW phase. We suggest to perform the corresponding experiments in SDW phases of the real Q1D organic conductors with chemical formula (TMTSF)$_2$X (X=PF$_6$, ClO$_4$, etc.).

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arXiv2018

Orbital Effect for the Fulde-Ferrell-Larkin-Ovchinnikov Phase in a Quasi-Two-Dimensional Superconductor in a Parallel Magnetic Field

We theoretically study the orbital destructive effect against superconductivity in a parallel magnetic field in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) phase at zero temperature in a quasi-two-dimensional (Q2D) conductor. We demonstrate that at zero temperature a special parameter, $λ= l_{\perp}(H)/d$, is responsible for strength of the orbital effect, where $l_{\perp}(H)$ is a typical "size" of the quasi-classical electron orbit in a magnetic field and $d$ is the inter-plane distance. We discuss applications of our results to the existing experiments on the FFLO phase in the organic Q2D conductors $κ$-(ET)$_2$Cu(NCS)$_2$ and $κ$-(ET)$_2$Cu[N(CN)$_2$]Cl.

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arXiv2018

Quantum limit in a quasi-one-dimensional conductor in a high tilted magnetic field

Recently, we have suggested Fermi-liquid - non-Fermi-liquid angular crossovers which may exist in quasi-one-dimensional (Q1D)conductors in high tilted magnetic fields [see A.G. Lebed, Phys. Rev. Lett. $\textbf{115}$, 157001 (2015).] All calculations in the Letter, were done by using the quasi-classical Peierls substitution method, whose applicability in high magnetic fields was questionable. Here, we solve a fully quantum mechanical problem and show that the main qualitative conclusions of the above mentioned Letter are correct. In particular, we show that in high magnetic fields, applied along one of the two main crystallographic axis, we have 2D electron spectrum, whereas, for directions of high magnetic fields far from the axes, we have 1D electron spectrum. The later is known to promote non-Fermi-liquid properties. As a result, we expect the existence of Fermi-liquid - non-Fermi-liquid angular crossovers or phase transitions. Electronic parameters of Q1D conductor (Per)$_2$Pt(mnt)$_2$ show that such transitions can appear in feasible high magnetic fields of the order of $H \simeq 20-25 \ T$.

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arXiv2017

Inequivalence between gravitational mass and energy due to quantum effects at microscopic and macroscopic levels

We review recent theoretical results, demonstrating breakdown of the equivalence between active and passive gravitational masses and energy due to quantum effects in General Relativity. In particular, we discuss the simplest composite quantum body - a hydrogen atom - and define its gravitational masses operators. Using Gedanken experiment, we show that the famous Einstein's equation, $E =mc^2$, is broken with small probability for passive gravitational mass of the atom. It is important that the expectation values of both active and passive gravitational masses satisfy the above mentioned equation for stationary quantum states. Nevertheless, we stress that, for quantum superpositions of stationary states in a hydrogen atom, where the expectation values of energy are constant, the expectation values of the masses oscillate in time and, thus, break the Einstein's equation. We briefly discuss experimental possibility to observe the above-mentioned time-dependent oscillations. In this review, we also improve several drawbacks of the original pioneering works.

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