arXiv subjects

Naoum Karchev

Publications and source records attributed to Naoum Karchev.

At least 19 recordsLinked to original sources

arXiv2025

Time dependent Ginzburg-Landau theory for design of superconducting wire

In the present paper, we study the superconducting wire. It is known from Maxwell equations that the current creates magnetic field that suppresses superconductivity and wire starts to conduct with resistance. We consider the time dependent Ginzburg-Landau theory to resolve the problem. The solutions of the system of equations show that applied electric field, perpendicular to the axis of the wire and to the magnetic field restores superconductivity and wire starts to conduct without resistivity. We can increase the current to a new suppression of superconductivity and to restore superconductivity increasing the applied electric field. The aforementioned results permit us to conclude that the superconducting wire can transport a very strong current if the novel design discussed in the paper is applied.

cond-mat.supr-con↗

arXiv2024

Overview of superconductivity in field-cooled magnetic materials

Considerable experimental skills have been accumulated in the preparation of field-cooled (FC) magnetic materials. This stimulates the search for FC magnetic materials that are superconductors. The article overviews the recent proposed mechanism of superconductivity in field-cooled magnetic materials. It is based on previously published results for magnon-induced superconductivity in field-cooled spin-1/2 antiferromagnets $[PRB96,214409]$ (arXiv:1712.02983) and Sequence of superconducting states in field cooled $FeCr_2S_4$ $[JPCM33,495604]$ (arXiv:2111.02765). Shortened version of arXiv:2308.00470.

cond-mat.supr-con↗

arXiv2022

Impact of the electric field on superconductivity in Bose-Einstein Condensation regime

In the strong coupling Bose-Einstein Condensation (BEC) regime the superconductors have two characteristic temperatures: $T^*$- onset of fermion pairing and $T_{sc}$- onset of superconductivity, such that $T_{sc} T_{sc}$. Important consequence is the fact that arbitrary temperature within the interval ($T_{sc},T^*$) is a critical temperature of superconductor transition if an appropriate electric field is applied. This means that if we set the temperature of the system within the above mentioned interval and increase the applied electric field the system undergoes an electric field induced transition to superconductor. We also show the existence of critical value of the applied electric field at which $T^*=T^E_{sc}$. This means that although the system is in BEC regime, away from the BCS one, we can apply an electric field that moves the system to a state with $T^E_{sc}=T^*$, characteristic of BCS regime. The results indicate that applied electric field experiments are a suitable tool to identify the BEC regime of the superconductors. The experiment can determine $T^*$ as a temperature below which the electric field Bose condenses the Cooper pairs, while above it the electrons screen the field and it cannot penetrate.

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arXiv2022

Applied electric field instead of pressure in H-based superconductors

In our desire to give a new suggestion for H-based superconductors experiments we present a theoretical framework for understanding the impact of an applied electric field on pressured hydride superconductors. We study a material at pressure $p$, when it possesses insulator-superconductor transition, at the respective superconducting critical temperature $T_{cr}$. The theory shows the applied electric field penetrates the material and forces the Cooper pairs to Bose condensate. If one applies an electric field and then increases the temperature, the theory predicts novel critical temperature $T^{el}_{cr}$ higher than $T_{cr}$. Therefore, the system has a higher superconducting critical temperature if we apply an electric field instead of increasing the pressure. The result shows that in the case of carbonaceous sulfur hydride at $234Gpa$ and near but below critical temperature $T_c=283K$, applying a sufficiently strong electric field, we can bring the superconducting critical temperature close to 300K.

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arXiv2021

Sequence of superconducting states in field cooled $FeCr_2S_4$

In the present article we discuss theoretically the emergence of superconductivity in field cooled $FeCr_2S_4$. The chromium electrons form a triplet $t_{2g}$ states and due to antiferromagnetic exchange with the iron spins have Zeeman splitting. Applied, during preparation, magnetic field along the moment of iron ions, successively compensates the Zeeman splittings. The chromium electrons with zero Zeeman energy form Cooper pairs induced by iron magnons. In that way, we predict theoretically the existence of sequence of superconducting states in field cooled $FeCr_2S_4$. Actually there are three different superconductors prepared applying, during preparation, different magnetic fields. In these compounds superconductivity coexist with the saturated magnetism of iron ions.

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arXiv2020

Theory of the superconductivity of $UGe_2$ revisited

We present a unified theory of magnetism and superconductivity of $UGe_2$. To this end, we consider part of $5f$ uranium electrons as mostly itinerant and other ones as mostly localized. The main feature that distinguishes the localized from the itinerant electrons is the effect of the pressure on them. The pressure strongly screens the itinerant electrons while the localized ones are almost unaffected. The screening of itinerant electrons leads to decreasing of their Coulomb repulsion, therefore to formation of doubly occupied and empty states. These states are spin-singlet and the effective spin of itinerant electrons, the zero-temperature magnetization in units of Bohr magneton, decreases. We obtain an effective two-spin Heisenberg model, which explains the magnetization-temperature diagram of $UGe_2$. It is shown that the experimentally observed characteristic temperature $T_x$, is a partial order transition temperature. Below the Curie temperature $(T_x<T_C)$ the system undergoes a transition from high temperature phase, were only localized electrons contribute the magnetization, to the low temperature one, where both itinerant and localized electrons contribute the magnetization. The characteristic temperature decreases when pressure increases. At the quantum partial order point $T_x=0$, the Zeeman splitting of the itinerant electrons is zero. This permits formation of Cooper pairs and an onset of superconductivity induced by the transversal fluctuations of the localized electrons. Small deviation from the quantum partial ordered state leads to suppression of superconductivity. This explains the dome form of the superconducting transition temperature. The very low superconducting critical temperature is a consequence of the Ising ferromagnetism.

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arXiv2020

Spin-nematic order induced superconductivity

We explore a spin-fermion model with fermion-spin-quadrupolar interaction. In a nematic phase, this interaction reduces to a four-fermion interaction that is a basis of superconductivity. When the coupling constant is positive the superconductivity is p-wave with spin-parallel paired fermions. When it is negative the superconductivity is p-wave and fermions are spin-antiparallel paired. For a system with zero chemical potential, even a very small coupling can bind fermions into bound state that leads to the superconductivity. When the chemical potential is non-zero the system possesses quantum critical transition from normal spin-nematic phase to phase where superconductivity coexists with spin-nematicity. The value of the quantum critical fermion-spin-nematicity coupling constant depends on the chemical potential.

cond-mat.supr-con↗

arXiv2017

Magnon-induced superconductivity in field-cooled spin-1/2 antiferromagnets

If, during the preparation, an external magnetic field is applied upon cooling we say it has been field cooled. A novel mechanism for insulator-metal transition and superconductivity in field-cooled spin-$1/2$ antiferromagnets on bcc lattice is discussed. Applying a magnetic field along the sublattice B magnetization, we change the magnetic and transport properties of the material. There is a critical value $H_{cr1}$. When the magnetic field is below the critical one $H H_{cr1}$ the sublattice A electrons are delocalized and the material is metal. There is a second critical value $H_{cr2}>H_{cr1}$. When $H=H_{cr2}$, it is shown that the Zeeman splitting of the sublattice A electrons is zero and they do not contribute to the magnetization of the system. At this quantum partial order point (QPOP) the sublattice B transversal spin fluctuations (magnons) interact with sublattice A electrons inducing spin anti-parallel \emph{p}-wave superconductivity which coexists with magnetism. At zero temperature the magnetic moment of sublattice B electrons is maximal. Below the Néel temperature $(T_N)$ the gap is approximately constant with a small increase when the system approaches $T_N$. It abruptly falls down to zero at temperatures above $T_N$.

cond-mat.supr-con↗

arXiv2016

Numerical solution of Maxwell equations for s-wave superconductors

We report the numerical solutions of the system of equations, which describes the electrodynamics of s-wave superconductors, for time independent fields and half-plane superconductor geometry. The results are: i)the applied magnetic field increases the Ginzburg-Landau (GL) coherence length and suppresses the superconductivity, ii)the applied electric field decreases GL coherence length and supports the superconductivity, iii) if the applied magnetic field is fixed and the applied electric field increases the London penetration depth of the magnetic field decreases. The main conclusion is that applying electric field at very low temperature one increases the critical magnetic field. This result is experimentally testable.

cond-mat.supr-con↗

arXiv2016

Electrodynamics of s-wave superconductors

In this paper we give a derivation of a system of equations to describe the electrodynamics of s-wave superconductors. First, we consider a relativistically covariant theory in terms of gauge four-vector electromagnetic potential and scalar complex field. We use the first-order formalism to obtain the supplemented Maxwell equations for gauge invariant electric, magnetic, four-vector fields and the modulus of the superconducting order parameter. The new four-vector field appears in some of the equations as a gauge invariant super-current and in other ones, while gauge invariant, as a four-vector electromagnetic potential. This dual contribution of the new four-vector field is the basis of the electrodynamics of superconductors. We focus on the system of equations with time-independent fields. The qualitative analysis shows that the applied magnetic field suppresses the superconductivity, while the applied electric field impacts appositely, supporting it. Second we consider time-dependent non-relativistic Ginzburg-Landau theory.

cond-mat.supr-con↗

arXiv2014

Coexistence of superconductivity and magnetism in spin-fermion model of ferrimagnetic spinel in an external magnetic field

A two-sublattice spin-fermion model of ferrimagnetic spinel, with spin-$1/2$ itinerant electrons at the sublattice $A$ site and spin-$s$ localized electrons at the sublattice $B$ site is considered. The exchange between itinerant and localized electrons is antiferromanetic. As a result the external magnetic field, applied along the magnetization of the localized electrons, compensates the Zeeman splitting due to the spin-fermion exchange and magnon-fermion interaction induces spin anti-parallel p-wave superconductivity which coexists with magnetism. We have obtained five characteristic values of the applied field (in units of energy) $H_{cr1}<H_3<H_0<H_4<H_{cr2}$. At $H_0$ the external magnetic field compensates the Zeeman splitting. When $H_{cr1}<H<H_{cr2}$ the spin antiparallel p-wave superconductivity with $T_{1u}$ configuration coexists with magnetism. The superconductor to normal magnet transition at finite temperature is second order when $H$ runs the interval $(H_3,H_4)$. It is an abrupt transition when $H_{cr1}<H<H_3$ or $H_4<H<H_{cr2}$. This is proved calculating the temperature dependence of the gap for three different values of the external magnetic field $H_{cr1}<H<H_3$, $H_4<H<H_{cr2}$ and $H=H_0$. In the first two cases the abrupt fall to zero of the gap at superconducting critical temperature shows that the superconductor to normal magnet transition is first order. The Hubbard term (Coulomb repulsion), in a weak coupling regime, does not affect significantly the magnon induced superconductivity. Relying on the above results one can formulate a recipe for preparing a superconductor from ferrimagnetic spinel: i) hydrostatic pressure above the critical value of insulator-metal transition. ii) external magnetic field along the sublattice magnetization with higher amplitude.

cond-mat.str-el↗

arXiv2014

Leggett's Modes in Magnetic Systems with Jahn-Teller distortion

Leggett's mode is a collective excitation corresponding to the oscillation of the relative phase of the order parameters in a two band superconductor, with frequency proportional to interband coupling. We report on the existence of modes, similar to Leggett's mode, in magnetic systems with Jahn-Teller distortion. The minimal Kugel-Khomskii model, which describes simultaneously both the spin and the orbital order, is studied. The dynamical degrees of freedom are spin-$s$ operators of localized spins and pseudospin-$τ$ operators, which respond to the orbital degeneracy and satisfy the similar commutation relation with those of the spin operators. In the case of "antiferro" spin and pseudospin order the system possesses two antiferromagnetic magnons with equal spin-wave velocities and two Leggett's modes with equal gaps proportional to the square root of the spin-pseudospin interaction constant. In the case of "ferro" spin and pseudospin order the system possesses one ferromagnetic magnon and one Leggett's mode with gap proportional to the spin-pseudospin interaction constant.

cond-mat.str-el↗

arXiv2013

Quantum critical behavior in three-dimensional one-band Hubbard model at half filling

One-band Hubbard model with hopping parameter $t$ and Coulomb repulsion $U$ is considered at half filling. By means of the Schwinger bosons and slave Fermions representation of the electron operators and integrating out the spin-singlet Fermi fields an effective Heisenberg model with antiferromagnetic exchange constant is obtained for vectors which identifies the local orientation of the spin of the itinerant electrons. The amplitude of the spin vectors is an effective spin of the itinerant electrons accounting for the fact that some sites, in the ground state, are doubly occupied or empty. Accounting adequately for the magnon-magnon interaction the Néel temperature is calculated. When the ratio $\frac tU$ is small enough ($\frac tU\leq 0.09$) the effective model describes a system of localized electrons. Increasing the ratio increases the density of doubly occupied states which in turn decreases the effective spin and Néel temperature. The phase diagram in plane of temperature $\frac {T_N}{U}$ and parameter $\frac tU$ is presented. The quantum critical point ($T_N=0$) is reached at $\frac tU=0.9$. The magnons in the paramagnetic phase are studied and the contribution of the magnons' fluctuations to the heat capacity is calculated. At Néel temperature the heat capacity has a peak which is suppressed when the system approaches quantum critical point.

cond-mat.str-el↗

arXiv2012

Path integral representation for spin systens

The present paper is a short review of different path integral representations of the partition function of quantum spin systems. To begin with, I consider coherent states for SU(2) algebra. Different parameterizations of the coherent states lead to different path integral representations. They all are unified within an U(1) gauge theory of quantum spin systems.

cond-mat.str-el↗

arXiv2012

Ferromagnetic Quantum critical behavior in three-dimensional Hubbard model with transverse anisotropy

One-band Hubbard model with transverse anisotropy is considered at density of electrons $n=0.4$. It is shown that when the anisotropy is appropriately chosen, the ground state is ferromagnetic with magnetic order perpendicular to the anisotropy. The increasing of the ratio $\frac tU$, where $t$ is the hopping parameter and $U$ is the Coulomb repulsion, decreases the Curie temperature, and the system arrives at the quantum critical point $(T_C=0)$. The result is obtained introducing Schwinger bosons and slave Fermions representation of the electron operators. Integrating out the spin-singlet Fermi fields an effective Heisenberg model with ferromagnetic exchange constant is obtained for vectors which identifies the local orientation of the spin of the itinerant electrons. The amplitude of the spin vectors is an effective spin of the itinerant electrons accounting for the fact that some sites, in the ground state, are doubly occupied or empty. Owing to the anisotropy, the magnon fluctuations drive the system to quantum criticality and when the effective spin is critically small these fluctuations suppress the magnetic order.

cond-mat.str-el↗

arXiv2011

Phase diagrams of $\rm La_{1-x}Ca_xMnO_3$ in Double Exchange Model with added antiferromagnetic and Jahn-Teller interaction

The phase diagram of the multivalent manganites $\rm La_{1-x}Ca_xMnO_3$, in space of temperature and doping $x$, is a challenge for the theoretical physics. It is an important test for the model used to study these compounds and the method of calculation. To obtain theoretically this diagram for $x<0.5$, we consider the two-band Double Exchange Model for manganites with added Jahn-Teller coupling and antiferromagnetic Heisenberg term. In order to calculate Curie and Néel temperatures we derive an effective Heisenberg model for a vector which describes the local orientation of the total magnetization of the system. The exchange constants of this model are different for different space directions and depend on the density of $e_g$ electrons, antiferromagnetic constants and the Jahn-Teller energy. To reproduce the well known phase transitions from A-type antiferromagnetism to ferromagnetism at low $x$ and C-type antiferromagnetism to G-type antiferromagnetism at large $x$, we argue that the antiferromagnetic exchange constants should depend on the lattice direction. We show that ferromagnetic to A-type antiferromagnetic transition results from the Jahn-Teller distortion. Accounting adequately for the magnon-magnon interaction, Curie and Néel temperatures are calculated. The results are in very good agreement with the experiment and provide values for the model parameters, which best describe the behavior of the critical temperature for $x<0.5$.

cond-mat.str-el↗

arXiv2010

Ferromagnetism of $UGe_2$

Magnetism of $UGe_2$ is due to the magnetic ordered moments of $5f$ uranium electrons. The strong spin-orbit coupling splits them into two groups. The magnetization is investigated in terms of two vector fields ${\bf M}_{1i}$ and ${\bf M}_{2i}$ which identify the local orientation of the magnetization of the two groups of $f$ electrons. Renormalized spin-wave theory, which accounts for the magnon-magnon interaction, and its extension are developed to describe two ferromagnetic phases in the system: low temperature large moment phase $0<T<T^{*}$ (FM2), where all $5f$ electrons contribute the ordered ferromagnetic moment, and high temperature low-moment phase $T^{*}<T<T_C$ (FM1), where $f$ electrons are partially ordered. Both phases are strictly ferromagnetic in accordance with experiment. The magnetization as a function of temperature is calculated. The anomalous temperature dependence of the ordered moment, known from the experiments with $UGe_2$, is very well reproduced theoretically. Below $T_x$ ($T^*$ in the present paper) the ferromagnetic moment increases in an anomalous way. The new understanding of the anomalous $FM2\to FM1$ transition, as a result of the magnetic order of two well separated groups of $f$ electrons, yields the key to an understanding of the ferromagnetism and transport properties in these compounds.

cond-mat.str-el↗

arXiv2009

Towards the theory of ferrimagnetism II

The present paper is a sequel to the paper by Karchev (2008 J.Phys.:Condens.Matter {\bf 20} 325219). A two-sublattice ferrimagnet, with spin-$s_1$ operators $\bf{S_{1i}}$ at the sublattice $A$ site and spin-$s_2$ operators $\bf{S_{2i}}$ at the sublattice $B$ site, is considered. Renormalized spin-wave theory, which accounts for the magnon-magnon interaction, and its extension are developed to describe the two ferrimagnetic phases $(0,T^*)$ and $(T^*,T_N)$ in the system, and to calculate the magnetization as a function of temperature. The influence of the parameters in the theory on the characteristic temperatures $T_N$ and $T^*$ is studied. It is shown that, increasing the inter-sublattice exchange interaction, the ratio $T_N/T^*>1$ decreases approaching one, and above some critical value of the exchange constant there is only one phase $T_N = T^*$, and the magnetization-temperature curve has the typical Curie-Weiss profile. When the intra-exchange constant of sublattice with stronger intra-exchange interaction increases the $Ne\grave{e}l$ temperature increases while $T^*$ remains unchanged. Finally, when the magnetic order of the sublattice with smaller magnetic order decreases, $T^*$ decreases. The theoretical predictions are utilize to interpret the experimentally measured magnetization-temperature curves.

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