arXiv subjects

Diana V. Shopova

Publications and source records attributed to Diana V. Shopova.

14 recordsLinked to original sources

arXiv2022

Infuence of pressure on magnetic phase transitions in the ferromagnetic superconductor UGe$_2$ -- phenomenological approach

We propose a thermodynamic model of free energy expansion up to eighth order of magnetisation to describe the complex magnetic phase transitions in ferromagnetic superconductor UGe$_2$. The model successfully describes transitions between ordered phases which take place without changing of magnetic structure but only of magnetisation which is the case of UGe$_2$ where two magnetic phases with same structure but with different magnitude occur with decreasing of temperature. We consider the infuence of pressure on magnetic transitions in the simplest form of including it only in pressure dependence of Curie temperature for the transition between the paramagnetic and themagnetic phase with the lower magnetic moment FM1. Our results show that for pressures lower than some limiting pressure the transition betweenlow-and high-magnetisation phase remains of crossover type. Above this limiting pressure this transition changes to real thermodynamic transition of first order.

cond-mat.supr-con↗

arXiv2021

Thermodynamic analysis of magnetic phase transitions in the ferromagnetic superconductor UGe$_2$ at ambient pressure

We study the possibility to apply phenomenological approach to the description of magnetic transitions in UGe$_2$ at ambient pressure with the help of Landau free energy expanded to 8th order in magnetisation. The analysis shows that for certain values of parameters in front of M4, and M6 terms in the free energy there is possibility for the appearance of two successive phase transitions between the low-magnetisation and high-magnetisation phases with the same magnetic structure. We establish the relation of the parameters in Landau energy, for which the phase transition from the disordered to low-magnetisation phase is of second order as the experimental data shows. We have also found the relations between the Landau coefficients which result in transition of crossover type between the low- and high-magnetisation phases when temperature is lowered.

cond-mat.supr-con↗

arXiv2020

Application of phenomenological Landau approach to the description of magnetic phase transitions in the ferromagnetic superconductor at ambient pressure

We study the possibility to apply phenomenological approach to the description of magnetic transitions in UGe2 with the help of Landau free energy expanded to 8-th order in magnetisation. The analysis shows that for certain values of parameters in front of M^4, and M^6 terms in the free energy there id possibility for the appearance of two successive phase transitions between the low-magnetisation and high-magnetisation phase with the same structure. We establish the relation of the parameters in Landau energy, for which the phase transition from the disordered to low-magnetisation phase is of second order as the experimental data shows. Within our approximation the transition between two magnetically ordered phases is of first order

cond-mat.supr-con↗

arXiv2018

Application of two-sublattice bilinearly coupled Heisenberg model to the description of certain ferrimagnetic materials

We study phenomenologically on the basis of two bilinearly coupled Heisen- berg models the phase diagram of some ferrimagnetic substances. Calculations are performed with the help of Landau energy obtained through applying the Hubbard-Stratonovich transformation to the initial microscopic Heisenberg Hamiltonian. The phase transitions within the model are of second order with the emergence of a compensation point at lower temperatures for some values of parameters of the system. The main phase is a two-sublattice collinear ferrimagnet but also a metastable non-collinear phase is present within the exchange approximation presented here. The numerical results give a detailed description of temperature dependence of magnetization on the strength of in- tersublattice interaction and the difference between the effective exchanges of two ferromagnetically ordered sublattices.

cond-mat.other↗

arXiv2014

Phenomenological description of anisotropy effects in some ferromagnetic superconductors

We study phenomenologically by using the previously derived Landau free energy, the role of anisotropy in ferromagnetic superconductors UGe2, URhGe, and UCoGe. The three compounds are separately discussed with the special stress on UGe2. The main effect comes from the strong uniaxial anisotropy of magnetization while the anisotropy of Cooper pairs and crystal anisotropy only slightly change the phase diagram in the vicinity of Curie temperature. The limitations of this approach are also discussed.

cond-mat.supr-con↗

arXiv2013

Some remarks on the magnetic phase diagram of ferromagnetic superconductor UGe2

The phase transition to ferromagnetic order in itinerant ferromagnetic superconductor UGe$_2$ as function of pressure shows change of phase transition order. At low temperature and high pressure the transition is of first order and at pressure of about 1.42 GPa the order changes to second. On the basis of Landau expansion of free energy up to the sixth order in magnetization we calculate the phase diagram taking into account the magneto-elastic interaction as the mechanism responsible for this change. We propose a simple Stoner-like dependence of the Curie temperature on the pressure and present the results for measurable thermodynamic quantities as the entropy jump at the first order phase transition.

cond-mat.supr-con↗

arXiv2010

Diamagnetic susceptibility of spin-triplet ferromagnetic superconductors

We calculate the diamagnetic susceptibility in zero external magnetic field above the phase transition from ferromagnetic phase to phase of coexistence of ferromagnetic order and unconventional superconductivity. For this aim we use generalized Ginzburg-Landau free energy of unconventional ferromagnetic superconductor with spin-triplet electron pairing. A possible application of the result to some intermetallic compounds is briefly discussed.

cond-mat.supr-con↗

arXiv2009

Thermodynamics of ferromagnetic superconductors with spin-triplet electron pairing

We present a general thermodynamic theory that describes phases and phase transitions of ferromagnetic superconductors with spin-triplet electron Cooper pairing. The theory is based on extended Ginzburg-Landau expansion in powers of superconducting and ferromagnetic order parameters. We propose a simple form for the dependence of theory parameters on the pressure that allows correct theoretical outline of the temperature-pressure phase diagram for which at low temperatures a stable phase of coexistence of p-wave superconductivity and itinerant ferromagnetism appears. We demonstrate that the theory is in an agreement with the experimental data for some intermetallic compounds that are experimentally proven to be itinerant ferromagnetic exhibiting spin-triplet superconductivity. Some basic features of quantum phase transitions in such systems are explained and clarified. We propose to group the spin-triplet ferromagnetic superconductors in two different types of thermodynamic behavior, on the basis of quantitative criterion deduced from the present theory and the analysis of experimental data.

cond-mat.supr-con↗

arXiv2008

On the phase diagrams of the ferromagnetic superconductors ZrZn2 and UGe2

A general phenomenological theory is presented for the phase behavior of ferromagnetic superconductors with spin-triplet electron Cooper pairing. The theory accounts in detail for the temperature-pressure phase diagram of ZrZn$_2$, while the main features of the diagram for UGe$_2$ are also described. Quantitative criteria are deduced for the U-type (type I) and Zr-type (type II) unconventional ferromagnetic superconductors with spin-triplet Cooper electron pairing. Some basic properties of quantum phase transitions are also elucidated.

cond-mat.supr-con↗

arXiv2007

Fluctuation and gauge effects on the critical behavior of superconductors

Gauge effects on the fluctuation properties of the normal-to-superconducting phase transition in bulk and thin film superconductors are reviewed. Similar problems in the description of other natural systems (liquid crystals, quantum field theory, early universe) are also discussed. The relatively strong gauge effects on the fluctuations of the ordering field at low spatial dimensionality $D$ and, in particular, in thin (quasi-2D) films are considered in details. A special attention is paid to the fluctuations of the gauge field. It is shown that the mechanism, in which these gauge fluctuations affect the phase transition order and other phase transition properties varies with the variation of spatial dimensionality $D$. The problem for the experimental confirmation of theoretical predictions about the order of phase transitions in gauge systems is discussed. Related topics: gauge effects on the critical behavior of unconventional superconductors, disorder, quantum fluctuations in a close vicinity of ultra-low phase transition temperatures, are also briefly discussed.

cond-mat.supr-con↗

arXiv2005

Meissner superconductivity in itinerant ferromagnets

Recent results about the coexistence of ferromagnetism and unconventional superconductivity with spin-triplet Cooper pairing are reviewed on the basis of the quasi-phenomenological Ginzburg-Landau theory. The superconductivity in the mixed phase of coexistence of ferromagnetism and unconventional superconductivity is triggered by the spontaneous magnetization. The mixed phase is stable whereas the other superconducting phases that usually exist in unconventional superconductors are either unstable or metastable at relatively low temperatures in a quite narrow domain of the phase diagram and the stability properties are determined by the particular values of Landau parameters. The phase transitions from the normal phase to the phase of coexistence is of first order while the phase transition from the ferromagnetic phase to the coexistence phase can be either of first or second order depending on the concrete substance. The Cooper pair and crystal anisotropy are relevant to a more precise outline of the phase diagram shape and reduce the degeneration of the ground states of the system. The results are discussed in view of application to itinerant ferromagnetic compounds as UGe2, ZrZn2, URhGe.

cond-mat.supr-con↗

arXiv2005

Meissner phases in spin-triplet ferromagnetic superconductors

We present new results for the properties of phases and phase transitions in spin-triplet ferromagnetic superconductors. The superconductivity of the mixed phase of coexistence of ferromagnetism and unconventional superconductivity is triggered by the presence of spontaneous magnetization. The mixed phase is stable but the other superconducting phases that usually exist in unconventional superconductors are either unstable or for particular values of the parameters of the theory some of them are metastable at relatively low temperatures in a quite narrow domain of the phase diagram. Phase transitions from the normal phase to the phase of coexistence is of first order while the phase transition from the ferromagnetic phase to the coexistence phase can be either of first or second order depending on the concrete substance. Cooper pair and crystal anisotropies determine a more precise outline of the phase diagram shape and reduce the degeneration of ground states of the system but they do not change drastically phase stability domains and thermodynamic properties of the respective phases. The results are discussed in view of application to metallic ferromagnets as UGe2, ZrZn2, URhGe.

cond-mat.supr-con↗

arXiv2004

Phenomenological study of spin-triplet ferromagnetic superconductors

Unconventional superconductivity with spin-triplet Cooper pairing is reviewed on the basis of the quasi-phenomenological Ginzburg-Landau theory. The superconductivity, in particular, the mixed phase of coexistence of ferromagnetism and unconventional superconductivity is triggered by the spontaneous magnetization. The mixed phase is stable whereas the other superconducting phases that usually exist in unconventional superconductors are either unstable, or, for particular values of the parameters of the theory, some of these phases are metastable at relatively low temperatures in a quite narrow domain of the phase diagram. The phase transitions from the normal phase to the phase of coexistence is of first order while the phase transition from the ferromagnetic phase to the coexistence phase can be either of first or second order depending on the concrete substance. The Cooper pair and crystal anisotropy are relevant to a more precise outline of the phase diagram shape and reduce the degeneration of the ground states of the system but they do not drastically influence the phase stability domains and the thermodynamic properties of the respective phases.

cond-mat.supr-con↗

arXiv2004

Phase transitions to spin-triplet ferromagnetic superconductivity in neutron stars

Effects of the anisotropy of Cooper pairs in spin-triplet ferromagnetic superconductors are investigated on the basis of the Ginzburg-Landau theory. A special attention is paid to the triggering of the superconducting state by the ferromagnetic order. The ground states of these superconductors are outlined and discussed. The idea about a possible coexistence of ferromagnetism and spin-triplet superconductivity in neutron stars is introduced.

astro-ph↗