arXiv subjects

Miodrag L. Kulic

Publications and source records attributed to Miodrag L. Kulic.

At least 19 recordsLinked to original sources

arXiv2021

High Pressure RTSC-Hydrides are Extreme Hard Type-II Superconductors

It is argued that most of the RTSC hydrides are intrinsic hard type-II superconductors with strong pinning effects. The pinning centers are long columnar-like defects, with the radius of the order of the superconducting coherence length. The core and electromagnetic pinning are both equally operative, thus giving maximal pinning potential when vortices are oriented along the columnar axis. The theory predicts: (i) large critical currents, (ii) huge decrease of the temperature broadening of the resistance in magnetic field compared with standard superconductors, (iii) large magnetization hysteresis, (iv) the magnetic irreversible line is pushed toward the second critical field and magnetization relaxation is much slower. That the RTSC-hydrides are hard type-II SC can give rise to new physics in these materials.

cond-mat.supr-con↗

arXiv2017

Comment on paper arXiv:17070526v1 "Electronic structure of FeSe monolayer superconductors: shallow bands and correlations" by Sadovskii group

We comment two incorrect statements given in Ref.1. (A) - In order to show that the electron-phonon interaction (EPI) is very small and irrelevant for superconductivity in 1UC FeSe/SrTiO3 system, the authors of Ref.1 use an EPI coupling constant which does not enter in any theory of superconductivity. So, their conclusion on the smallness of the EPI in 1UC FeSe/SrTiO3 is incorrect. Accordingly, their coupling constant has also nothing to do with the EPI coupling with the forward scattering peak (EPI-FSP), which is proposed recently in order to explain high Tc in 1UC FeSe/SrTiO3. (B) - In Ref.1 it is claimed that the experimentally resolved ARPES replica bands can be explained by the LDA+DMFT method. We show that this statement is also incorrect, i.e. the LDA+DMFT method in Ref.1 is unable to explain the replica bands.

cond-mat.supr-con↗

arXiv2014

Theory of Coherent Van der Waals Matter

We explain in depth the previously proposed theory of the coherent Van der Waals(cVdW) interaction - the counterpart of Van der Waals (VdW) force - emerging in spatially coherently fluctuating electromagnetic fields. We show that cVdW driven matter is dominated by many body interactions, which are significantly stronger than those found in standard Van der Waals (VdW) systems. Remarkably, the leading 2- and 3-body interactions are of the same order with respect to the distance $(\propto R^{-6})$, in contrast to the usually weak VdW 3-body effects ($\propto R^{-9}$). From a microscopic theory we show that the anisotropic cVdW many body interactions drive the formation of low-dimensional structures such as chains, membranes and vesicles with very unusual, non-local properties. In particular, cVdW chains display a logarithmically growing stiffness with the chain length, while cVdW membranes have a bending modulus growing linearly with their size. We argue that the cVdW anisotropic many body forces cause local cohesion but also a negative effective "surface tension". We conclude by deriving the equation of state for cVdW materials and propose new experiments to test the theory, in particular the unusual 3-body nature of cVdW.

cond-mat.soft↗

arXiv2013

Self-Assembly of Colloidal Superstructures in Coherently Fluctuating Fields

From microscopic fluid clusters to macroscopic droplets, the structure of fluids is governed by the Van der Waals force, a force that acts between polarizable objects. In this Letter, we derive a general theory that describes the non-equilibrium counterpart to the Van der Waals force, which emerges in spatially coherently fluctuating electromagnetic fields. We describe the formation of a novel and complex hierarchy of self-organized morphologies in magnetic and dielectric colloid systems. Most striking among these morphologies are dipolar foams - colloidal superstructures that swell against gravity and display a high sensitivity to the applied field. We discuss the dominance of many body forces and derive the equation of state for a material formed by the coherent Van der Waals force. Our theory is applied to recent experiments in paramagnetic colloidal systems and a new experiment is suggested to test the theory.

cond-mat.soft↗

arXiv2009

Possible strong electron-lattice interaction and giant magneto-elastic effects in Fe-pnictides

The possibility for an appreciable many-body contribution to the electron-phonon interaction (EPI) in Fe-pnictides is discussed in the model where EPI is due to the electronic polarization of As- ions. The EPI-pol coupling ismuch larger than the one obtained in the LDA band structure calculations. It contributes significantly to the intra-band s-wave pairing and an appreciable positive As-isotope effect in the superconducting critical temperature is expected. In the Fe-breathing mode the linear (in the Fe-displacements) EPI-pol coupling vanishes, while the non-linear (quadratic) one is very strong. The part of the EPI-pol coupling, which is due to the "potential" energy (the Hubbard U) changes, is responsible for the giant magneto-elastic effects in MFe_{2}As_{2}, M=Ca, Sr, Ba since it gives much larger contribution to the magnetic pressure than the band structure effects do. This mechanism is contrary to the LDA prediction where the magneto-elastic effects are due to the "kinetic" energy effects, i.e. the changes in the density of states by the magneto-elastic effects. The proposed $EPI-pol is expected to be operative (and strong) in other Fe-based superconductors with electronically polarizable ions such as Se, Te, S etc., and in high-temperature superconductors due to the polarizability of the O-ions.

cond-mat.supr-con↗

arXiv2009

Multiple-quantized vortices in rotating LOFF state of ultracold Fermi superfluid gas

A rotating ultracold S-wave superfluid Fermi gas is considered, when the population imbalance (or equivalently the mismatch in chemical potentials) corresponds to the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state in the vicinity of the Lifshitz critical point. It is shown that under these conditions the critical angular velocity in two-dimensional systems is an oscillating function of temperature and population imbalance giving rise to reentrant superfluid phases. This leads to vortex lattices with multiple-quantized circulation quanta. The reason for this behavior is the population by Cooper pairs of the Landau levels above the lowest one.

cond-mat.quant-gas↗

arXiv2007

ARPES Spectra of Bi2212 give the Coulomb Coupling $λ^{C}\approx 1$ and the Electron-Phonon Coupling $λ^{EP}=2-3$

We show that the double kink-structure in the electronic self-energy of Bi2212 near the nodal point at low energy $ω_{1}\approx 50-70$ $meV$ and at high energy at $ω_{2}\approx 350$ $meV$, observed recently in the ARPES measurements by Valla et al \cite{Valla-2006}, gives that the electron-phonon (EPI) coupling constant $λ_{z}^{EP}$in the normal part of the self-energy $Σ(ω)$ is twice larger than the Coulomb coupling $λ_{z}^{C}$. The experimental data for $\func{Re}Σ(ω)$ up to energies $\sim 350$ $meV$ can be satisfactory explained by $% λ_{z}^{EP}\approx 2.1$ and $λ_{z}^{C}\approx 1.1$. Additionally the low energy slope of the ARPES $\func{Re}Σ(ω)$ at $ω<20$ $meV$ \cite{Valla-2006} gives a hint that the low energy phonons might contribute significantly to the EPI coupling, i.e. $λ_{z}^{low,EP}>1$, thus giving the total EPI coupling constant $λ_{z,tot}^{EP}=λ_{z}^{EP}+λ_{z}^{low,EP}>3$. In order to test the role of low frequency phonons by ARPES measurements a much better momentum resolution is needed than that reported in \cite{Valla-2006}. Possible pairing scenarios based on ARPES, tunnelling and magnetic neutron scattering measurements are discussed.

cond-mat.supr-con↗

arXiv2006

Pi-Josephson Junction and Spontaneous Superflow in Rings from Ultracold Fermionic Atomic Gases

he BCS-like pairing in ultracold fermionic atomic ($UCFAG$) gases is studied in the model of "isotopic-spin" pairing proposed in 1991 \cite% {Ku-Hof-SSC}. This model assumes a mismatch ($δ$) in chemical potentials of pairing fermionic atoms. It is shown that a $π$-Josephson junction can be realized in $UCFAG$ systems, where the left and right banks $% S$ are the $UCFAG$ superfluids. The weak link $M$ consists from the normal $% UCFAG$ with the finite mismatch $δ$. If the $π$-junction is a part of a closed ring the superfluid mass-current flows spontaneously in the ring, i.e., the time-reversal symmetry is broken spontaneously. This is realized if the radius of the ring $R$ is larger than the critical one $% R_{c} $. All these effects exist also in the case when $δ\gg Δ$, where $Δ$ is the superfluid gap, but with the reduced thickness of the weak link. It is also discussed, that if junctions $SM_{1}M_{2}S$ and trilayers $% M_{1}SM_{2}$ from $UCFAG$ are realizable this renders a possibility for a novel electronics - \textit{hypertronics}.

cond-mat.stat-mech↗

arXiv2006

Pi-Josephson Junction and Spontaneous Superflow in Rings from Ultracold Fermionic Atomic Gases

The BCS-like pairing in ultracold fermionic atomic ($UCFAG$) gases is studied in the model of "isotopic-spin" pairing proposed in 1991 \cite% {Ku-Hof-SSC}. This model assumes a mismatch ($δ$) in chemical potentials of pairing fermionic atoms. It is shown that a $π$-Josephson junction can be realized in $UCFAG$ systems, where the left and right banks $% S$ are the $UCFAG$ superfluids. The weak link $M$ consists from the normal $% UCFAG$ with the finite mismatch $δ$. If the $π$-junction is a part of a closed ring the superfluid mass-current flows spontaneously in the ring, i.e., the time-reversal symmetry is broken spontaneously. This is realized if the radius of the ring $R$ is larger than the critical one $% R_{c} $. All these effects exist also in the case when $δ\gg Δ$, where $Δ$ is the superfluid gap, but with the reduced thickness of the weak link. It is also discussed, that if junctions $SM_{1}M_{2}S$ and trilayers $% M_{1}SM_{2}$ from $UCFAG$ are realizable this renders a possibility for a novel electronics - \textit{hypertronics}.

cond-mat.supr-con↗

arXiv2006

Importance of the Electron-Phonon Interaction with the Forward Scattering Peak for Superconducting Pairing in Cuprates

Basic experimental facts related to ARPES, tunnelling, optics ad neutron scattering measurements are discussed. They give evidence for the relevance of the electron-phonon interaction (EPI) in pairing mechanism of HTSC cuprates. A controllable theory for strong correlations and their effects on EPI is discussed which is based on the 1/N expansion method in the t-J model. Strong correlations renormalize EPI and other charge-fluctuation properties (by including nonmagnetic impurity scattering) and the forward scattering peak (FSP) appears in the effective interactions. Pronounced FSP in EPI of HTSC cuprates reconciles many puzzling results. The theory of EPI with FSP gives that the couplings in the s- and d-wave pairing channel are of the same magnitude near and below the optimal hole doping. FSP in the nonmagnetic impurity scattering potential is responsible for robustness of d-wave pairing in cuprates. The ARPES kink and the isotope effect in the nodal and anti-nodal points and the collapse of the elastic impurity scattering in the superconducting state, are explained by this theory. It also explains why the nodal kink is not-shifted in the superconducting state while the anti-nodal kink is shifted by the maximal superconducting gap. In systems with FSP in EPI besides the classical phase fluctuations there are also internal fluctuations of Cooper pairs giving rise to an additional contribution to the pseudogap.

cond-mat.supr-con↗

arXiv2005

Conventional Magnetic Superconductors: Coexistence of Singlet Superconductivity and Magnetic Order

The basic physics of bulk magnetic superconductors (MS) related to the problem of the coexistence of singlet superconductivity (SC) and magnetic order is reviewed. The interplay between exchange (EX) and electromagnetic (EM) interaction is discussed and argued that the singlet SC and uniform ferromagnetic (F) order practically never coexist. In case of their mutual coexistence the F order is modified into a domain-like or spiral structure depending on magnetic anisotropy. It turns out that this situation is realized in several superconductors such as $ErRh_{4}B_{4}$, $HoMo_{6}S_{8}$, $HoMo_{6}Se_{8}$ with electronic and in $AuIn_{2}$ with nuclear magnetic order. The later problem is also discussed here. The coexistence of SC and antiferromagnetism is more favorable than with the modified F order. Very interesting physics is realized in systems with SC and weak-ferromagnetism which results in an very reach phase diagram. The properties of magnetic superconductors in magnetic field are very peculiar, especially near the (ferro)magnetic transition temperature where the upper critical field becomes smaller than the thermodynamical critical field. The extremely interesting physics of Josephson junctions based on MS with spiral magnetic order is also discussed. The existence of the triplet pairing amplitude $F_{\uparrow \uparrow}$ ($F_{\downarrow \downarrow}$) in MS with rotating magnetization (the effect recently rediscovered in SFS junctions) gives rise to the so called $π$-contact. Furthermore, the interplay of the superconducting and magnetic phase in such a contact renders possibilities for a new type of coupled Josephson-qubits.

cond-mat.supr-con↗

arXiv2004

Can the bosonic coupling constant be extracted from the ARPES scattering rate in cuprate superconductors?

The recent ARPES results for the imaginary part of the self-energy obtained on a number of HTSC bismuthates \cite{Kordyuk} are analyzed. By accepting the ''Fermi-Bose'' {\it division-procedure} of the self-energy into the Fermi-liquid and bosonic parts - which is proposed in \cite{Kordyuk}, one obtains very small bosonic coupling constant $λ_{B, Im} <0.2$. If this procedure would be correct then the standard Eliashberg theory makes any bosonic mechanism of pairing irrelevant. As a consequence we are confronted with a trilemma: (1) to abandon the ``Fermi-Bose'' division-procedure \cite{Kordyuk}; (2) to abandon the Eliashberg theory; (3) to abandon the interpretation of ARPES data within the three-step model, where the ARPES intensity is proportional to the quasiparticle spectral function $A(\mathbf{k},ω)$. However, since the bosonic coupling constant extracted from the ARPES nodal kink at 70 meV \cite{Lanzara}, which measures the real part of the self-energy is much larger than the one extracted from the ARPES line-width this means that the ``Fermi-Bose'' division procedure done in \cite{Kordyuk} is ambiguous.

cond-mat.supr-con↗

arXiv2004

Electron Phonon Interaction and Strong Correlations in High-Temperature Superconductors: One can not avoid unavoidable

The important role of the electron-phonon interaction (EPI) in explaining the properties of the normal state and pairing mechanism in high-T$_{c}$ superconductors (HTSC) is discussed. A number of experimental results are analyzed such as: dynamical conductivity, Raman scattering, neutron scattering, ARPES, tunnelling measurements, isotope effect and etc. They give convincing evidence that the EPI is strong and dominantly contributes to pairing in HTSC oxides. It is argued that strong electronic correlations in conjunction with the pronounced (in relatively weakly screened materials) EPI are unavoidable ingredients for the microscopic theory of pairing in HTSC oxides. I present the well defined and controllable theory of strong correlations and the EPI. It is shown that strong correlations give rise to the pronounced \textit{forward scattering peak} in the EPI - the FSP theory. The FSP theory explains in a consistent way several (crucial) puzzles such as much smaller transport coupling constant than the pairing one ($λ_{tr}\ll λ$), which are present if one interprets the results in HTSC oxides by the old Migdal-Eliashberg theory for the EPI. The ARPES shift puzzle where the nodal kink at 70 meV is unshifted in the superconducting state, while the anti-nodal one at 40 meV is shifted can be explained at present only by the FSP theory. A number of other interesting predictions of the FSP theory are also discussed.

cond-mat.supr-con↗

arXiv2002

High-T_{c} Superconductors with AF Order: Limitations on Spin-Fluctuation Pairing Mechanism

The very intriguing antagonistic interplay of antiferromagnetism (AF) and superconductivity (SC), recently discovered in high-temperature superconductors, is studied in the framework of a microscopic theory. We explain the surprisingly large increase of the magnetic Bragg peak intensity $I_{Q}$ at $Q\sim (π,π)$ in the magnetic field $H\ll H_{c2}$ at low temperatures $0<T\ll T_{c},T_{AF}$ in $La_{2-x}Sr_{x}CuO_{4}$. Good agreement with experimental results is found. The theory predicts large anisotropy of the relative intensity $R_{Q}(H)=(I_{Q}(H)-I_{Q}(0))/I_{Q}(0)$%, i.e. $R_{Q}(H\parallel c-axis)\gg R_{Q}(H\perp c-axis)$. The quantum (T=0) phase diagram at H=0 is constructed. The theory also predicts: (i) the magnetic field induced AF order in the SC state; (ii) small value for the spin-fluctuation coupling constant $g<(0.025-0.05)$ $eV$. The latter gives very small SC critical temperature $T_{c}(\ll 40$ $K)$, thus questioning the spin-fluctuation mechanism of pairing in HTS oxides.

cond-mat.supr-con↗

arXiv2001

Decoupling of superconducting layers in magnetic superconductor RuSr_{2}GdCu_{2}O_{8}

We propose the model for magnetic properties of the magnetic superconductor RuSr$_{2}$GdCu$_{2}$O$_{8}$, which incorporates the theory of the superconducting/ferromagnetic multilayers. The transition line $T_{d}(h)$, on which the Josephson coupled superconducting planes are decoupled, i.e. $% j_{c}(T_{d})=0$, is calculated as a function of the exchange energy $h$. As the result of this decoupling a nonmonotonic behavior of magnetic properties, like the lower critical field $H_{c1}$, Josephson plasma frequency, etc. is realized near (or by crossing) the $T_{d}(h)$ line. The obtained results are used in analyzing the newly discovered antiferromagnetic ruthenocuprate RuSr$_{2}$GdCu$_{2}$O$_{8}$ with possible weak ferromagnetic order in the RuO planes.

cond-mat.supr-con↗

arXiv2000

Thermoelectric Effects in S-N-S Weak Links with Heavy Fermions as The Normal Metal: A Possibility for Thermosensors

It is shown that S-N-S weak links with an heavy-fermion metal as the weak link (N) can be useful thermosensors. This property is due to the large thermopower of heavy-fermion metals which is of the order 10^(-6) V/K. The longitudinal sound can also generate voltage oscillation due to the large Grueneisen parameter, $Ω_{hf}\sim 10^{2}$, in heavy-fermions. Similar effects are expected with other Kondo systems as the weak link.

cond-mat.supr-con↗

arXiv2000

Josephson Effect in Magnetic Superconductors with Spiral Magnetic Order

It is shown that in magnetic superconductors with spiral magnetic order the Josephson current has an additional contribution which depends: (i) on the relative orientation (magnetic phase) $θ=θ_{L}-θ_{R}$ of magnetizations on the left ($L$) and right ($R$) banks of the contact, (ii) on the junction helicity $χ=χ_{L}χ_{R}$, (with spiral helicity $χ_{L(R)}=\pm 1$), i.e. $J=[J_{c}-J_χ\cos θ]\sin ϕ$ with $ϕ=ϕ_{L}-ϕ_{R}$. The ratio $R_χ\equiv J_χ/J_{c}$ is calculated as a function of the superconducting order parameter $Δ$, the exchange field energy $h$ and the wave vector $Q$ of the spiral magnetic structure. The $π$-Josephson contact can be realized in such a system in some region of parameters. Some possible consequences of this new phase relation is also analyzed.

cond-mat.str-el↗

arXiv1999

Ferromagnetic Semiconductor - Singlet (or Triplet) Superconductor - Ferromagnetic Semiconductor Systems as Possible Logic Circuits and Switches

We consider thin superconducting (S) films of thickness d $\ll ξ_{0}$, sandwiched between two ferromagnetic semiconducting insulators (FI) with differently orientated magnetizations - the FI-S-FI system. We calculate the dependence of the superconducting critical temperature on the orientation of the magnetization in the insulators and on the thickness of the superconducting film. The calculations are done for singlet as well as triplet superconductors. In the singlet case T_{c} depends on the relative orientation of the left and right magnetization only, while in the triplet case T_{c} depends on the absolute orientation of magnetization. The latter property can serve as a kind of spin-spectroscopy of triplet and unconventional superconductors, for instance in resolving the structure of the triplet order parameter in the recently discovered layered superconductor Sr_{2}RuO_{4}. The possibility of logic circuits and switches, which are based on the FI-S-FI systems with arbitrary orientation of magnetizations in FI films, is analyzed too.

cond-mat↗