Vortex structures of rotating Bose-Einstein condensates in anisotropic harmonic potential

We found an analytical solution for the vortex structure in a rapidly rotating trapped Bose-Einstein condensate in the lowest Landau level approximation. This solution is exact in the limit of a large number of vortices and is obtained for the case of anisotropic harmonic potential. For the case of symmetric harmonic trap when the rotation frequency is equal to the trapping frequency, the solution coincides with the Abrikosov triangle vortex lattice in type-II superconductors. In a general case the coarse grained density is found to be close to the Thomas-Fermi profile, except the vicinity of edges of a condensate cloud.Comment: 7 pages, 3 figure

Impurity induced bound states and proximity effect in a bilayer exciton condensate

The effect of impurities which induce local interlayer tunneling in bilayer exciton condensates is discussed. We show that a localized single fermion bound state emerges inside the gap for any strength of impurity scattering and calculate the dependence of the impurity state energy and wave function on the potential strength. We show that such an impurity induced single fermion state enhances the interlayer coherence around it, and is similar to the superconducting proximity effect. As a direct consequence of these single impurity states, we predict that a finite concentration of such impurities will increase the critical temperature for exciton condensation.Comment: 4 pages, 2 figure

Pressure-induced spin-state transition of iron in magnesiow\"ustite (Fe,Mg)O

We present a detailed theoretical study of the electronic, magnetic, and structural properties of magnesiow\"ustite Fe$_{1-x}$Mg$_x$O with $x$ in the range between 0$-$0.875 using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we compute the electronic structure and phase stability of the rock-salt B1-structured (Fe,Mg)O at high pressures relevant for the Earth's lower mantle. We obtain that upon compression paramagnetic (Fe,Mg)O exhibits a spin-state transition of Fe$^{2+}$ ions from a high-spin to low-spin (HS-LS) state which is accompanied by a collapse of local magnetic moments. The HS-LS transition results in a substantial drop of the lattice volume by about 4$-$8 %, implying a complex interplay between electronic and lattice degrees of freedom. Our results reveal a strong sensitivity of the calculated transition pressure $P_{\rm tr.}$ upon addition of Mg. While for Fe-rich magnesiow\"ustite, Mg $x < 0.5$, $P_{\rm tr.}$ exhibits a rather weak variation at $\sim$80 GPa, for Fe-poor (Fe,Mg)O it drops, e.g., by about 35 % to 52 GPa for Mg $x=0.75$. This behavior is accompanied by a substantial change of the spin transition range from 50$-$140 GPa in FeO to 30$-$90 GPa for $x=0.75$. In addition, the calculated bulk modulus (in the HS state) is found to increase by $\sim$12 % from 142 GPa in FeO to 159 GPa in (Fe,Mg)O with Mg $x=0.875$. We find that the pressure-induced HS-LS transition has different consequences for the electronic properties of the Fe-rich and poor (Fe,Mg)O. For the Fe-rich (Fe,Mg)O, the transition is found to be accompanied by a Mott insulator to (semi-) metal phase transition. In contrast to that, for $x>0.25$, (Fe,Mg)O remains insulating up to the highest studied pressures, implying a Mott insulator to band insulator phase transition at the HS-LS transformation.Comment: 9 pages, 9 figure

Effect of disorder on a Pomeranchuk instability

We study the effect of disorder on the order parameter equation and transition temperature of a Pomeranchuk-type Fermi-surface instability using replica mean field theory. We consider the example of a phase transition to a $d_{x^2 +y^2}$ type Fermi surface distortion, and show that, in the regime where such a transition is second order, the transition temperature is reduced by disorder in essentially the same way as that for a d-wave superconductor. We argue that observing this disorder dependence of metal-to-metal transition might be a useful indicator of a finite angular momentum Fermi surface distortion.Comment: 4.1 pages, 3 figs. Version as published in EPL. Added data of Sr3Ru2O7 to theory curves of Fig.2, and some clarification of derivation of result

Magnetic Collapse and the Behavior of Transition Metal Oxides at High Pressure

We report a detail theoretical study of the electronic structure and phase stability of transition metal oxides MnO, FeO, CoO, and NiO in their paramagnetic cubic B1 structure by employing dynamical mean-field theory of correlated electrons combined with \emph{ab initio} band structure methods (DFT+DMFT). Our calculations reveal that under pressure these materials exhibit a Mott insulator-metal transition (IMT) which is accompanied by a simultaneous collapse of local magnetic moments and lattice volume, implying a complex interplay between chemical bonding and electronic correlations. Moreover, our results for the transition pressure show a monotonous decrease from ~ 145 GPa to 40 GPa, upon moving from MnO to CoO. In contrast to that, in NiO, magnetic collapse is found to occur at remarkably higher pressure of ~ 429 GPa. We provide a unified picture of such a behavior and suggest that it is primary a localized to itinerant moment behavior transition at the IMT that gives rise to magnetic collapse in transition metal oxides.Comment: 6 pages, 3 figure

Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN

We use first-principles calculations to study the effect of magnetic disorder and electron correlations on the structural and thermodynamic properties of CrN. We illustrate the usability of a special quasirandom structure supercell treatment of the magnetic disorder by comparing with coherent potential approximation calculations and with a complementary magnetic sampling method. The need of a treatment of electron correlations effects beyond the local density approximation is proven by a comparison of LDA+U calculations of structural and electronic properties with experimental results. When magnetic disorder and strong electron correlations are taken into account simultaneously, pressure and temperature induced structural and magnetic transitions in CrN can be understood.Comment: 23 pages, 7 figure

Cyclotron resonance and Faraday rotation in graphite

The optical conductivity of graphite in quantizing magnetic fields is analytically evaluated for frequencies in the range of 10--300 meV, where the electron relaxation processes can be neglected and the low-energy excitations at the "Dirac lines" are more essential. The conductivity peaks are explained in terms of the electron transitions in graphite. Conductivity calculated per one graphite layer tends on average to the universal conductivity of graphene while the frequency is larger than the Landau spacing. The (semi)metal-insulator transformation is possible under doping in high magnetic fields.Comment: 4 pages, 5 figure

Coexistence of ferromagnetism and singlet superconductivity via kinetic exchange

We propose a novel mechanism for the coexistence of metallic ferromagnetism and singlet superconductivity assuming that the magnetic instability is due to kinetic exchange. Within this scenario, the unpaired electrons which contribute to the magnetization have a positive feedback on the gain of the kinetic energy in the coexisting phase by undressing the effective mass of the carriers involved into the pairing. The evolution of the magnetization and pairing amplitude, and the phase diagram are first analyzed for a generic kinetic exchange model and then are determined within a specific case with spin dependent bond-charge occupation.Comment: 4 pages, 2 figure

Clustering in disordered ferromagnets: The Curie temperature in diluted magnetic semiconductors

We theoretically investigate impurity correlation and magnetic clustering effects on the long-range ferromagnetic ordering in diluted magnetic semiconductors, such as $\textrm{Ga}_{1-x}\textrm{Mn}_{x}\textrm{As}$, using analytical arguments and direct Monte Carlo simulations. We obtain an analytic formula for the ferromagnetic transition temperature $T_{c}$ which becomes asymptotically exact in the strongly disordered, highly dilute (i.e. small $x$) regime. We establish that impurity correlations have only small effects on $T_{c}$ with the neutrally correlated random disorder producing the nominally highest $T_{c}$. We find that the ferromagnetic order is approached from the high temperature paramagnetic side through a random magnetic clustering phenomenon consistent with the percolation transition scenario.Comment: 5 pages, 4 figure

Surface impedance of superconductors with magnetic impurities

Motivated by the problem of the residual surface resistance of the superconducting radio-frequency (SRF) cavities, we develop a microscopic theory of the surface impedance of s-wave superconductors with magnetic impurities. We analytically calculate the current response function and surface impedance for a sample with spatially uniform distribution of impurities, treating magnetic impurities in the framework of the Shiba theory. The obtained general expressions hold in a wide range of parameter values, such as temperature, frequency, mean free path, and exchange coupling strength. This generality, on the one hand, allows for direct numerical implementation of our results to describe experimental systems (SRF cavities, superconducting qubits) under various practically relevant conditions. On the other hand, explicit analytical expressions can be obtained in a number of limiting cases, which makes possible further theoretical investigation of certain regimes. As a feature of key relevance to SRF cavities, we show that in the regime of "gapless superconductivity" the surface resistance exhibits saturation at zero temperature. Our theory thus explicitly demonstrates that magnetic impurities, presumably contained in the oxide surface layer of the SRF cavities, provide a microscopic mechanism for the residual resistance.Comment: 9 pages, 3 figs; v2: published versio