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Mario Cuoco

Publications and source records attributed to Mario Cuoco.

At least 109 records · Page 6Linked to original sources
arXiv2015

Analogy between Jahn-Teller distortion and Rashba spin splitting, and Jahn-Teller counterpart of spin texture

In developing physical theories analogical reasoning has been found to be very powerful, as attested by a number of important historical examples. An analogy between two apparently different phenomena, once established, allows one to transfer information and bring new concepts from one phenomenon to the other. Here we discuss an important analogy between two widely different physical problems, namely, the Jahn-Teller distortion in molecular physics and the Rashba spin splitting in condensed matter physics. By exploring their conceptual and mathematical features and by searching for the counterparts between them, we examine the orbital texture in Jahn-Teller systems, as the counterpart of the spin texture of the Rashba physics, and put forward a possible way of experimentally detecting the orbital texture. Finally, we discuss the analogy by comparing the coexistence of linear Rashba+Dresselhaus effects and Jahn-Teller problems for specific symmetries, which allow for non-trivial spin and orbital textures, respectively.

cond-mat.mtrl-sci↗
arXiv2015

Novel Spin-Orbital Phases Induced by Orbital Dilution

We demonstrate that magnetic $3d$ impurities with $S=3/2$ spins and no orbital degree of freedom induce changes of spin-orbital order in a $4d^4$ Mott insulator with $S=1$ spins. Impurities act either as spin defects which decouple from the surrounding ions, or trigger orbital polarons along $3d$-$4d$ bonds. The $4d$-$4d$ superexchange in the host $J_{\rm host}$ competes with $3d$-$4d$ superexchange $J_{\rm imp}$ --- it depends on which orbital is doubly occupied. The spin-orbital order within the host is totally modified at doping $x=1/4$. Our findings provide new perspective for future theoretical and experimental studies of doped transition-metal oxides.

cond-mat.str-el↗
arXiv2015

Edge states and topological insulating phases generated by curving a nanowire with Rashba spin-orbit coupling

We prove that curvature effects in low-dimensional nanomaterials can promote the generation of topological states of matter by considering the paradigmatic example of quantum wires with Rashba spin-orbit coupling, which are periodically corrugated at the nanometer scale. The effect of the periodic curvature generally results in the appearance of insulating phases with a corresponding novel butterfly spectrum characterized by the formation of fine measure complex regions of forbidden energies. When the Fermi energy lies in the gaps, the system displays localized end states protected by topology. We further show that for certain corrugation periods the system possesses topologically non-trivial insulating phases at half-filling. Our results suggest that the local curvature and the topology of the electronic states are inextricably intertwined in geometrically deformed nanomaterials.

cond-mat.mes-hall↗
arXiv2015

Spin-Orbital Order Modified by Orbital Dilution in Transition Metal Oxides: From Spin Defects to Frustrated Spins Polarizing Host Orbitals

We study the $3d$ substitution in $4d$ transition metal oxides in the cases of $3d^3$ doping at either $3d^2$ or $4d^4$ sites which realize orbital dilution. We derive the effective $3d-4d$ (or $3d-3d$) superexchange in a Mott insulator with different ionic valencies, underlining the emerging structure of the spin-orbital coupling between the impurity and the host sites and demonstrate that it is qualitatively different from that encountered in the host itself. This derivation shows that the interaction between the host and the impurity depends in a crucial way on the type of doubly occupied $t_{2g}$ orbital. One finds that in some cases, due to the quench of the orbital degree of freedom at the $3d$ impurity, the spin and orbital order within the host is drastically modified by doping. The impurity acts either as a spin defect accompanied by an orbital vacancy in the spin-orbital structure when the host-impurity coupling is weak, or it favors doubly occupied active orbitals (orbital polarons) along the $3d-4d$ bond leading to antiferromagnetic or ferromagnetic spin coupling. This competition between different magnetic couplings leads to quite different ground states. We find that magnetic frustration and spin degeneracy can be lifted by the quantum orbital flips of the host but they are robust in special regions of the incommensurate phase diagram. The spin-orbit coupling can lead to anisotropic spin and orbital patterns along the symmetry directions and cause a radical modification of the order imposed by the spin-orbital superexchange. Our findings are expected to be of importance for future theoretical understanding of experimental results for doped $4d$ transition metal oxides doped with $3d^3$ ions. We suggest how the local or global changes of the spin-orbital order induced by such impurities could be detected experimentally.

cond-mat.str-el↗
arXiv2014

Control of Edge Currents at a Ferromagnet - Triplet Superconductor Interface by Multiple Helical Majorana Modes

We study the spin and charge currents flowing at the interface of an itinerant ferromagnet with a topological spin-triplet superconductor having different number of time-reversal-invariant Majorana helical modes. Depending on the number of helical modes, the capacity of carrying spin and charge currents is shown to be directly related to the amplitude and orientation of the ferromagnetic magnetization with respect to the superconducting $\vec{d}$-vector. Differently from the one-helical mode spin-triplet superconductor, we find that the presence of a finite amount of electronic hybridization with the two pairs of Majorana helical modes leads to nonvanishing charge current independently of the ferromagnetic exchange. The competition between the two pairs of Majorana helical modes remarkably yields a spin-current response that is almost constant in the range of weak to intermediate ferromagnetism. The behavior of the spin current is tightly linked to the direction of the spin-polarization in the ferromagnet and tends to be flatten for a magnetization that is coplanar to the spin-triplet $\vec{d}$-vector independently of the number of helical modes.

cond-mat.supr-con↗
arXiv2013

Magnetic Intragap States and Mixed Parity Pairing at the Edge of Spin-Triplet Superconductors

We show that a spontaneous magnetic moment may appear at the edge of a spin-triplet superconductor if the system allows for pairing in a subdominant channel. To unveil the microscopic mechanism behind such effect we combine numerical solution of the Bogoliubov-De Gennes equations for a tight-binding model with nearest-neighbor attraction, and the symmetry based Ginzburg-Landau approach. We find that a potential barrier modulating the electronic density near the edge of the system leads to a non-unitary superconducting state close to the boundary where spin-singlet pairing coexists with the dominant triplet superconducting order. We demonstrate that the spin polarization at the edge appears due to the inhomogeneity of the non-unitary state and originates in the lifting of the spin-degeneracy of the Andreev bound-states.

cond-mat.supr-con↗
arXiv2013

Spin-orbital coupling in a triplet superconductor-ferromagnet junction

We study a novel type of coupling between spin and orbital degrees of freedom which appears at triplet superconductor-ferromagnet interfaces. Using a self-consistent spatially-dependent mean-field theory, we show that increasing the angle between the ferromagnetic moment and the triplet vector order parameter enhances or suppresses the p-wave gap close to the interface, according as the gap antinodes are parallel or perpendicular to the boundary, respectively. The associated change in condensation energy establishes an orbitally-dependent preferred orientation for the magnetization. When both gap components are present, as in a chiral superconductor, we observe a first-order transition between different moment orientations as a function of the exchange field strength.

cond-mat.supr-con↗
arXiv2013

Curvature-induced Rashba spin-orbit interaction in strain-driven nanostructures

We derive the effective dimensionally reduced Schrödinger equation with spin-orbit interaction in low-dimensional electronic strain driven nanostructures. A method of adiabatic separation among fast normal quantum degrees of freedom and slow tangential quantum degrees of freedom is used to show the emergence of a strain-induced Rashba-like spin-orbit interaction (SOI). By applying this analysis to one-dimensional curved quantum wires we demonstrate that the curvature-induced Rashba SOI leads to enhanced spin-orbit effects.

cond-mat.mes-hall↗
arXiv2013

Proximity effects in spin-triplet superconductor-ferromagnet heterostucture with spin-active interface

We study the physical properties of a ballistic heterostructure made of a ferromagnet (FM) and a spin-triplet superconductor (TSC) with a layered structure stacking along the direction perpendicular to the planes where a chiral px+ipy pairing occurs and assuming spin dependent processes at the interface. We use a self-consistent Bogoliubov-de Gennes approach on a three-dimensional lattice to obtain the spatial profiles of the pairing amplitude and the magnetization. We find that, depending on the strength of the ferromagnetic exchange field, the ground state of the system can have two distinct configurations with a parallel or anti-parallel collinearity between the magnetic moments in the bulk and at the interface. We demonstrate that a magnetic state having non coplanar interface, bulk and Cooper pairs spins may be stabilized if the bulk magnetization is assumed to be fixed along a given direction. The study of the density of states reveals that the modification of the electronic spectrum in the FM plays an important role in the setting of the optimal magnetic configuration. Finally, we find the existence of induced spin-polarized pair correlations in the FM-TSC system.

cond-mat.supr-con↗
arXiv2012

Structural and electronic properties of Sr2RuO4-Sr3Ru2O7 heterostructure

By means of first-principles calculations we study the structural and electronic properties of a superlattice made of Sr2RuO4 and Sr3Ru2O7 ruthenate oxides. Due to the symmetry mismatch of the two systems a significant structural rearrangement occurs within the superlattice. We find that at the interface the RuO6 octahedra get elongated for the Sr2RuO4 while tend to be compressed for the Sr3Ru2O7 as compared to inner layers and the bulk phases. The positions of the Sr-atoms in the Sr-O layers at the interface are strongly modified and influence the alignment of the Ru atoms with respect to the planar oxygens as well as the Ru-O-Ru in-plane bond angles. Such structural rearrangement leads to a modification of the electronic structure close to the Fermi level. The main changes occurring at the interface and in the inner layers of the heterostructure are analyzed and compared with the bulk phases of the Sr2RuO4 and Sr3Ru2O7 compounds. We show that the positions of the peaks in the density of states close to the Fermi levels get shifted and renormalized in the spectral weight. The balance between the renormalization of the bandwidth of the dxy band and that of the crystal field splitting results into a minor change of the Van Hove singularities position within the superlattice. The effective tight-binding parameters for the 4d Ru bands are determined by means of a maximally localized Wannier functions approach and used to discuss the modification of the electronic structure of the superlattice with respect to the bulk phases. Consequences on the modification of the superconducting and metamagnetic behaviour of the superlattice with respect to the bulk phases are discussed.

cond-mat.mtrl-sci↗
arXiv2011

Odd-frequency triplet pairing in mixed-parity superconductors

We show that mixed-parity superconductors may exhibit equal-spin pair correlations that are odd-in-time and can be tuned by means of an applied field. The direction and the amplitude of the pair correlator in the spin space turn out to be strongly dependent on the symmetry of the order parameter, and thus provide a tool to identify different types of singlet-triplet mixed configurations. We find that odd-in-time spin-polarized pair correlations can be generated without magnetic inhomogeneities in superconducting/ferromagnetic hybrids when parity mixing is induced at the interface.

cond-mat.supr-con↗
arXiv2011

Josephson effect in S/F/S junctions: spin bandwidth asymmetry vs. Stoner exchange

We analyze the dc Josephson effect in a ballistic superconductor/ferromagnet/superconductor junction by means of the Bogoliubov-de Gennes equations in the quasiclassical Andreev approximation. We consider the possibility of ferromagnetism originating from a mass renormalization of carriers of opposite spin, i.e. a spin bandwidth asymmetry. We provide a general formula for Andreev levels which is valid for arbitrary interface transparency, exchange interaction, and bandwidth asymmetry. We analyze the current-phase relation, the critical current, and the free energy in the short junction regime. We demonstrate that a larger number of $0-π$ transitions caused by a change in junction width or polarization magnitude is expected when ferromagnetism is driven by spin bandwidth asymmetry compared to Stoner magnetism, and that the same mechanism is likely responsible for the flowing of a larger critical current. We compare the phase diagrams of two identical junctions differing only in the mechanism by which the mid layer becomes magnetic, i.e. Stoner magnetism or bandwidth asymmetry. In particular, we point out that for these two cases, the phase difference across the junction in the ground state need not be the same, even if the polarizations are equal.

cond-mat.supr-con↗
arXiv2011

Does a ferromagnet with spin-dependent masses produce a spin-filtering effect in a ferromagnetic/insulator/superconductor junction?

We analyze charge transport through a ballistic ferromagnet/insulator/superconductor junction by means of the Bogoliubov-de Gennes equations. We take into account the possibility that ferromagnetism in the first electrode may be driven by a mass renormalization of oppositely polarized carriers, i.e. by a spin bandwidth asymmetry, rather than by a rigid splitting of up-and down-spin electron bands as in a standard Stoner ferromagnet. By evaluating the averaged charge conductance for both an s- and a $d_{x^2-y^2}$-wave order parameter for the S side, we show that the mass mismatch in the ferromagnetic electrode may mimic a spin active barrier. Indeed, in the $s$-wave case we show that under suitable conditions the spin dependent conductance of minority carriers below the energy gap $Δ_0$ can be larger than for majority carriers, and lower above $Δ_0$. On the other hand, for a d_{x^2-y^2}-wave superconductor similar spin-dependent effects give rise to an asymmetric peak splitting in the conductance. These results suggest that the junction may work as a spin-filtering device.

cond-mat.supr-con↗
arXiv2011

Charge and spin transport through a ferromagnet/insulator/unconventional superconductor junction

We analyze the charge and spin transport through a ballistic ferromagnet/insulator/superconductor junction by means of the Bogoliubov-de Gennes equations. For the ferromagnetic side we assume that ferromagnetism may be driven by an unequal mass renormalization of oppositely polarized carriers, i.e. a spin bandwidth asymmetry, and/or by a rigid splitting of up-and down-spin electron bands, as in a standard Stoner ferromagnet, whereas the superconducting side is assumed to exhibit a d-wave symmetry of the order parameter, which can be pure or accompanied by a minority component breaking time-reversal symmetry. Several remarkable features in the charge conductance arise in this kind of junction, providing useful information about the mechanism of ferromagnetism in the ferromagnetic electrode, as well as of the order parameter symmetry in the superconducting one. In particular, we show that when a time-reversal symmetry breaking superconductor is considered, the use of the two kinds of ferromagnet mentioned above represents a valuable tool to discriminate between the different superconducting mixed states. We also explain how this junction may mimic a switch able to turn on and off a spin current, leaving the charge conductance unchanged, and we show that for a wide range of insulating barrier strengths, a spin bandwidth asymmetry ferromagnet may support a spin current larger than a standard Stoner one.

cond-mat.supr-con↗
arXiv2010

Spin-sensitive Long-ranged Proximity Effect for Triplet Superconductors

The discovery of noncentrosymmetric superconductors, such as CePt$_3$Si, and chiral superconductors, such as Sr$_2$RuO$_4$, calls for experimental methods to identify the presence of spin-triplet pairing. We here demonstrate a method which accomplishes this in an appealingly simple manner: a spin-sensitive proximity effect in a ferromagnet$\mid$triplet superconductor bilayer. It is shown how the orientation of the field can be used to unambiguously distinguish between different spin-triplet states. Moreover, the proximity effect becomes long-ranged in spite of the presence of an exchange field and even without any magnetic inhomogeneities, in contrast to conventional S$\mid$F junctions. Our results can be verified by STM-spectroscopy and could be useful as a tool to characterize the pairing state in unconventional superconducting materials.

cond-mat.supr-con↗
arXiv2010

Spin-active interfaces and unconventional pairing in half-metal$\mid$superconductor junctions

We study the physical properties of a half-metallic ferromagnet$\mid$superconductor (HM$\mid$S) bilayer, allowing for an arbitrary bulk pairing symmetry of the superconductor and spin-dependent processes at the interface. In particular, we study how the possibility of unconventional pairing such as $p$- and d-wave and a spin-active interface influence the \textit{(i)} conductance spectra, \textit{(ii)} proximity effect, and \textit{(iii)} local density of states of such a bilayer. Our calculation is done both analytically and numerically in the ballistic limit, using both a continuum- and lattice-model. It is found that the spin-dependent phase-shifts occuring at the HM$\mid$S interface seriously influence all of the aforementioned phenomena. We explain our results in terms of Andreev reflection in the presence of a spin-active interface, allowing for both spin-filtering and spin-mixing processes. We demonstrate how the surface-bound states induced by the anisotropy of the superconducting order parameter at the HM$\mid$S interface are highly sensitive to these spin-dependent processes. Our results can be directly tested experimentally using STM-measurements and/or point-contact spectroscopy.

cond-mat.supr-con↗
arXiv2008

Coexistence of itinerant ferromagnetism and a non-unitary superconducting state with line nodes: possible application to UGe$_2$

We construct a mean-field theory for itinerant ferromagnetism coexisting with a non-unitary superconducting state, where only the majority-spin band is gapped and contains line nodes, while the minority-spin band is gapless at the Fermi level. Our study is motivated by recent experimental results indicating that this may be the physical situation realized in the heavy-fermion compound UGe$_2$. We investigate the stability of the mean-field solution of the magnetic and superconducting order parameters. Also, we provide theoretical predictions for experimentally measurable properties of such a non-unitary superconductor: the specific heat capacity, the Knight shift, and the tunneling conductance spectra. Our study should be useful for direct comparison with experimental results and also for further predictions of the physics that may be expected in ferromagnetic superconductors.

cond-mat.supr-con↗