arXiv subjects

Aline Ramires

Publications and source records attributed to Aline Ramires.

34 records · Page 2Linked to original sources

arXiv2021

Superconductivity in disordered locally noncentrosymmetric materials: an application to CeRh$_2$As$_2$

Layered three-dimensional centrosymmetric crystals can exhibit characteristics of noncentrosymmetric materials. This happens when each individual layer alone lacks inversion, but, when combined, inversion symmetry is restored; hence the designation: locally noncentrosymmertic superconductors (LNCSs). In LNCSs, the effects of impurities and subdominant magnetic field induced pairing channels remain unexplored. Using a minimal model, we examine all pairing channels and show that there is always a subdominant superconducting instability that is favored at high magnetic fields, which can substantially alter the magnetic field -- temperature phase diagram. Also, we find that the phase diagram responds to disorder in a non-monotonic way, which can be subjected to experimental verification. We apply these ideas to the recently unveiled two-phase superconducting phase diagram of CeRh$_2$As$_2$. We identify the two phases as singlet-triplet mixed even- and odd-parity states at low and a high fields, respectively. Furthermore, we predict the presence of two superconducting phases also for in-plane magnetic fields in cleaner samples, since a high-field phase could have been so far hindered by impurity effects.

cond-mat.supr-con↗

arXiv2021

Emulating heavy fermions in twisted trilayer graphene

Twisted van der Waals materials have been shown to host a variety of tunable electronic structures. Here we put forward twisted trilayer graphene (TTG) as a platform to emulate heavy fermion physics. We demonstrate that TTG hosts extended and localized modes with an electronic structure that can be controlled by interlayer bias. In the presence of interactions, the existence of localized modes leads to the development of local moments, which are Kondo coupled to coexisting extended states. By electrically controlling the effective exchange between local moments, the system can be driven from a magnetic into a heavy fermion regime, passing through a quantum critical point. Our results put forward twisted graphene multilayers as a platform for the realization of strongly correlated heavy fermion physics in a purely carbon-based platform.

cond-mat.mes-hall↗

arXiv2021

Two scenarios for superconductivity in CeRh$_2$As$_2$

CeRh$_2$As$_2$, a non-symmorphic heavy fermion material, was recently reported to host a remarkable phase diagram with two superconducting phases. In this material, the two inequivalent Ce sites per unit cell, related by inversion symmetry, introduce a sublattice structure corresponding to an extra internal degree of freedom. Here we propose a classification of the possible superconducting states in CeRh$_2$As$_2$ from the two Ce-sites perspective. Based on the superconducting fitness analysis and the quasiclassical Eilenberger equations, we discuss two limits: Rashba spin-orbit coupling and inter-layer hopping dominated normal state. In both limits, we are able find two scenarios that generate phase diagrams in qualitative agreement with experiments: i) intra-sublattice pairing with an even-odd transition under magnetic field, and ii) inter-sublattice pairing with an odd-odd transition under magnetic field.

cond-mat.supr-con↗

arXiv2020

Stabilizing Even-Parity Chiral Superconductivity in Sr$_2$RuO$_4$

Strontium ruthenate (Sr$_2$RuO$_4$) has long been thought to host a spin-triplet chiral $p$-wave superconducting state. However, the singletlike response observed in recent spin-susceptibility measurements casts serious doubts on this pairing state. Together with the evidence for broken time-reversal symmetry and a jump in the shear modulus $c_{66}$ at the superconducting transition temperature, the available experiments point towards an even-parity chiral superconductor with $k_z(k_x\pm ik_y)$-like $E_g$ symmetry, which has consistently been dismissed based on the quasi-two-dimensional electronic structure of Sr$_2$RuO$_4$. Here, we show how the orbital degree of freedom can encode the two-component nature of the $E_g$ order parameter, allowing for a local orbital-antisymmetric spin-triplet state that can be stabilized by on-site Hund's coupling. We find that this exotic $E_g$ state can be energetically stable once a complete, realistic three-dimensional model is considered, within which momentum-dependent spin-orbit coupling terms are key. This state naturally gives rise to Bogoliubov Fermi surfaces.

cond-mat.supr-con↗

arXiv2019

Superconducting order parameter of Sr$_2$RuO$_4$: a microscopic perspective

The character of the superconducting phase of Sr$_2$RuO$_4$, is topic of a longstanding discussion. The classification of the symmetry allowed order parameters has relied on the tetragonal symmetry of the lattice and on cylindrical Fermi surfaces, usually taken to be featureless, not including the non-trivial symmetry aspects related to their orbital content. Here we show how the careful account of the orbital degree of freedom in Sr$_2$RuO$_4$, leads to a much richer classification of order parameters. We analyse the stability and degeneracy of these new order parameters from the perspective of the concept of superconducting fitness and propose a new best order parameter candidate.

cond-mat.supr-con↗

arXiv2019

Generalized Anderson's theorem for superconductors derived from topological insulators

A well-known result in unconventional superconductivity is the fragility of nodal superconductors against nonmagnetic impurities. Despite this common wisdom, Bi$_2$Se$_3$-based topological superconductors have recently displayed unusual robustness against disorder. Here we provide a theoretical framework which naturally explains what protects Cooper pairs from strong scattering in complex superconductors. Our analysis is based on the concept of superconducting fitness and generalizes the famous Anderson's theorem into superconductors having multiple internal degrees of freedom. For concreteness, we report on the extreme example of the Cu$_x$(PbSe)$_5$(Bi$_2$Se$_3$)$_6$ superconductor, where thermal conductivity measurements down to 50 mK not only give unambiguous evidence for the existence of nodes, but also reveal that the energy scale corresponding to the scattering rate is orders of magnitude larger than the superconducting energy gap. This provides a most spectacular case of the generalized Anderson's theorem protecting a nodal superconductor.

cond-mat.supr-con↗

arXiv2019

Impurity-induced triple point fermions in twisted bilayer graphene

Triple point fermions are elusive electronic excitations that generalize Dirac and Weyl modes beyond the conventional high energy paradigm. Yet, finding real materials naturally hosting these excitations at the Fermi energy has remained challenging. Here we show that twisted bilayer graphene is a versatile platform to realize robust triple point fermions in two dimensions. In particular, we establish that the introduction of localized impurities lifts one of the two degenerate Dirac cones, yielding triple point fermions at charge neutrality. Furthermore, we show that the valley polarization is preserved for certain impurity locations in the moire supercell for both weak and strong impurity potentials. We finally show that in the presence of interactions, a symmetry broken state with local magnetization can develop out of the triple point bands, which can be selectively controlled by electrostatic gating. Our results put forward twisted bilayer graphene as a simple solid-state platform to realize triple point fermions at charge neutrality, and demonstrate the non-trivial role of impurities in moire systems.

cond-mat.mes-hall↗

arXiv2018

Tailoring $T_c$ by symmetry principles: The concept of Superconducting Fitness

We propose a generalization of the concept of superconducting fitness, which allows us to make statements analogous to Anderson's theorems concerning the stability of different superconducting states. This concept can be applied to complex materials with several orbital, layer, sublattice or valley degrees of freedom. The superconducting fitness parameters $F_A(\bf{k})$ and $F_C(\bf{k})$ give a direct measure of the robustness of the weak coupling instability and of the presence of detrimental terms in the Hamiltonian, respectively. These two parameters can be employed as a guide to engineer normal state Hamiltonians in order to favour or suppress superconducting order parameters with different symmetries and topological properties. To illustrate the applicability and power of this concept we study three cases: the non-centrosymmetric heavy fermion $\text{CePt}_3\text{Si}$, the hole doped iron pnictide $\text{KFe}_2\text{As}_2$ and the doped topological insulator $\text{Cu}_x\text{Bi}_2\text{Se}_3$.

cond-mat.supr-con↗

arXiv2017

The Symplectic Fermi Liquid and its realization in cold atomic systems

In this work we study a system of interacting fermions with large spin and SP(N) symmetry. We contrast their behaviour with the case of SU(N) symmetry by analysing the conserved quantities and the dynamics in each case. We also develop the Fermi liquid theory for fermions with SP(N) symmetry. We find that the effective mass and inverse compressibility are always enhanced in the presence of interactions, and that the N-dependence of the enhancement is qualitatively different in distinct parameter regimes. The Wilson ratio can be enhanced, indicating that the system can be made closer to a magnetic instability, in contrast to the SU(N) scenario. We conclude discussing what are the experimental routes to SP(N) symmetry within cold atoms and the exciting possibility to realize physics in higher dimensions in these systems.

cond-mat.other↗

arXiv2016

A note on the upper critical field of Sr$_2$RuO$_4$ under strain

In the light of the recently discussed mechanism for suppression of superconductivity in multi-orbital systems called inter-orbital effect, here we extend our analysis of the upper critical field in Sr$_2$RuO$_4$ now under strain. We show that the presence of the standard orbital effect and the new mechanism can consistently account for the qualitative changes observed in the upper critical field for the strained system, in particular, combining the overall enhancement of the critical field with the reduction in the anisotropy. The proposed picture holds for a triplet superconducting state, showing that a singlet state is not the only possibility to account for the observations. We suggest further experiments in order to clarify our understanding about the superconducting phase of Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2016

Identifying detrimental effects for multi-orbital superconductivity - Application to Sr$_2$RuO$_4$

We propose a general scheme to probe the compatibility of arbitrary pairing states with a given normal state Hamiltonian by the introduction of a concept called superconducting fitness. This new quantity gives a direct measure of the suppression of the superconducting critical temperature in presence of key symmetry breaking fields. A merit of the superconducting fitness is that it can be used as a tool to identify non-trivial mechanisms to suppress superconductivity under various external influences, in particular, magnetic fields or distortions, even in complex multi-orbital systems. In the light of this new concept we analyse the multi-band superconductor Sr$_2$RuO$_4$ and propose a new mechanism for the suppression of superconductivity in multi-orbital systems, which we call inter-orbital effect, as a possible explanation for the unusual limiting feature observed in the upper critical field of this material.

cond-mat.supr-con↗

arXiv2015

Supersymmetric approach to heavy fermion systems

We propose a generalization of the supersymmetric representation of spins with symplectic symmetry, generalizing the rotation group of the spin from SU(2) to SP(N). As a test application of this new representation, we consider two toy models involving a competition of the Kondo effect and antiferromagnetism: a two-impurity model and a frustrated three-impurity model. Exploring an ensemble of L-shaped representations with a fixed number of boxes in their respective Young tableaux, we allow the system to choose which representation is energetically more favorable in each region in parameter space. We discuss how the features of these preliminary applications can generalize to Kondo lattice models.

cond-mat.str-el↗

arXiv2014

Theory of the Electron Spin Resonance in the Heavy Fermion Metal β-YbAlB4

The heavy fermion metal β-YbAlB4 exhibits a bulk room temperature conduction electron ESR signal which evolves into an Ising-anisotropic f-electron signal exhibiting hyperfine features at low temperatures. We develop a theory for this phenomenon based on the development of resonant scattering off a periodic array of Kondo centers. We show that the hyperfine structure arises from the scattering off the Yb atoms with non zero nuclear spin, while the constancy of the ESR intensity is a consequence of the presence of CEF excitations of the order of the hybridization strength.

cond-mat.str-el↗

arXiv2012

β-YbAlB4: a critical nodal metal

We propose a model for the intrinsic quantum criticality of β-YbAlB4, in which a vortex in momentum space gives rise to a new type of Fermi surface singularity. The unquenched angular momentum of the |J = 7/2,m_J = \pm5/2> Yb 4f-states generates a momentum-space line defect in the hybridization between 4f and conduction electrons, leading to a quasi-two dimensional Fermi surface with a k\perp4 dispersion and a singular density of states proportional to E^{-1/2}. We discuss the implications of this line-node in momentum space for our current understanding of quantum criticality and its interplay with topology.

cond-mat.str-el↗

arXiv2010

Quantum criticality in inter-band superconductors

In fermionic systems with different types of quasi-particles, attractive interactions can give rise to exotic superconducting states, as pair density wave (PDW) superconductivity and breached pairing. In the last years the search for these new types of ground states in cold atom and in metallic systems has been intense. In the case of metals the different quasi-particles may be the up and down spin bands in an external magnetic field or bands arising from distinct atomic orbitals that coexist at a common Fermi surface. These systems present a complex phase diagram as a function of the difference between the Fermi wave-vectors of the different bands. This can be controlled by external means, varying the density in the two-component cold atom system or, in a metal, by applying an external magnetic field or pressure. Here we study the zero temperature instability of the normal system as the Fermi wave-vectors mismatch of the quasi-particles (bands) is reduced and find a second order quantum phase transition to a PDW superconducting state. From the nature of the quantum critical fluctuations close to the superconducting quantum critical point (SQCP), we obtain its dynamic critical exponent. It turns out to be $z=2$ and this allows to fully characterize the SQCP for dimensions $d \ge 2$.

cond-mat.str-el↗

arXiv2010

Fluctuations in a superconducting quantum critical point of multi-band metals

In multi-band metals quasi-particles arising from different atomic orbitals coexist at a common Fermi surface. Superconductivity in these materials may appear due to interactions within a band (intra-band) or among the distinct metallic bands (inter-band). Here we consider the suppression of superconductivity in the intra-band case due to hybridization. The fluctuations at the superconducting quantum critical point (SQCP) are obtained calculating the response of the system to a fictitious space and time dependent field, which couples to the superconducting order parameter. The appearance of superconductivity is related to the divergence of a generalized susceptibility. For a single band superconductor this coincides with the \textit{Thouless criterion}. For fixed chemical potential and large hybridization, the superconducting state has many features in common with breached pair superconductivity with unpaired electrons at the Fermi surface. The T=0 phase transition from the superconductor to the normal state is in the universality class of the density-driven Bose-Einstein condensation. For fixed number of particles and in the strong coupling limit, the system still has an instability to the normal sate with increasing hybridization.

cond-mat.str-el↗