arXiv subjects

Jose Lorenzana

Publications and source records attributed to Jose Lorenzana.

17 recordsLinked to original sources

arXiv2025

Ternary nickel hydrides: a new platform for unconventional superconductivity and quantum magnetism

In this letter, we propose ternary nickel hydrides MNiH2 (M = Li, Na) as new materials that mimic cuprate physics but have important differences and interesting properties. Ni-H bands are wider than in oxides due to shorter bond lengths and covalency is larger than in Ni oxides which leads to a large scale of magnetic interactions. The charge transfer energy is smaller than in LaNiO2 which in cuprates translates to a larger Tc. We notice the formation of the electride band close to the Fermi surface which appears due to H vacancy along the c lattice vector. The considerable difference with cuprates arises from dz2 orbitals hybridization with interstitial orbitals allowing charge transfer to an apical vacancy state and self-doping the cuprate like Ni dx2-y2 H s antibonding bands which suggests that stoichiometric NaNiH2 may already be metallic and superconducting.

cond-mat.str-el↗

arXiv2025

Violation of Pauli Limit at KTaO3(110) Interfaces

The superconducting order parameter at the KTaO3 interfaces and its dependence on interface orientation remains a subject of debate. The superconductivity at these interfaces exhibits strong resilience against in-plane magnetic field and violates Pauli limit. The interface orientation dependence of critical field and violation of Pauli limit, however, have not been investigated. To address this problem, we grew epitaxial LaMnO3/KTaO3 heterostructures using molecular beam epitaxy. We show that superconductivity is extremely robust against the in-plane magnetic field. Our results indicate that the interface orientation, despite impacting the critical temperature, does not affect the ratio of critical field to the Pauli limiting field. These results offer opportunities to engineer superconductors which are resilient against magnetic field.

cond-mat.supr-con↗

arXiv2024

Controlling Orbital Ordering of Intergrowth Structures with Flat [Ag(II)F2] Layers to Mimic Oxocuprates(II)

Based on the Density Functional Theory calculations, we propose a new pathway toward compounds featuring flat [AgF2] layers which mimic [CuO2] layers in high-temperature oxocuprate superconductor precursors. Calculations predict the dynamic (phonon) and energetic stability of the new phases over diverse substrates. For some compounds with ferro orbital ordering, we find a gigantic intrasheet superexchange constant of up to minus 211 meV (DFT+U) and minus 256 meV (SCAN), calculated for hypothetical (CsMgF3)2KAgF3 intergrowth. Semiempirical calculations show that at optimum doping, the expected superconducting critical temperature should reach 200 K. The partial substitution of K+ with Ba2+ leads to noticeable electron doping of [AgF2] sublattice, as revealed by progressive population of the Upper-Hubbard band. On the other hand, modest 10 to 15% hole-doping through partial substitution of Mg2+ with Li+, primarily leads to the depopulation of p(z) orbitals of apical F atoms. We also find structures with an undesired antiferrodistortive structural ordering and discuss the structural factors that determine the transition from buckled to flat planes and from different types of orbital ordering using Landau theory of phase transitions.

cond-mat.mtrl-sci↗

arXiv2024

Rearrangement of orbitals in KAgF3 due to Kugel-Khomskii mechanism: a Neutron diffraction and Density Functional Theory study

The crystal structure of KAgF3 was studied by powder neutron diffraction. KAgF3 exhibits at all temperatures an orthorhombic symmetry in space group Pnma that allows for several distortions with respect to the ideal cubic perovskite structure. At all temperatures there is a strong splitting of Ag-F distances parallel to the a,c planes that documents alternating occupation of holes in x2-y2 and z2-y2 orbitals. The orientation of the octahedron elongation and thereby the orbital order flips at a structural phase transition occurring around Ts=240K which is accompanied by a suppression of magnetic susceptibility. The orbital ordering is further enhanced in the low-temperature phase and the twisting of the AgF4 plaquettes forming the antiferromagnetic chains changes. DFT calculations show an enhancement of the magnetic interaction in the low temperature phase indicating that the transition and the orbital order are partially driven by the Kugel-Khomskii mechanism.

cond-mat.str-el↗

arXiv2023

Thermodynamic phase diagram of the competition between superconductivity and charge order in cuprates

We argue that there is a special doping point in the phase diagram of cuprates, such that the condensation of holes into a charge-ordered and into a superconducting phase are degenerate in energy but with an energy barrier in between. We present a Monte Carlo simulation of this problem without and with quenched disorder. While in the clean case charge order and superconductivity are separated by a first-order line which is nearly independent of temperature, in the presence of quenched disorder, charge order is fragmented into domains separated by superconducting filaments reminiscent of the supersolid behavior in $^4$He. The resulting phase diagram is in good agreement with the experiments.

cond-mat.supr-con↗

arXiv2023

Enhanced Critical Field of Superconductivity at an Oxide Interface

The nature of superconductivity and its interplay with strong spin-orbit coupling at the KTaO3(111) interfaces remains a subject of debate. To address this problem, we grew epitaxial LaMnO3/KTaO3(111) heterostructures. We show that superconductivity is robust against the in-plane magnetic field, with the critical field of superconductivity reaching 25 T in optimally doped heterostructures. The superconducting order parameter is highly sensitive to carrier density. We argue that spin-orbit coupling drives the formation of anomalous quasiparticles with vanishing magnetic moment, providing the condensate significant immunity against magnetic fields beyond the Pauli paramagnetic limit. These results offer design opportunities for superconductors with extreme resilience against magnetic field.

cond-mat.supr-con↗

arXiv2023

Generalized Rashba Electron-Phonon Coupling and Superconductivity in Strontium Titanate

SrTiO3 is known for its proximity to a ferroelectric phase and for showing an 'optimal' doping for superconductivity with a characteristic dome-like behaviour resembling systems close to a quantum critical point. Several mechanisms have been proposed to link these phenomena, but the abundance of undetermined parameters prevents a definite assessment. Here, we use ab initio computations supplemented with a microscopic model to study the linear coupling between conduction electrons and the ferroelectric soft transverse modes allowed in the presence of spin-orbit coupling. We find a robust Rashba-like coupling, which can become surprisingly strong for particular forms of the polar eigenvector. We characterize this sensitivity for general eigenvectors and, for the particular form deduced by hyper-Raman scattering experiments, we find a BCS pairing coupling constant of the right order of magnitude to support superconductivity. The ab initio computations enable us to go beyond the linear-in-momentum conventional Rashba-like interaction and naturally explain the dome behaviour including a characteristic asymmetry. The dome is attributed to a momentum dependent quenching of the angular momentum due to a competition between spin-orbit and hopping energies. The optimum density for having maximum Tc results in rather good agreement with experiments without free parameters. These results make the generalized Rashba dynamic coupling to the ferroelectric soft mode a compelling pairing mechanism to understand bulk superconductivity in doped SrTiO3.

cond-mat.supr-con↗

arXiv2022

Anisotropic Rashba coupling to polar modes in KTaO3

Motivated by the discovery of superconductivity in KTaO3-based heterostructures, we study a pairing mechanism based on spin-orbit assisted coupling between the conduction electrons and the ferroelectric modes present in the material. We use ab initio frozen-phonon computations to show a linear-in-momentum Rashba-like coupling with a strong angular dependence in momentum for the lower j=3/2 manifold, deviating from the conventional isotropic Rashba model. This implies the Rashba-like interaction with the polar modes has substantial L=3 cubic harmonic corrections, which we quantify for each electronic band. The strong anisotropy of the Rashba interaction is captured by a microscopic toy model for the t2g electrons. We find its origin to be the angular dependence in electronic momentum imposed by the kinetic term on the degenerate j=3/2 manifold. A comparison between the toy model and ab initio results indicate that additional symmetry allowed terms beyond odd-parity spin-conserving inter-orbital hopping processes are needed to describe the Rashba-like polar interaction between the electrons and the soft ferroelectric mode.

cond-mat.supr-con↗

arXiv2022

Theory of superconductivity mediated by Rashba coupling in incipient ferroelectrics

Experimental evidence suggests that superconductivity in SrTiO$_3$ is mediated by a soft transverse ferroelectric mode which, according to conventional theories, has negligible coupling with electrons. A phenomenological Rashba type coupling has been proposed on symmetry arguments but a microscopic derivation is lacking. Here we fill this gap and obtain a linear coupling directly from a minimal microscopic model of the electronic structure. We find that the effective electron-electron pairing interaction has a strong momentum dependence. This yields an unusual situation in which the leading $s$-wave channel is followed by a sub-leading $p$-wave state which shows a stronger pairing instability than the $d$-wave state. The bare Rashba coupling constant is estimated for the lowest band of doped SrTiO$_3$ with the aid of first-principles computations. Extrapolating the estimation to the multi-band regime we show that it can explain bulk superconductivity. For densities where the Meissner effect has not been observed an extra softening due to inhomogeneities is needed to explain the zero resistance state.

cond-mat.supr-con↗

arXiv2021

Multiple-magnon excitations shape the spin spectrum of cuprate parent compounds

Thanks to high resolution and polarization analysis, resonant inelastic x-ray scattering (RIXS) magnetic spectra of La2CuO4, Sr2CuO2Cl2 and CaCuO2 reveal a rich set of properties of the spin 1/2 antiferromagnetic square lattice of cuprates. The leading single-magnon peak energy dispersion is in excellent agreement with the corresponding inelastic neutron scattering measurements. However, the RIXS data unveil an asymmetric lineshape possibly due to odd higher order terms. Moreover, a sharp bimagnon feature emerges from the continuum at (1/2,0), coincident in energy with the bimagnon peak detected in optical spectroscopy. These findings show that the inherently complex spin spectra of cuprates, an exquisite manifestation of quantum magnetism, can be effectively explored by exploiting the richness of RIXS cross sections.

cond-mat.supr-con↗

arXiv2020

Fate of doped carriers in silver fluoride cuprate analogues

AgF$_2$ is a correlated charge-transfer insulator with properties remarkably similar to insulating cuprates which have raised hope that it may lead to a new family of unconventional superconductors upon doping. We use ab initio computations to study doping strategies leading to metallization. Because the upper Hubbard band is very narrow electron doping leads to undesired strongly self-trapped states (polarons). For the hole-doped case, polaron tendency is stronger than for cuprates but still moderate enough to expect that heavily doped compounds may become metallic. Since the strong electron lattice coupling originates in the strong buckling we study also an hypothetically flat allotrope and show that it has excellent prospect to become metallic. We compare the AgF2 behavior with that for the hole-doped conventional cuprate La$_2$CuO$_4$ and electron-doped Nd$_2$CuO$_4$. Our results show a clear path to achieve high temperature superconductivity in silver fluorides.

cond-mat.str-el↗

arXiv2020

Gigantic work function in layered AgF2

AgF2 is a layered material and a correlated charge transfer insulator with an electronic structure very similar to the parent compounds of cuprate high-Tc superconductors. It is also interesting for being a powerful oxidizer. Here we present a first principles computation of its electronic properties in a slab geometry including its work function for the (010) surface (7.76 eV) which appears to be one of the highest among known materials surpassing even that of fluorinated diamond (7.24 eV). We demonstrate that AgF2 will show a broken-gap type alignment becoming electron doped and promoting injection of holes in many wide band gap insulators if chemical reaction can be avoided. Novel junction devices involving p type and n type superconductors are proposed. The issue of chemical reaction is discussed in connection with the possibility to create flat AgF2 monolayers achieving high-Tc superconductivity. As a first step in this direction, we study the stability and properties of an isolated AgF2 monolayer.

cond-mat.mtrl-sci↗

arXiv2020

Epitaxial engineering of flat silver fluorides cuprate analogs

As-grown AgF2 has a remarkably similar electronic structure as insulating cuprates, but it is extremely electronegative, which makes it hard to handle and dope. Furthermore, buckling of layers reduces magnetic interactions and enhances unwanted self-trapping lattice effects. We argue that epitaxial engineering can solve all these problems. By using a high throughput approach and first principle computations, we find a set of candidate substrates which can sustain the chemical aggressiveness of AgF2 and at the same time have good lattice parameter matching for heteroepitaxy, enhancing AgF2 magnetic and transport properties and opening the possibility of field-effect carrier injection to achieve a new generation of high-Tc superconductors. Assuming a magnetic mechanism and extrapolating from cuprates we predict that the superconducting critical temperature of a single layer can reach 195 K.

cond-mat.str-el↗

arXiv2016

Phase nucleation in curved space

Nucleation and growth is the dominant relaxation mechanism driving first order phase transitions. In two-dimensional at systems nucleation has been applied to a wide range of problems in physics, chemistry and biology. Here we study nucleation and growth of two-dimensional phases lying on curved surfaces and show that curvature modify both, critical sizes of nuclei and paths towards the equilibrium phase. In curved space nucleation and growth becomes inherently inhomogeneous and critical nuclei form faster on regions of positive Gaussian curvature. Substrates of varying shape display complex energy landscapes with several geometry-induced local minima, where initially propagating nuclei become stabilized and trapped by the underlying curvature.

cond-mat.soft↗

arXiv2010

High-Tc ferrolectricity emerging from magnetic degeneracy in cupric oxide

Cupric oxide is multiferroic at unusual high temperatures. From density functional calculations we find that the low-T magnetic phase is paraelectric and the higher-T one ferroelectric, with a size and direction of polarization in good agreement with experiment. By mapping of the ab initio results on an effective spin model we find with Monte Carlo that in the high-T magnetic state non-collinearity and inversion symmetry breaking stabilize each other via the Dzyaloshinskii Moriya interaction. This leads to a novel mechanism for multiferroicity, with the particular property that non-magnetic impurities enhance the effect.

cond-mat.str-el↗

arXiv2005

Viscoelasticity and metastability limit in supercooled liquids

A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp exists below which the liquid relaxation time exceeds the crystal nucleation time. We revisit classical nucleation theory taking into account the viscoelastic response of the liquid to the formation of crystal nuclei and find that the kinetic spinodal is strongly influenced by elastic effects. We introduce a dimensionless parameter λ, which is essentially the ratio between the infinite frequency shear modulus and the enthalpy of fusion of the crystal. In systems where λis larger than a critical value λ_c the metastability limit is totally suppressed, independently of the surface tension. On the other hand, if λ< λ_c a kinetic spinodal is present and the time needed to experimentally observe it scales as exp[ω/(λ_c-λ)^2], where ωis roughly the ratio between surface tension and enthalpy of fusion.

cond-mat.stat-mech↗

arXiv2000

The induced charge in a Frohlich polaron: Sum rule and spatial extent

Within the path integral formalism, we derive exact expressions for correlation functions measuring the lattice charge induced by an electron and associated polarization in Frohlich polaron problem. We prove that a sum rule for the total induced charge, already obtained within approximated approaches is indeed exact. As a consequence the total induced charge is shown rigorously to be temperature independent. In addition we perform path integral Monte Carlo calculations of the correlation functions and we compare with variational results based on Feynman method. As the temperature increases the polaron radius decreases. On the other hand at high temperatures the electron motion is not hindered by the lattice. These apparently contradictory results are discussed.

cond-mat.str-el↗