arXiv subjects

William J. Herrera

Publications and source records attributed to William J. Herrera.

At least 19 recordsLinked to original sources

arXiv2026

Visualizing impurity-driven scattering phase textures in EuCd2As2

Understanding how disorder modifies electronic states in magnetic semiconductors is important for controlling spin-dependent transport and topological responses. Here we use scanning tunneling microscopy to visualize scattering phase textures in EuCd2As2. By isolating a single surface wavevector we reconstruct spatial phase maps of the local density of states and identify phase dislocations characterized by 2pi winding around impurity sites. These phase singularities emerge systematically within charge puddles generated by Eu interstitials and their positions evolve with bias voltage. We show that their spatial structure is consistent with interference between multiple scattering channels, including contributions from spin-orbit coupling. We provide a model which reproduces phase dislocations and relates the decay of the phase gradient to the relative strength of spin-orbit and scalar scattering. Our results establish a route to access the phase of electronic scattering in real space and study the role of local disorder and spin-orbit interactions in shaping electronic states in quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Topological superconductivity in a dimerized Kitaev chain revealed by nonlocal transport

Artificial Kitaev chains engineered from semiconducting quantum dots coupled by superconducting segments offer a promising route to realize and control Majorana bound states for topological quantum computation. We study a dimerized Kitaev chain--equivalent to a superconducting Su-Schrieffer-Heeger model--and analyze the behavior of the resulting two coupled chains. We show that interference between Majorana edge modes from each chain gives rise to observable signatures in nonlocal conductance. Additionally, we identify a parity effect in the system length that governs the coupling of edge states, supported by an analytical model. Our results provide experimentally accessible probes for Majorana hybridization in mesoscopic topological superconductors.

cond-mat.mtrl-sci↗

arXiv2024

Long-range Cooper pair splitting by chiral Majorana edge states

We analyze the transport properties of a Cooper pair splitter device composed of two-point electrodes in contact with a ferromagnetic/superconductor (F/S) junction constructed on the surface of a topological insulator (TI). For the pair potential in the S region, we consider s- and d-wave symmetries, while for the F region, we focus on a magnetization vector normal to the TI surface. Non-local transport along the F/S interface is mediated by chiral Majorana edge states, with chirality controlled by the polarization of the magnetization vector. We demonstrate that crossed Andreev reflections slowly decays with the separation of the electrodes in standard clean samples. Our system exhibits a maximum Cooper pair-splitting efficiency of 80% for a symmetrical voltage configuration, even in high-temperature superconductor devices.

cond-mat.supr-con↗

arXiv2023

Dynamics of a quantum polariton vortex: Low excitation scenario

Quantum vorticity in polariton systems has been traditionally investigated within the frame of many-body phenomena under the mean-field or coherent approaches. In the present work, we show that the fully quantized picture describes richer dynamics for the vortex core at the quantum coupling limit, where two systems exchange an indivisible excitation. The quantum correlations intrinsic to our formalism account for the emergence of a family of trajectories that differ from the circular paths known in macroscopic vorticity phenomena. These results indicate that there exists a criterion to differentiate the behavior at the edge between the quantum and classical polariton vortex dynamics.

cond-mat.mes-hall↗

arXiv2023

Microscopic Green's function approach for generalized Dirac Hamiltonians

The rising interest in Dirac materials, condensed matter systems where low-energy electronic excitations are described by the relativistic Dirac Hamiltonian, entails a need for microscopic effective models to analytically describe their transport properties. Specifically, for the study of quantum transport, these effective models must take into account the effect of atomic-scale interfaces and the presence of well-defined edges while reproducing the correct band structure. We develop a general method to analytically compute the microscopic Green's function of Dirac materials valid for infinite, semi-infinite, and finite two-dimensional layers with zigzag or armchair edge orientations. We test our method by computing the density of states and scattering probabilities of germanene and some transition metal dichalcogenides, obtaining simple analytical formulas. Our results provide a useful analytical tool for the interpretation of transport experiments on Dirac materials and could be extended to describe additional degrees of freedom like extra layers, superconductivity, etc.

cond-mat.mes-hall↗

arXiv2021

Green's functions in quantum mechanics courses

Green's functions in Physics have proven to be a valuable tool for understanding fundamental concepts in different branches, such as electrodynamics, solid-state and many -body problems. In quantum mechanics advanced courses, Green's functions usually are explained in the context of the scattering problem by a central force. However, their use for more basic problems is not often implemented. The present work introduces Green's Function in quantum mechanics courses with some examples that can be solved with essential tools. For this, the general aspects of the theory are shown, emphasizing the solution of different fundamental issues of quantum mechanics from this approach. In particular, we introduce the time-independent Green's functions and the Dyson equation to solve problems with an external potential. As examples, we show the scattering by a Dirac delta barrier, where the reflection and transmission coefficients are found. In addition, the infinite square potential well energy levels, and the local density of states, are calculated.

physics.ed-ph↗

arXiv2020

A Green's function approach to Topological Insulator junctions with magnetic and superconducting regions

This work presents a Green's function approach, originally implemented in graphene with well-defined edges, to the surface of a strong 3D Topological Insulator (TI) with a sequence of proximitized superconducting (S) and ferromagnetic (F) surfaces. This consists of the derivation of the Green's functions for each region by the asymptotic solutions method, and their coupling by a tight-binding Hamiltonian with the Dyson equation to obtain the full Green's functions of the system. These functions allow the direct calculation of the momentum-resolved spectral density of states, the identification of subgap interface states, and the derivation of the differential conductance for a wide variety of configurations of the junctions. We illustrate the application of this method for some simple systems with two and three regions, finding the characteristic chiral state of the Quantum Anomalous Hall Effect (QAHE) at the NF interfaces, and chiral Majorana modes at the NS interfaces. Finally, we discuss some geometrical effects present in three-region junctions such as weak Fabry-Pérot resonances and Andreev bound states.

cond-mat.mes-hall↗

arXiv2020

One-dimensional moire charge density wave in the hidden order state of URu2Si2 induced by fracture

Moiré patterns can lead to fundamentally new electronic behavior when formed between two atomic lattices slightly shifted with respect to each other. A solid is however not just characterized by the atomic lattice, but also by charge or magnetic excitations that do not need to be commensurate to the lattice. This raises the question if one can obtain a moiré by combining periodic electronic modulations and the atomic lattice. Here we report on the discovery of a one-dimensional charge density wave (1D-CDW) which is a moiré pattern between the atomic lattice and a hot spot for electronic scattering in the bandstructure of the hidden order (HO) state of URu$_2$Si$_2$. The moiré is produced by fracturing the crystal at low temperatures. Our results suggest that charge interactions are among the most relevant features competing with HO in URu$_2$Si$_2$.

cond-mat.str-el↗

arXiv2019

Dirac point formation revealed by Andreev tunneling in superlattice-graphene/superconductor junctions

A graphene superlattice is formed by a one-dimensional periodic potential and is characterized by the emergence of new Dirac points in the electronic structure. The group velocity of graphene's massless Dirac fermions at the new points is drastically reduced, resulting in a measurable effect in the conductance spectroscopy. We show here that tunnel spectroscopy using a superconducting hybrid junction is more sensitive to the formation of Dirac points in the spectrum of graphene superlattices due to the additional contribution of Andreev processes. We examine the transport properties of a graphene-based superlattice--superconductor hybrid junction and demonstrate that a superlattice potential can coexist with proximity-induced superconducting correlations. Both effects contribute to change graphene's spectrum for subgap energies and, as a result, the normalized tunneling conductance features sharp changes for voltages proportional to the energy separation between the original and the newly generated Dirac points.

cond-mat.supr-con↗

arXiv2018

Proximity induced time-reversal topological superconductivity in Bi2Se3 films without phase tuning

Many proposals to generate a time-reversal invariant topological superconducting phase are based on imposing a $π$ phase difference between the superconducting leads proximitizing a nanostructure. We show that this phase can be induced on a thin film of a topological insulator like Bi$_2$Se$_3$ in proximity to a single s-wave superconductor. In our analysis we take into account the parity degree of freedom of the electronic states which is not included in effective Dirac-like surface theories. We find that the topological phase can be reached when the induced interparity pairing dominates over the intraparity one. Application of an electric field perpendicular to the film extends the range of parameters where the topological phase occurs.

cond-mat.supr-con↗

arXiv2018

Subgap states in two dimensional spectroscopy of unconventional superconductors using graphene

The two-dimensional nature of graphene makes it an ideal platform to explore proximity-induced unconventional planar superconductivity and the possibility of topological superconductivity. Using Green's functions techniques, we study the transport properties of a finite size ballistic graphene layer placed between a normal state electrode and a graphene lead with proximity-induced unconventional superconductivity. Our microscopic description of such a junction allows us to consider the effect of edge states in the graphene layer and the imperfect coupling to the electrodes. The tunnel conductance through the junction and the spectral density of states feature a rich interplay between graphene's edge states, interface bound states formed at the graphene-superconductor junction, Fabry-Pérot resonances originated from the finite size of the graphene layer, and the characteristic Andreev surface states of unconventional superconductors. Within our analytical formalism, we identify the separate contribution from each of these subgap states to the conductance and density of states. Our results show that graphene provides an advisable tool to determine experimentally the pairing symmetry of proximity-induced unconventional superconductivity.

cond-mat.mes-hall↗

arXiv2017

Noise cross-correlation and Cooper pair splitting efficiency in multi-teminal superconductor junctions

We analyze the non-local shot noise in a multi-terminal junction formed by two Normal metal leads con- nected to one superconductor. Using the cross Fano factor and the shot noise, we calculate the efficiency of the Cooper pair splitting. The method is applied to d-wave and iron based superconductors. We de- termine that the contributions to the noise cross-correlation are due to crossed Andreev reflections (CAR), elastic cotunneling, quasiparticles transmission and local Andreev reflections. In the tunneling limit, the CAR contribute positively to the noise cross-correlation whereas the other processes contribute negatively. Depending on the pair potential symmetry, the CAR are the dominant processes, giving as a result a high efficiency for Cooper pair split. We propose the use of the Fano factor to test the efficiency of a Cooper pair splitter device.

cond-mat.supr-con↗

arXiv2015

Solutions of Laplace's equation with simple boundary conditions, and their applications for capacitors with multiple symmetries

We find solutions of Laplace's equation with specific boundary conditions (in which such solutions take either the value zero or unity in each surface) using a generic curvilinear system of coordinates. Such purely geometrical solutions (that we shall call Basic Harmonic Functions BHF's) are utilized to obtain a more general class of solutions for Laplace's equation, in which the functions take arbitrary constant values on the boundaries. On the other hand, the BHF's are also used to obtain the capacitance of many electrostatic configurations of conductors. This method of finding solutions of Laplace's equation and capacitances with multiple symmetries is particularly simple, owing to the fact that the method of separation of variables becomes much simpler under the boundary conditions that lead to the BHF's. Examples of application in complex symmetries are given. Then, configurations of succesive embedding of conductors are also examined. In addition, expressions for electric fields between two conductors and charge densities on their surfaces are obtained in terms of generalized curvilinear coordinates. It worths remarking that it is plausible to extrapolate the present method to other linear homogeneous differential equations.

physics.class-ph↗

arXiv2013

Study of the apsidal precession of the Physical Symmetrical Pendulum

We study the apsidal precession of a Physical Symmetrical Pendulum (Allais' precession) as a generalization of the precession corresponding to the Ideal Spherical Pendulum (Airy's Precession). Based on the Hamilton-Jacobi formalism and using the technics of variation of parameters along with the averaging method, we obtain approximate solutions, in terms of which the motion of both systems admits a simple geometrical description. The method developed in this paper is considerably simpler than the standard one in terms of elliptical functions and the numerical agreement with the exact solutions is excellent. In addition, the present procedure permits to show clearly the origin of the Airy's and Allais' precession, as well as the effect of the spin of the Physical Pendulum on the Allais' precession. Further, the method can be extended to the study of the asymmetrical pendulum in which an exact solution is not possible anymore.

physics.class-ph↗

arXiv2012

Shot noise in HTc superconductor quantum point contact system

We study the electrical transport properties of a quantum point contact between a lead and a Hight Tc superconductor. For this, we use the Hamiltonian approach and non-equilibrium Green functions of the system. The electrical current and the shot noise are calculated with this formalism. We consider $d_{x^2-y^2}$, $d_{xy}$, $d_{x^2-y^2}+is$ and $d_{xy}+is$ symmetries for the pair potential. Also we explore the $s_{+-}$ and $s_{++}$ symmetries describing the behavior of the ferropnictides superconductors. We found that for $d_{xy}$ symmetry there is not a zero bias conductance peak and for $d+is$ symmetries there is a displacement of the transport properties. From shot noise and current, the Fano factor is calculated and we found that it takes values of effective charge between $e$ and $2e$, this is explained by the diffraction of quasiparticles in the contact. For the $s_{+-}$ and $s_{++}$ symmetries the results show that the electrical current and the shot noise depend on the mixing coefficient, furthermore the effective electric charge can take values between 0 and $2e$, in contrast with the results obtained for $s$ wave superconductors.

cond-mat.supr-con↗

arXiv2012

Work and energy in rotating systems

Literature analyzes the way in which Newton's second law can be used when non-inertial rotating systems are used. However, the treatment of the work and energy theorem in rotating systems is not considered in textbooks. In this paper, we show that the work and energy theorem can still be applied to a closed system of particles in a rotating system, as long as the work of fictitious forces is properly included in the formalism. The coriolis force does not contribute to the work coming from fictitious forces. It worths remarking that real forces that do not do work in an inertial reference frame can do work in the rotating reference frame and viceversa. The combined effects of acceleration of the origin and rotation of the non-inertial system are also studied.

physics.class-ph↗

arXiv2011

The positivity and other properties of the matrix of capacitance: physical and mathematical implications

We prove that the matrix of capacitance in electrostatics is a positive-singular matrix with a non-degenerate null eigenvalue. We explore the physical implications of this fact, and study the physical meaning of the eigenvalue problem for such a matrix. Many properties are easily visualized by constructing a "potential space" isomorphic to the euclidean space. The problem of minimizing the internal energy of a system of conductors under constraints is considered, and an equivalent capacitance for an arbitrary number of conductors is obtained. Moreover, some properties of systems of conductors in successive embedding are examined. Finally, we discuss some issues concerning the gauge invariance of the formulation.

physics.class-ph↗

arXiv2011

The role of the virtual work in Faraday's law

In the context of Faraday's induction law, we show that the concept of virtual work provides another point of view to clarify the nature of the induced electric field, as well as the fact that the integral over a closed path of the induced electric field is not the work performed by a unit charge. The usefulness of the concept of virtual conservativity is discussed. Further, we study the relation between the electromotive force and the real work done by an external agent to keep a circuit at constant velocity. From this discussion it is observed that magnetic forces can be non-conservative from the mathematical point of view, but can be treated as conservative for all practical purposes.

physics.class-ph↗