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Jose L. Lado

Publications and source records attributed to Jose L. Lado.

At least 73 records · Page 4Linked to original sources
arXiv2024

A Non-Hermitian Moiré Valley Filter

A valley filter capable of generating a valley-polarized current is a crucial element in valleytronics, yet its implementation remains challenging. Here, we propose a valley filter made of a graphene bilayer which exhibits a 1D moiré pattern in the overlapping region of the two layers controlled by heterostrain. In the presence of a lattice modulation between layers, electrons propagating in one layer can have valley-dependent dissipation due to valley asymmetric interlayer coupling, thus giving rise to a valley-polarized current. Such a process can be described by an effective non-Hermitian theory, in which the valley filter is driven by a valley-resolved non-Hermitian skin effect. Nearly 100\% valley-polarization can be achieved within a wide parameter range and the functionality of the valley filter is electrically tunable. The non-Hermitian topological scenario of the valley filter ensures high tolerance against imperfections such as disorder and edge defects. Our work opens a new route for efficient and robust valley filters while significantly relaxing the stringent implementation requirements.

cond-mat.mes-hall↗
arXiv2024

Tuning spinaron and Kondo resonances via quantum confinement

Controlling zero bias anomalies in magnetic atoms provides a promising strategy to engineer tunable quantum many-body excitations. Here we show how two different quantum impurities featuring spinaron and Kondo excitations can be controlled via quantum confinement engineering by using circular quantum corrals on a Ag(111) surface. In corrals built from both Ag and Co adatoms, the width of the zero bias anomaly in the central Co adatom oscillates as a function of corral radius with a period of half of the Ag(111) surface state wavelength. Parameters extracted for Co/Ag(111) show only small differences in extracted spinaron zero-bias anomaly between corral walls built from Ag or Co adatoms. In quantum corrals occupied with metal-free phthalocyanine, a $S=1/2$ Kondo system, we observe notable changes in the zero bias anomaly lineshape as a function of corral radius. Our results offer insight into many-body Kondo and spinaron resonances where the electronic density is controlled by confinement engineering.

cond-mat.mes-hall↗
arXiv2024

Non-Hermitian topology and criticality in photonic arrays with engineered losses

Integrated photonic systems provide a flexible platform where artificial lattices can be engineered in a reconfigurable fashion. Here, we show that one-dimensional photonic arrays with engineered losses allow the realization of topological excitations stemming from non-Hermiticity and bulk mode criticality. We show that a generalized modulation of the local photonic losses allows the creation of topological modes both in the presence of periodicity and even in the quasiperiodic regime. We demonstrate that a localization transition of all the bulk photonic modes can be engineered in the presence of a quasiperiodic loss modulation, and we further demonstrate that such a transition can be created in the presence of both resonance frequency modulation and loss modulation. We finally address the robustness of this phenomenology to the presence of next to the nearest neighbor couplings and disorder in the emergence of criticality and topological modes. Our results put forward a strategy to engineer topology and criticality solely from engineered losses in a photonic system, establishing a potential platform to study the impact of nonlinearities in topological and critical photonic matter.

physics.optics↗
arXiv2024

Doped Mott phase and charge correlations in monolayer 1T-NbSe$_2$

The doped Hubbard model is one of the paradigmatic platforms to engineer exotic quantum many-body states, including charge-ordered states, strange metals and unconventional superconductors. While undoped and doped correlated phases have been experimentally realized in a variety twisted van der Waals materials, experiments in monolayer materials, and in particular 1T transition metal dichalcogenides, have solely reached the conventional insulating undoped regime. Correlated phases in monolayer two-dimensional materials have much higher associated energy scales than their twisted counterparts, making doped correlated monolayers an attractive platform for high temperature correlated quantum matter. Here, we demonstrate the realization of a doped Mott phase in a van der Waals dichalcogenide 1T-NbSe$_2$ monolayer. The system is electron doped due to electron transfer to a monolayer van der Waals substrate via proximity, leading to a correlated triangular lattice with both half-filled and fully-filled sites. We analyze the distribution of the half-filled and filled sites and show the arrangement is unlikely to be controlled by disorder alone, and we show that the presence of competing non-local many-body correlations would account for the charge correlations found experimentally. Our results establish 1T-NbSe$_2$ as a potential monolayer platform to explore correlated doped Mott physics in a frustrated lattice.

cond-mat.str-el↗
arXiv2024

Hamiltonian learning with real-space impurity tomography in topological moire superconductors

Extracting Hamiltonian parameters from available experimental data is a challenge in quantum materials. In particular, real-space spectroscopy methods such as scanning tunneling spectroscopy allow probing electronic states with atomic resolution, yet even in those instances extracting effective Hamiltonian is an open challenge. Here we show that impurity states in modulated systems provide a promising approach to extracting non-trivial Hamiltonian parameters of a quantum material. We show that by combining the real-space spectroscopy of different impurity locations in a moire topological superconductor, modulations of exchange and superconducting parameters can be inferred via machine learning. We demonstrate our strategy with a physically-inspired harmonic expansion combined with a fully-connected neural network that we benchmark against a conventional convolutional architecture. We show that while both approaches allow extracting exchange modulations, only the former approach allows inferring the features of the superconducting order. Our results demonstrate the potential of machine learning methods to extract Hamiltonian parameters by real-space impurity spectroscopy as local probes of a topological state.

cond-mat.mes-hall↗
arXiv2024

Nature of the unconventional heavy fermion Kondo state in monolayer CeSiI

CeSiI has been recently isolated in the ultrathin limit, establishing CeSiI as the first intrinsic two-dimensional van der Waals heavy-fermion material up to 85 K. We show that, due to the strong Ce spin-orbit coupling, the local moments develop a multipolar real-space magnetic texture, leading to local pseudospins with a nearly vanishing net moment. To elucidate its Kondo-screened regime, we extract from first principles the parameters of the Kondo lattice model describing this material. We develop a fractional pseudofermion methodology in combination with ab initio calculations to reveal the nature of the Kondo-screened heavy fermion state in CeSiI. Using this formalism, we analyze the competing magnetic interactions leading to a heavy-fermion order as a function of the magnetic exchange between the localized f-electrons and the strength of the Kondo coupling. Our results show that the magnetic exchange interactions promote an unconventional momentum-dependent heavy-fermion Kondo screened phase, establishing the nature of the heavy-fermion behavior of ultrathin CeSiI observed experimentally.

cond-mat.str-el↗
arXiv2023

Local Probe Structure Isomerization in a One-Dimensional Molecular Array

Synthesis of one-dimensional molecular arrays with tailored stereoisomers is challenging yet has a great potential for application in molecular opto-, electronic- and magnetic-devices, where the local array structure plays a decisive role in the functional properties. Here, we demonstrate construction and characterization of dehydroazulene isomer and diradical units in three-dimensional organometallic compounds on Ag(111) with a combination of low-temperature scanning tunneling microscopy and density functional theory calculations. Tip-induced voltage pulses firstly result in the formation of a diradical species via successive homolytic fission of two C-Br bonds in the naphthyl groups, which are subsequently transformed into chiral dehydroazulene moieties. The delicate balance of the reaction rates among the diradical and two stereoisomers, arising from an in-line configuration of tip and molecular unit, allows directional azulene-to-azulene and azulene-to-diradical local probe structural isomerization in a controlled manner. Furthermore, our theoretical calculations suggest that the diradical moiety hosts an open-shell singlet with antiferromagnetic coupling between the unpaired electrons, which can undergo an inelastic spin transition of 91 meV to the ferromagnetically coupled triplet state.

cond-mat.mtrl-sci↗
arXiv2023

Atomic-scale visualization of multiferroicity in monolayer NiI$_2$

Progress in layered van der Waals materials has resulted in the discovery of ferromagnetic and ferroelectric materials down to the monolayer limit. Recently, evidence of the first purely two-dimensional multiferroic material was reported in monolayer NiI$_2$. However, probing multiferroicity with scattering-based and optical bulk techniques is challenging on 2D materials, and experiments on the atomic scale are needed to fully characterize the multiferroic order at the monolayer limit. Here, we use scanning tunneling microscopy (STM) supported by theoretical calculations based on density functional theory (DFT) to probe and characterize the multiferroic order in monolayer NiI$_2$. We demonstrate that the type-II multiferroic order displayed by NiI$_2$, arising from the combination of a magnetic spin spiral order and a strong spin-orbit coupling, allows probing the multiferroic order in the STM experiments. Moreover, we directly probe the magnetoelectric coupling of NiI$_2$ by external electric field manipulation of the multiferroic domains. Our findings establish a novel point of view to analyse magnetoelectric effects at the microscopic level, paving the way towards engineering new multiferroic orders in van der Waals materials and their heterostructures.

cond-mat.mtrl-sci↗
arXiv2023

Transfer learning from Hermitian to non-Hermitian quantum many-body physics

Identifying phase boundaries of interacting systems is one of the key steps to understanding quantum many-body models. The development of various numerical and analytical methods has allowed exploring the phase diagrams of many Hermitian interacting systems. However, numerical challenges and scarcity of analytical solutions hinder obtaining phase boundaries in non-Hermitian many-body models. Recent machine learning methods have emerged as a potential strategy to learn phase boundaries from various observables without having access to the full many-body wavefunction. Here, we show that a machine learning methodology trained solely on Hermitian correlation functions allows identifying phase boundaries of non-Hermitian interacting models. These results demonstrate that Hermitian machine learning algorithms can be redeployed to non-Hermitian models without requiring further training to reveal non-Hermitian phase diagrams. Our findings establish transfer learning as a versatile strategy to leverage Hermitian physics to machine learning non-Hermitian phenomena.

quant-ph↗
arXiv2023

Lee-Yang theory of quantum phase transitions with neural network quantum states

Predicting the phase diagram of interacting quantum many-body systems is a central problem in condensed matter physics and related fields. A variety of quantum many-body systems, ranging from unconventional superconductors to spin liquids, exhibit complex competing phases whose theoretical description has been the focus of intense efforts. Here, we show that neural network quantum states can be combined with a Lee-Yang theory of quantum phase transitions to predict the critical points of strongly-correlated spin lattices. Specifically, we implement our approach for quantum phase transitions in the transverse-field Ising model on different lattice geometries in one, two, and three dimensions. We show that the Lee-Yang theory combined with neural network quantum states yields predictions of the critical field, which are consistent with large-scale quantum many-body methods. As such, our results provide a starting point for determining the phase diagram of more complex quantum many-body systems, including frustrated Heisenberg and Hubbard models.

cond-mat.str-el↗
arXiv2023

Control of molecular orbital ordering using a van der Waals monolayer ferroelectric

Two-dimensional (2D) ferroelectric materials provide a promising platform for the electrical control of quantum states. In particular, due to their 2D nature, they are suitable for influencing the quantum states of deposited molecules via the proximity effect. Here, we report electrically controllable molecular states in phthalocyanine molecules adsorbed on monolayer ferroelectric material SnTe. In particular, we demonstrate that the strain and ferroelectric order in SnTe creates a transition between two distinct orbital orders in the adsorbed phthalocyanine molecules. By controlling the polarization of the ferroelectric domain using scanning tunneling microscopy (STM), we have successfully demonstrated that orbital order can be manipulated electrically. Our results show how ferroelastic coupling in 2D systems allows control of molecular states, providing a starting point for ferroelectrically switchable molecular orbital ordering and ultimately, electrical control of molecular magnetism.

cond-mat.mtrl-sci↗
arXiv2023

Hamiltonian inference from dynamical excitations in confined quantum magnets

Quantum-disordered models provide a versatile platform to explore the emergence of quantum excitations in many-body systems. The engineering of spin models at the atomic scale with scanning tunneling microscopy and the local imaging of excitations with electrically driven spin resonance has risen as a powerful strategy to image spin excitations in finite quantum spin systems. Here, focusing on $S=1/2$ lattices as realized by Ti in MgO, we show that dynamical spin excitations provide a robust strategy to infer the nature of the underlying Hamiltonian. We show that finite-size interference of the dynamical many-body spin excitations of a generalized long-range Heisenberg model allows the underlying spin couplings to be inferred. We show that the spatial distribution of local spin excitations in Ti islands and ladders directly correlates with the underlying ground state in the thermodynamic limit. Using a supervised learning algorithm, we demonstrate that the different parameters of the Hamiltonian can be extracted by providing the spatially and frequency-dependent local excitations that can be directly measured by electrically driven spin resonance with scanning tunneling microscopy. Our results put forward local dynamical excitations in confined quantum spin models as versatile witnesses of the underlying ground state, providing an experimentally robust strategy for Hamiltonian inference in complex real spin models.

cond-mat.mes-hall↗
arXiv2023

Evidence of nodal superconductivity in monolayer 1H-TaS$_2$ with hidden order fluctuations

Unconventional superconductors represent one of the fundamental directions in modern quantum materials research. In particular, nodal superconductors are known to appear naturally in strongly correlated systems, including cuprate superconductors and heavy-fermion systems. Van der Waals materials hosting superconducting states are well known, yet nodal monolayer van der Waals superconductors have remained elusive. Here, using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments, we show that pristine monolayer 1H-TaS$_2$ realizes a nodal superconducting state. By including non-magnetic disorder, we drive the nodal superconducting state to a conventional gapped s-wave state. Furthermore, we observe the emergence of many-body excitations close to the gap edge, signalling a potential unconventional pairing mechanism. Our results demonstrate the emergence of nodal superconductivity in a van der Waals monolayer, providing a building block for van der Waals heterostructures exploiting unconventional superconducting states.

cond-mat.supr-con↗
arXiv2023

Adversarial Hamiltonian learning of quantum dots in a minimal Kitaev chain

Determining Hamiltonian parameters from noisy experimental measurements is a key task for the control of experimental quantum systems. An experimental platform that recently emerged, and where knowledge of Hamiltonian parameters is crucial to fine-tune the system, is that of quantum dot-based Kitaev chains. In this work, we demonstrate an adversarial machine learning algorithm to determine the parameters of a quantum dot-based Kitaev chain. We train a convolutional conditional generative adversarial neural network (Conv-cGAN) with simulated differential conductance data and use the model to predict the parameters at which Majorana bound states are predicted to appear. In particular, the Conv-cGAN model facilitates a rapid, numerically efficient exploration of the phase diagram describing the transition between elastic co-tunneling and crossed Andreev reflection regimes. We verify the theoretical predictions of the model by applying it to experimentally measured conductance obtained from a minimal Kitaev chain consisting of two spin-polarized quantum dots coupled by a superconductor-semiconductor hybrid. Our model accurately predicts, with an average success probability of $97$\%, whether the measurement was taken in the elastic co-tunneling or crossed Andreev reflection-dominated regime. Our work constitutes a stepping stone towards fast, reliable parameter prediction for tuning quantum-dot systems into distinct Hamiltonian regimes. Ultimately, our results yield a strategy to support Kitaev chain tuning that is scalable to longer chains.

cond-mat.mes-hall↗
arXiv2023

Moiré-driven multiferroic order in twisted CrCl$_3$, CrBr$_3$ and CrI$_3$ bilayers

Layered van der Waals materials have risen as a powerful platform to engineer artificial competing states of matter. Here we show the emergence of multiferroic order in twisted chromium trihalide bilayers, an order fully driven by the moiré pattern and absent in aligned multilayers. Using a combination of spin models and ab initio calculations, we show that a spin texture is generated in the moiré supercell of the twisted system as a consequence of the competition between stacking-dependent interlayer magnetic exchange and magnetic anisotropy. An electric polarization arises associated with such a non-collinear magnetic state due to the spin-orbit coupling, leading to the emergence of a local ferroelectric order following the moiré. Among the stochiometric trihalides, our results show that twisted CrBr$_3$ bilayers give rise to the strongest multiferroic order. We further show the emergence of a strong magnetoelectric coupling, which allows the electric generation and control of magnetic skyrmions. Our results put forward twisted chromium trihalide bilayers, and in particular CrBr$_3$ bilayers, as a powerful platform to engineer artificial multiferroic materials and electrically tunable topological magnetic textures.

cond-mat.mtrl-sci↗
arXiv2023

Andreev Reflection and Klein Tunneling in High-Temperature Superconductor/Graphene Junctions

Scattering processes in quantum materials emerge as resonances in electronic transport, including confined modes, Andreev states, and Yu-Shiba-Rusinov states. However, in most instances, these resonances are driven by a single scattering mechanism. Here we show the appearance of resonances due to the combination of two simultaneous scattering mechanisms, one from superconductivity and the other from graphene p-n junctions. These resonances stem from Andreev reflection and Klein tunneling that occur at two different interfaces of a hole-doped region of graphene formed at the boundary with superconducting graphene due to proximity effects from Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_{8+δ}$. The resonances persist with gating the from p$^+$-p and p-n configurations. The suppression of the oscillation amplitude above the bias energy comparable to the induced superconducting gap indicates the contribution from Andreev reflection. Our experimental measurements are supported by quantum transport calculations in such interfaces, leading to analogous resonances. Our results put forward a hybrid scattering mechanism in graphene/high-temperature superconductor heterojunctions of potential impact for graphene-based Josephson junctions.

cond-mat.supr-con↗
arXiv2023

Atomic scale imaging of sign-changing order parameter in superconducting FeSe with Tunneling Andreev Reflection

The pairing symmetry of the superconducting order parameter in iron-based superconductors has been a subject of debate, with various models proposing s-wave, d-wave, and mixed combinations as possible candidates. Here we probe the pairing symmetry of FeSe utilizing the new methodology of Tunneling Andreev Reflection (TAR). TAR directly exploits the transparency-dependence of tunneling current to disentangle contributions of single-particle current and Andreev reflection. These measurements provided new direct evidence in favor of the sign-changing nature of the superconducting order in FeSe, in a distinctly complementary approach to nanoscale imaging of quasiparticle interference. Crucially TAR can also probe higher-order contributions to Andreev reflection. Quantitative comparison of the experimental signatures of higher-order Andreev reflections with those in concomitant tight-binding simulations revealed new evidence in support of the nodal gap structure of superconductivity in FeSe. Finally, the effect of structural topological defects can be directly probed with TAR owing to its atomic spatial resolution. In particular, we find that superconductivity is completely suppressed along the twin boundary while its electronic structure is characterized by a V-shaped signature of a pseudogap state. Our findings provide new insight into the pairing symmetry of an unconventional superconductor, demonstrating the potential of differential tunneling Andreev reflection to reveal microscopic properties of emerging quantum materials.

cond-mat.supr-con↗
arXiv2023

Topological spin excitations in non-Hermitian spin chains with a generalized kernel polynomial algorithm

Spectral functions of non-Hermitian Hamiltonians can reveal the existence of topologically non-trivial line gaps and the associated topological edge modes. However, the computation of spectral functions in a non-Hermitian many-body system remains an open challenge. Here, we put forward a numerical approach to compute spectral functions of a non-Hermitian many-body Hamiltonian based on the kernel polynomial method and the matrix-product state formalism. We show that the local spectral functions computed with our algorithm reveal topological spin excitations in a non-Hermitian spin model, faithfully reflecting the non-trivial line gap topology in a many-body model. We further show that the algorithm works in the presence of the non-Hermitian skin effect. Our method offers an efficient way to compute local spectral functions in non-Hermitian many-body systems with tensor-networks, allowing to characterize line gap topology in non-Hermitian quantum many-body models.

quant-ph↗