arXiv subjects

J. A. Galvis

Publications and source records attributed to J. A. Galvis.

17 recordsLinked to original sources

arXiv2026

Defect-induced multiferroicity in bulk solid solutions of WSe$_2$ and WTe$_2$

Transition metal dichalcogenides provide a versatile platform for tunable ferroic phenomena at the atomic scale owing to their reduced dimensionality. Here we investigate the structural, magnetic, and ferroelectric properties of bulk solid solution W(Se1-xTex)2(1-delta) single crystals synthesized by chemical vapor transport. The room temperature behavior is analyzed as a function of tellurium concentration (x) and chalcogen defect fraction (delta). X ray diffraction and Raman spectroscopy reveal lattice expansion and symmetry reduction with increasing x, consistent with a 2H to 1Td structural transition above a critical composition xc about 18 percent. Piezoresponse force microscopy identifies piezoelectricity near stoichiometric compositions (delta less than 5 percent) and switchable ferroelectricity in the chalcogen deficient regime (delta greater than 20 percent). Magnetometry measurements show a corresponding evolution from paramagnetic to ferromagnetic behavior with increasing delta. Near stoichiometric Te poor samples exhibit piezoelectric and paramagnetic responses, whereas multiferroic states characterized by the coexistence of ferroelectric and ferromagnetic responses emerge at high vacancy concentrations. The performed characterizations indicate that x primarily governs structural symmetry, while delta controls the emergence of both ferromagnetic and ferroelectric responses. These trends are summarized in a configurational phase diagram highlighting the cooperative influence of dopants and defects on ferroic behavior. Overall, controlled stoichiometry and vacancy engineering offer an effective strategy to tailor ferroic responses in transition metal dichalcogenides.

cond-mat.mtrl-sci↗

arXiv2026

Lattice dynamics of the charge density wave compounds TaTe$_4$ and NbTe$_4$ and their evolution across solid solutions

Understanding lattice dynamics is central to elucidating the microscopic origin of charge density waves (CDWs), particularly in materials where electron-phonon coupling can play a dominant role. Raman spectroscopy, combined with first-principles calculations, offers a direct means to identify the vibrational modes involved and to monitor their evolution under controlled perturbations. In this work, we combine density functional theory calculations and Raman spectroscopy measurements to investigate the vibrational properties of the quasi-one-dimensional transition metal tetrachalcogenides TaTe$_4$ and NbTe$_4$, as well as their solid solutions Ta$_{1-x}$Nb$_{x}$Te$_4$ ($x$ = 0.0 - 1.0). For the stoichiometric compounds, first-principles calculations predict a phonon instability consistent with the trimerization associated with the CDW phase, providing theoretical evidence for the lattice distortion driving the transition. The calculated Raman-active modes show good agreement with room-temperature experimental spectra, enabling a systematic assignment of the observed peaks. Across the solid solution, most Raman modes evolve smoothly with composition. In contrast, the highest-frequency E$_{g}$ mode, dominated by transition-metal motion, exhibits a distinct behavior: its frequency remains close to that of the parent compounds while its intensity redistributes with stoichiometry. This evolution highlights the short-range character of this vibrational mode and suggests its relevance to the CDW-related lattice distortion in these materials.

cond-mat.mtrl-sci↗

arXiv2026

Universality of linear in temperature and linear in field Planckian scattering rate in high temperature cuprate superconductors

One of the long standing puzzles in strongly correlated materials is the microscopic origin of the quantum critical Planckian strange metal phase with universal linear in temperature scattering rate from which unconventional superconductivity directly emerges by lowering temperatures. Recently, the linear in temperature and linear in field resistivity have been simultaneously observed in high temperature cuprate superconductors, manifested by the universal field to temperature scaling in magnetoresistivity. To date, there has been a lack of coherent and unified understanding of these coexisting linear behaviors and their possible link to quantum criticality. In this work, we establish the universality in linear in temperature and linear in field Planckian behaviors in underdoped LSCO near optimal doping. Experimentally, we observe the linear in field Planckian scattering rate and its relation to its linear in temperature counterpart. Theoretically, we propose a spin based common microscopic mechanism based on Kondo-like charge fluctuations near local quantum criticality of heavy fermion formulated tJ model subject to a Zeeman term. Similar to frequency to temperature scaling near quantum criticality, we find the magnetic field here effectively introduces a Zeeman energy, reminiscent of an external energy in the quantum critical regime, leading to field to temperature scaling. Our analytically predicted universal field to temperature scaling in isotropic scattering rate and the relation between the linear in temperature and linear in field Planckian coefficients, unifies these two phenomena over an extended doping range, pointing toward a unified quantum-critical origin of Planckian transport in cuprates.

cond-mat.supr-con↗

arXiv2025

Fermi Surface Reconstruction and Anisotropic Linear Magnetoresistance in the Charge Density Wave Topological Semimetal TaTe4

Understanding the interplay between topology and correlated electron states is central to the study of quantum materials. TaTe$_4$ is a quasi-one-dimensional charge density wave (CDW) compound predicted to host topological phases, which makes it a model platform to explore this interplay. Here, we combine high-field magnetotransport measurements with density functional theory calculations to provide a comprehensive mapping of the Fermi surface (FS) of TaTe$_4$ in its CDW phase. Using multiple current-field geometries, we resolve the four largest of six pockets of the FS predicted by theory and find no evidence of non-CDW bands, highlighting the full reconstruction of the FS in the bulk. We identify a previously unobserved quasi-cylindrical pocket and uncover a large size orbit consistent with magnetic breakdown between reconstructed FS sheets, from which we estimate a CDW gap of $\sim$0.29~eV. Moreover, we observe a robust linear magnetoresistance that persists across all field directions when current flows perpendicular to the 1D chains along which the CDW is formed, with a distinct high-field linear regime emerging when field is along the chains. These findings establish TaTe$_4$ as a prototypical material to study the coexistence of correlation-driven reconstruction and topological electronic states.

cond-mat.mtrl-sci↗

arXiv2023

Room temperature multiferroicity in a transition metal dichalcogenide

The coexistence of multiple ferroic orders, i.e. multiferroicity, is a scarce property to be found in materials. Historically, this state has been found mainly in 3-dimensional complex oxides, but so far this state has still been elusive for the most widely studied and characterized family of 2-dimensional compounds, the transition metal dichalcogenides. In this study we report the experimental realization of multiferroic states in this family of materials, at room temperature, in bulk single crystals of Te-doped WSe2. We observe the coexistence of ferromagnetism and ferroelectricity, evidenced in the presence of magnetization and piezoresponse force microscopy hysteresis loops. These findings open the possibility of widening the use and study of van der Waals-based multifunctional devices for new nanoelectronics and spintronics applications.

cond-mat.mtrl-sci↗

arXiv2022

Direct observation of discommensurate charge density wave modulation in the quasi-1D Weyl semimetal candidate NbTe$_4$

The transition-metal tetrachalcogenides are a model system to explore the conjunction of correlated electronic states such as charge density waves (CDW), with topological phases of matter. Understanding the connection between these phases requires a thorough understanding of the individual states, which for the case of the CDW in this system, is still missing. In this paper we combine phonon-structure calculations and scanning tunneling microscopy measurements of NbTe$_4$ in order to provide a full characterization of the CDW state. We find that, at short range, the superstructure formed by the CDW is fully commensurate with the lattice parameters. Moreover, our data reveals the presence of phase-slip domain-walls separating regions of commensurate-CDW in the nanoscale, indicating that the CDW in this compound is discommensurate at long-range. Our results solve a long-standing discussion about the nature of the CDW in these materials, and provide a strong basis for the study of the interplay between this state and other novel quantum electronic states.

cond-mat.mtrl-sci↗

arXiv2022

Raman spectroscopy of few-layers TaS$_2$ and Mo-doped TaS$_2$ with enhanced superconductivity

The use of simple, fast and economic experimental tools to characterize low-dimensional materials is an important step in the process of democratizing the use of such materials in laboratories around the world. Raman spectroscopy has arisen as a way of indirectly determining the thickness of nanolayers of transition metal dichalcogenides (TMDs), avoiding the use of more expensive tools such as atomic force microscopy, and it is therefore a widely used technique in the study of semiconducting TMDs. However, the study of many metallic TMDs in the limit of few atomic layers is still behind when compared to their semiconducting counterparts, partly due to the lack of similar alternative characterization studies. In this work we present the characterization of the Raman spectrum, specifically of the E$^1_{2g}$- and A$_{1g}$-modes, of mechanically exfoliated crystals of Ta$_{1-x}$Mo$_x$S$_2$, a metallic TMD which exhibits charge density wave formation and superconductivity. The clear identification of contributions to the Raman spectrum coming from the SiO$_2$/Si substrate, which overlap with the peaks coming from the sample, and which dominate in intensity in the few-layer-samples limit, allowed the isolation of the individual E$^1_{2g}$- and A$_{1g}$-modes of the samples and, for the first time, the observation of a clear evolution of the Raman shifts of both modes as a function of sample thickness. The evolution of such peaks qualitatively resembles the evolution seen in other TMDs, and provide a way of indirectly determining sample thickness in the limit of few atomic layers at a low cost. In addition, we observe a softening (red-shift) of both E$^1_{2g}$- and A$_{1g}$-modes with Mo-doping in the nanolayers, possibly related to the increased out-of-plane lattice parameter with respect to the pure compound.

cond-mat.mes-hall↗

arXiv2018

Mass enhancement in multiple bands approaching optimal doping in a high-temperature superconductor

Pnictides provide an opportunity to study the effects of quantum criticality in a multi-band high temperature superconductor. Quasiparticle mass divergence near optimal doping, observed in two major classes of high-temperature superconductors, pnictides and cuprates, is a direct experimental indicator of enhanced electronic interactions that accompany quantum criticality. Whether quasiparticles on all Fermi surface pockets in BaFe2(As1-xPx)2 are affected by quantum criticality is an open question, which specific heat measurements at high magnetic fields can directly address. Here we report specific heat measurements up to 35T in BaFe2(As1-xPx)2 over a broad doping range, 0.44 <= x <= 0.6. We observe saturation of C/T in the normal state at all dopings where superconductivity is fully suppressed. Our measurements demonstrate that quasiparticle mass increases towards optimal doping in multiple pockets, some of which exhibit even stronger mass enhancement than previously reported from quantum oscillations of a single pocket.

cond-mat.supr-con↗

arXiv2018

Scale-invariant magnetoresistance in a cuprate superconductor

The anomalous metallic state in high-temperature superconducting cuprates is masked by the onset of superconductivity near a quantum critical point. Use of high magnetic fields to suppress superconductivity has enabled a detailed study of the ground state in these systems. Yet, the direct effect of strong magnetic fields on the metallic behavior at low temperatures is poorly understood, especially near critical doping, $x=0.19$. Here we report a high-field magnetoresistance study of thin films of \LSCO cuprates in close vicinity to critical doping, $0.161\leq x\leq0.190$. We find that the metallic state exposed by suppressing superconductivity is characterized by a magnetoresistance that is linear in magnetic field up to the highest measured fields of $80$T. The slope of the linear-in-field resistivity is temperature-independent at very high fields. It mirrors the magnitude and doping evolution of the linear-in-temperature resistivity that has been ascribed to Planckian dissipation near a quantum critical point. This establishes true scale-invariant conductivity as the signature of the strange metal state in the high-temperature superconducting cuprates.

cond-mat.str-el↗

arXiv2017

Tilted vortex cores and superconducting gap anisotropy in 2H-NbSe2

Superconducting vortex cores have been extensively studied for magnetic fields applied perpendicular to the surface by mapping the density of states (DOS) through Scanning Tunneling Microscopy (STM). Vortex core shapes are often linked to the superconducting gap anisotropy---quasiparticle states inside vortex cores extend along directions where the superconducting gap is smallest. The superconductor 2H-NbSe$_2$ crystallizes in a hexagonal structure and vortices give DOS maps with a sixfold star shape for magnetic fields perpendicular to the surface and the hexagonal plane. This has been associated to a hexagonal gap anisotropy located on quasi two-dimensional Fermi surface tubes oriented along the $c$ axis. The gap anisotropy in another, three-dimensional, pocket is unknown. However, the latter dominates the STM tunneling conductance. Here we measure DOS in magnetic fields parallel to the surface and perpendicular to the $c$ axis. We find patterns of stripes due to in-plane vortex cores running nearly parallel to the surface. The patterns change with the in-plane direction of the magnetic field, suggesting that the sixfold gap anisotropy is present over the whole Fermi surface. Due to a slight misalignment between the vector of the magnetic field and the surface, our images also show outgoing vortices. Their shape is successfully compared to detailed calculations of vortex cores in tilted fields. Their features merge with the patterns due to in plane vortices, suggesting that they exit at an angle with the surface. Measuring the DOS of vortex cores in highly tilted magnetic fields with STM can thus be used to study the superconducting gap structure.

cond-mat.supr-con↗

arXiv2017

Subsurface bending and reorientation of tilted vortex lattices in the bulk due to Coulomb-like repulsion at the surface

We study vortex lattices (VLs) in superconducting weak-pinning platelet-like crystals of $β$-Bi$_{2}$Pd in tilted fields with a Scanning Tunneling Microscope. We show that vortices exit the sample perpendicular to the surface and are thus bent beneath the surface. The structure and orientation of tilted VL in the bulk are, for large tilt angles, strongly affected by Coulomb-type intervortex repulsion at the surface due to stray fields.

cond-mat.supr-con↗

arXiv2015

Magnetic field dependence of the density of states in the multiband superconductor $β$-Bi$_2$Pd

We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $β$-Bi$_2$Pd. We find a single fully isotropic superconducting gap. However, the magnetic field dependence of the intervortex density of states is higher than the one expected in a single gap superconductor, and the hexagonal vortex lattice is locked to the square atomic lattice. Such increase in the intervortex density of states and vortex lattice locking have been found in superconductors with multiple superconducting gaps and anisotropic Fermi surfaces. We compare the upper critical field $H_{c2}(T)$ obtained in our sample with previous measurements and explain available data within multiband supercondutivity. We propose that $β$-Bi$_2$Pd is a single gap multiband superconductor. We anticipate that single gap multiband superconductivity can occur in other compounds with complex Fermi surfaces.

cond-mat.supr-con↗

arXiv2014

Zero-bias conductance peak in detached flakes of superconducting 2H-TaS$_2$ probed by scanning tunneling spectroscopy

We report an anomalous tunneling conductance with a zero bias peak in flakes of superconducting 2H-TaS$_2$ detached through mechanical exfoliation. To explain the observed phenomenon, we construct a minimal model for a single unit cell layer of superconducting 2H-TaS$_2$ with a simplified 2D Fermi surface and sign-changing Cooper pair wavefunction induced by Coulomb repulsion. Superconductivity is induced in the central $Γ$ pocket, where it becomes nodal. We show that weak scattering at the nodal Fermi surface, produced by non-perturbative coupling between tip and sample, gives Andreev states that lead to a zero bias peak in the tunneling conductance. We suggest that reducing dimensionality down to a few atom thick crystals could drive a crossover from conventional to sign changing pairing in the superconductor 2H-TaS$_2$.

cond-mat.supr-con↗

arXiv2013

Scanning Tunneling Microscopy in the superconductor LaSb2

We present very low temperature (0.15 K) scanning tunneling microscopy and spectroscopy experiments in the layered superconductor LaSb$_2$. We obtain topographic microscopy images with surfaces showing hexagonal and square atomic size patterns, and observe in the tunneling conductance a superconducting gap. We find well defined quasiparticle peaks located at a bias voltage comparable to the weak coupling s-wave BCS expected gap value (0.17 meV). The amount of states at the Fermi level is however large and the curves are significantly broadened. We find T$_c$ of 1.2 K by following the tunneling conductance with temperature.

cond-mat.supr-con↗

arXiv2013

Scanning tunneling spectroscopy of layers of superconducting 2H-TaSe$_\textbf{2}$: Evidence for a zero bias anomaly in single layers

We report a characterization of surfaces of the dichalcogenide TaSe$_2$ using scanning tunneling microscopy and spectroscopy (STM/S) at 150 mK. When the top layer has the 2H structure and the layer immediately below the 1T structure, we find a singular spatial dependence of the tunneling conductance below 1 K, changing from a zero bias peak on top of Se atoms to a gap in between Se atoms. The zero bias peak is additionally modulated by the commensurate $3a_0 \times 3a_0$ charge density wave of 2H-TaSe$_2$. Multilayers of 2H-TaSe$_2$ show a spatially homogeneous superconducting gap with a critical temperature also of 1 K. We discuss possible origins for the peculiar tunneling conductance in single layers.

cond-mat.supr-con↗

arXiv2012

Magnetic and superconducting phase diagrams in ErNi2B2C

We present measurements of the superconducting upper critical field Hc2(T) and the magnetic phase diagram of the superconductor ErNi2B2C made with a scanning tunneling microscope (STM). The magnetic field was applied in the basal plane of the tetragonal crystal structure. We have found large gapless regions in the superconducting phase diagram of ErNi2B2C, extending between different magnetic transitions. A close correlation between magnetic transitions and Hc2(T) is found, showing that superconductivity is strongly linked to magnetism.

cond-mat.supr-con↗

arXiv2012

Scanning microscopies of superconductors at very low temperatures

We discuss basics of scanning tunneling microscopy and spectroscopy (STM/S) of the superconducting state with normal and superconducting tips. We present a new method to measure the local variations in the Andreev reflection amplitude between a superconducting tip and the sample. This method is termed Scanning Andreev Reflection Spectroscopy (SAS). We also briefly discuss vortex imaging with STM/S under an applied current through the sample, and show the vortex lattice as a function of the angle between the magnetic field and sample's surface.

cond-mat.supr-con↗