arXiv subjects

L. Balicas

Publications and source records attributed to L. Balicas.

At least 19 recordsLinked to original sources

arXiv2026

Charge dynamics in the Weyl semimetals NbIrTe$_4$ and TaIrTe$_4$ under pressure: Signatures of an electronic phase transition

A high-pressure investigation of the Weyl semimetals NbIrTe$_4$ and TaIrTe$_4$ is presented, using infrared spectroscopy supplemented by density functional theory calculations. The experimental optical conductivity spectra as a function of pressure suggest the occurrence of a pressure-induced phase transition at a critical pressure $P_\text{c}=7\text{--}8$ GPa. This transition is most likely electronic in nature, as Raman scattering measurements provide no evidence of a significant structural phase transition. Above $P_\text{c}$ a significant redistribution of spectral weight occurs in the optical conductivity spectrum for both materials. A Drude-Lorentz analysis of the optical data indicates a sharp reduction in the free carrier concentration at $P_\text{c}$, concomitant with the appearance of a low-energy phonon, which was initially screened by free charge carriers. A predominantly electronic origin of the phase transition is supported by the calculated electronic band structure, Fermi surface, and interband optical conductivity as a function of pressure. Our findings provide collective evidence for a pressure-induced, most likely electronic phase transition in both van der Waals materials at $P_\text{c}=7\text{--}8$ GPa, highlighting the tunability of their electronic band structure by hydrostatic pressure.

cond-mat.mtrl-sci↗

arXiv2026

Tunable Polariton Canalization in Natural van der Waals Oxide

Hyperbolic phonon polaritons (HPPs) are coupled oscillations of anisotropic lattice vibrations and electromagnetic fields that confine the latter to the nanoscale, enabling novel nano-polaritonic devices. While HPPs have been identified in multiple layered materials, achieving advanced control and manipulation - particularly polariton canalization for unidirectional energy flow - often necessitates complex device fabrications or crystal modifications. Here we visualize and elucidate the properties of in-plane hyperbolicity in alpha-V2O5, a layered compound with a highly anisotropic permittivity tensor. We show unidirectional Poynting-vector propagation of polaritons in alpha-V2O5 without additional treatments. Combined with theoretical modeling, our infrared nano-imaging studies unveil a novel form of polariton canalization, with its dispersion contour continuously tunable by the incident light frequency. Additionally, we provide a theoretically calculated permittivity phase diagram for tailoring polaritonic wavefronts. These findings suggest that the metal-oxide alpha-V2O5 holds great promise for on-demand light canalization and control at the nanoscale.

physics.optics↗

arXiv2025

Upper critical magnetic field and multiband superconductivity in artificial high-Tc superlattices of nano quantum wells

Artificial high-Tc superlattices (AHTS) composed of quantum building blocks with tunable superconducting critical temperature have been synthesized by engineering their nanoscale geometry using the Bianconi-Perali-Valletta (BPV) two gaps superconductivity theory. These quantum heterostructures consist of quantum wells made of superconducting, modulation-doped Mott insulators (S), confined by a metallic (N) potential barrier. The lattice geometry has been carefully engineered to induce the predicted Fano-Feshbach shape resonance between the gaps, near a topological Lifshitz transition. Here, we validate the BPV theory by providing compelling experimental evidence that AHTS samples, at the peak of the superconducting dome, exhibit resonant two-band, two-gap superconductivity. This is demonstrated by measuring the temperature dependence of the upper critical magnetic field,Bc2, in samples with superlattice periods 3.3<d<5.28 nm and L/d ratios close to the magic value 2/3 (where L is the thickness of the superconducting La2CuO4 layer and d is the superlattice period). The data reveal the predicted upward concavity in Hc2(T) and a characteristic kink in the coherence length as a function of temperature, confirming the predicted two-band superconductivity with Fermi velocity ratio 0.25 and significant pair exchange term among the two condensates.

cond-mat.mes-hall↗

arXiv2022

High-temperature superconductivity in hydrides: experimental evidence and details

Since the discovery of superconductivity at 200 K in H3S [1] similar or higher transition temperatures, Tcs, have been reported for various hydrogen-rich compounds under ultra-high pressures [2]. Superconductivity was experimentally proved by different methods, including electrical resistance, magnetic susceptibility, optical infrared, and nuclear resonant scattering measurements. The crystal structures of superconducting phases were determined by X-ray diffraction. Numerous electrical transport measurements demonstrate the typical behaviour of a conventional phonon-mediated superconductor: zero resistance below Tc, the shift of Tc to lower temperatures under external magnetic fields, and pronounced isotope effect. Remarkably, the results are in good agreement with the theoretical predictions, which describe superconductivity in hydrides within the framework of the conventional BCS theory. However, despite this acknowledgment, experimental evidence for the superconducting state in these compounds has recently been treated with criticism [3, 4], which apparently stems from misunderstanding and misinterpretation of complicated experiments performed under very high pressures. Here, we describe in greater detail the experiments revealing high-temperature superconductivity in hydrides under high pressures. We show that the arguments against superconductivity [3, 4] can be either refuted or explained. The experiments on the high-temperature superconductivity in hydrides clearly contradict the theory of hole superconductivity [4] and eliminate it [3].

cond-mat.supr-con↗

arXiv2021

Experimental observation and spin texture of Dirac node arcs in tetradymite topological metals

We report the observation of a non-trivial spin texture in Dirac node arcs, novel topological objects formed when Dirac cones of massless particles extend along an open one-dimensional line in momentum space. We find that such states are present in all the compounds of the tetradymite M$_2$Te$_2$X family (M$=$Ti, Zr or Hf and X$=$P or As), regardless of the weak or strong character of the topological invariant. The Dirac node arcs in tetradymites are thus the simplest possible, textbook example, of a type-I Dirac system with a single spin-polarized node arc.

cond-mat.mes-hall↗

arXiv2021

Interplay between strong correlations and electronic topology in the underlying kagome lattice of Na2/3CoO2

Electronic topology in metallic kagome compounds is under intense scrutiny. We present transport experiments in Na2/3CoO2 in which the Na order differentiates a Co kagome sub-lattice in the triangular CoO2 layers. Hall and magnetoresistance (MR) data under high fields give evidence for the coexistence of light and heavy carriers. At low temperatures, the dominant light carrier conductivity at zero field is suppressed by a B-linear MR suggesting Dirac like quasiparticles. Lifshitz transitions induced at large B and T unveil the lower mobility carriers. They display a negative B^2 MR due to scattering from magnetic moments likely pertaining to a flat band. We underline an analogy with heavy Fermion physics.

cond-mat.str-el↗

arXiv2020

Layer- and Gate-tunable Spin-Orbit Coupling in a High Mobility Few-Layer Semiconductor

Spin-orbit coupling (SOC) is a relativistic effect, where an electron moving in an electric field experiences an effective magnetic field in its rest frame. In crystals without inversion symmetry, it lifts the spin degeneracy and leads to many magnetic, spintronic and topological phenomena and applications. In bulk materials, SOC strength is a constant that cannot be modified. Here we demonstrate SOC and intrinsic spin-splitting in atomically thin InSe, which can be modified over an unprecedentedly large range. From quantum oscillations, we establish that the SOC parameter αis thickness-dependent; it can be continuously modulated over a wide range by an out-of-plane electric field, achieving intrinsic spin splitting tunable between 0 and 20 meV. Surprisingly, αcould be enhanced by an order of magnitude in some devices, suggesting that SOC can be further manipulated. Our work highlights the extraordinary tunability of SOC in 2D materials, which can be harnessed for in operando spintronic and topological devices and applications.

cond-mat.mes-hall↗

arXiv2019

The Fermi surface of PtCoO2 from quantum oscillations and electronic structure calculations

The delafossite series of layered oxides include some of the highest conductivity metals ever discovered. Of these, PtCoO2, with a room temperature resistivity of 1.8 microOhmcm for in-plane transport, is the most conducting of all. The high conduction takes place in triangular lattice Pt layers, separated by layers of Co-O octahedra, and the electronic structure is determined by the interplay of the two types of layer. We present a detailed study of quantum oscillations in PtCoO2, at temperatures down to 35 mK and magnetic fields up to 30 T. As for PdCoO2 and PdRhO2, the Fermi surface consists of a single cylinder with mainly Pt character, and an effective mass close to the free electron value. Due to Fermi-surface warping, two close-lying high frequencies are observed. Additionally, a pronounced difference frequency appears. By analysing the detailed angular dependence of the quantum-oscillation frequencies, we establish the warping parameters of the Fermi surface. We compare these results to the predictions of first-principles electronic structure calculations including spin-orbit coupling on Pt and Co and on-site correlation U on Co, and hence demonstrate that electronic correlations in the Co-O layers play an important role in determining characteristic features of the electronic structure of PtCoO2.

cond-mat.str-el↗

arXiv2019

Superconductivity up to 243 K in yttrium hydrides under high pressure

The discovery of high-temperature conventional superconductivity in H3S with a critical temperature of Tc=203 K was followed by the recent record of Tc ~250 K in the face-centered cubic (fcc) lanthanum hydride LaH10 compound. It was realized in a new class of hydrogen-dominated compounds having a clathrate-like crystal structure in which hydrogen atoms form a 3D framework and surround a host atom of rare earth elements. Yttrium hydrides are predicted to have even higher Tc exceeding room temperature. In this paper, we synthesized and refined the crystal structure of new hydrides: YH4, YH6, and YH9 at pressures up to 237 GPa finding that YH4 crystalizes in the I4/mmm lattice, YH6 in Im-3m lattice and YH9 in P63/mmc lattice in excellent agreement with the calculations. The observed very high-temperature superconductivity is comparable to that found in fcc-LaH10: the pressure dependence of Tc for YH9 also displays a "dome like shape" with the highest Tc of 243 K at 201 GPa. We also observed a Tc of 227 K at 237 GPa for the YH6 phase. However, the measured Tcs are notably lower by ~30 K than predicted. Evidence for superconductivity includes the observation of zero electrical resistance, a decrease of Tc under an external magnetic field and an isotope effect. The theoretically predicted fcc YH10 with the promising highest Tc>300 K was not stabilized in our experiments under pressures up to 237 GPa.

cond-mat.supr-con↗

arXiv2019

Origin of the butterfly magnetoresistance in a Dirac nodal-line system

We report a study on the magnetotransport properties and on the Fermi surfaces (FS) of the ZrSi(Se,Te) semimetals. Density Functional Theory (DFT) calculations, in absence of spin orbit coupling (SOC), reveal that both the Se and the Te compounds display Dirac nodal lines (DNL) close to the Fermi level $\varepsilon_F$ at symmorphic and non-symmorphic positions, respectively. We find that the geometry of their FSs agrees well with DFT predictions. ZrSiSe displays low residual resistivities, pronounced magnetoresistivity, high carrier mobilities, and a butterfly-like angle-dependent magnetoresistivity (AMR), although its DNL is not protected against gap opening. As in Cd$_3$As$_2$, its transport lifetime is found to be 10$^2$ to 10$^3$ times larger than its quantum one. ZrSiTe, which possesses a protected DNL, displays conventional transport properties. Our evaluation indicates that both compounds most likely are topologically trivial. Nearly angle-independent effective masses with strong angle dependent quantum lifetimes lead to the butterfly AMR in ZrSiSe.

cond-mat.str-el↗

arXiv2018

Superconductivity at 250 K in lanthanum hydride under high pressures

The discovery of superconductivity at 203 K in H3S brought attention back to conventional superconductors whose properties can be described by the Bardeen-Cooper-Schrieffer (BCS) and the Migdal-Eliashberg theories. These theories predict that high, and even room temperature superconductivity (RTSC) is possible in metals possessing certain favorable parameters such as lattice vibrations at high frequencies. However, these general theories do not suffice to predict real superconductors. New superconducting materials can be predicted now with the aid of first principles calculations based on Density Functional Theory (DFT). In particular, the calculations suggested a new family of hydrides possessing a clathrate structure, where the host atom (Ca, Y, La) is at the center of the cage formed by hydrogen atoms. For LaH10 and YH10 superconductivity, with critical temperatures Tc ranging between 240 and 320 K is predicted at megabar pressures. Here, we report superconductivity with a record Tc ~ 250 K within the Fm-3m structure of LaH10 at a pressure P ~ 170 GPa. We proved the existence of superconductivity at 250 K through the observation of zero-resistance, isotope effect, and the decrease of Tc under an external magnetic field, which suggests an upper critical magnetic field of 120 T at zero-temperature. The pressure dependence of the transition temperatures Tc (P) has a maximum of 250-252 K at the pressure of about 170 GPa. This leap, by ~ 50 K, from the previous Tc record of 203 K indicates the real possibility of achieving RTSC (that is at 273 K) in the near future at high pressures and the perspective of conventional superconductivity at ambient pressure.

cond-mat.supr-con↗

arXiv2018

Planar Hall-effect, Anomalous planar Hall-effect, and Magnetic Field-Induced Phase Transitions in TaAs

We evaluate the topological character of TaAs through a detailed study of the angular, magnetic-field and temperature dependence of its magnetoresistivity and Hall-effect(s), and of its bulk electronic structure through quantum oscillatory phenomena. At low temperatures, and for fields perpendicular to the electrical current, we extract an extremely large Hall angle $Θ_H$ at higher fields, that is $Θ_H \sim 82.5^{\circ}$, implying a very pronounced Hall signal superimposed into its magnetoresistivity. For magnetic fields and electrical currents perpendicular to the \emph{c}-axis we observe a very pronounced planar Hall-effect, when the magnetic field is rotated within the basal plane. This effect is observed even at higher temperatures, i.e. as high as $T = 100$ K, and predicted recently to result from the chiral anomaly among Weyl points. Superimposed onto this planar Hall, which is an even function of the field, we observe an anomalous planar Hall-signal akin to the one reported for that is an odd function of the field. Below 100 K, negative longitudinal magnetoresistivity (LMR), initially ascribed to the chiral anomaly and subsequently to current inhomogeneities, is observed in samples having different geometries and contact configurations, once the large Hall signal is subtracted. Our measurements reveal a phase transition upon approaching the quantum limit that leads to the reconstruction of the FS and to the concomitant suppression of the negative LMR indicating that it is intrinsically associated with the Weyl dispersion at the Fermi level. For fields along the \emph{a}-axis it also leads to a pronounced hysteresis pointing to a field-induced electronic phase-transition. This collection of unconventional tranport observations points to the prominent role played by the axial anomaly among Weyl nodes.

cond-mat.mes-hall↗

arXiv2018

Hundredfold Enhancement of Light Emission via Defect Control in Monolayer Transition-Metal Dichalcogenides

Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties, and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods - chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above $10^{13} /cm^2$ to below $10^{11} /cm^2$. Because these point defects act as centers for non-radiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.

cond-mat.mtrl-sci↗

arXiv2018

Detailed study on the Fermi surfaces of the type-II Dirac semimetallic candidates PdTe2 and PtTe2

We present a detailed quantum oscillatory study on the Dirac type-II semimetallic candidates PdTe$_{2}$ and PtTe$_{2}$ \emph{via} the temperature and the angular dependence of the de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects. In high quality single crystals of both compounds, i.e. displaying carrier mobilities between $10^3$ and $10^4$ cm$^2$/Vs, we observed a large non-saturating magnetoresistivity (MR) which in PtTe$_2$ at a temperature $T = 1.3$ K, leads to an increase in the resistivity up to $5 \times 10^{4}$ % under a magnetic field $μ_0 H = 62$ T. These high mobilities correlate with their light effective masses in the range of 0.04 to 1 bare electron mass according to our measurements. For PdTe$_{2}$ the experimentally determined Fermi surface cross-sectional areas show an excellent agreement with those resulting from band-structure calculations. Surprisingly, this is not the case for PtTe$_{2}$ whose agreement between calculations and experiments is relatively poor even when electronic correlations are included in the calculations. Therefore, our study provides a strong support for the existence of a Dirac type-II node in PdTe$_2$ and probably also for PtTe$_2$. Band structure calculations indicate that the topologically non-trivial bands of PtTe$_2$ do not cross the Fermi-level ($\varepsilon_F$). In contrast, for PdTe$_2$ the Dirac type-II cone does intersect $\varepsilon_F$, although our calculations also indicate that the associated cyclotron orbit on the Fermi surface is located in a distinct $k_z$ plane with respect to the one of the Dirac type-II node. Therefore it should yield a trivial Berry-phase.

cond-mat.str-el↗

arXiv2018

Bulk Fermi surfaces of the Dirac Type-II Semimetallic Candidates VAl3, NbAl3 and TaAl3

We report a de Haas-van Alphen (dHvA) effect study on the Dirac type-II semimetallic candidates \emph{M}Al$_3$ (where, \emph{M} = V, Nb and Ta). The angular-dependence of their Fermi surface (FS) cross-sectional areas reveals a remarkably good agreement with first-principle calculations. Therefore, dHvA supports the existence of tilted Dirac cones with Dirac type-II nodes located at 100, 230 and 250 meV above the Fermi level $\varepsilon_F$ for VAl$_3$, NbAl$_3$ and TaAl$_3$ respectively, in agreement with the prediction of broken Lorentz invariance in these compounds. However, for all three compounds we find that the cyclotron orbits on their FSs, including an orbit nearly enclosing the Dirac type-II node, yield trivial Berry phases. We explain this $via$ an analysis of the Berry phase where the position of this orbit, relative to the Dirac node, is adjusted within the error implied by the small disagreement between our calculations and the experiments. We suggest that a very small amount of doping could displace $\varepsilon_F$ to produce topologically non-trivial orbits encircling their Dirac node(s).

cond-mat.mtrl-sci↗

arXiv2018

Evidence for negative thermal expansion in the superconducting precursor phase SmFeAsO

The fluorine-doped rare-earth iron oxypnictide series SmFeAsO$_{1-x}$F$_x$ (0 $\leq x \leq$ 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies, the parent compound SmFeAsO exhibits a tetragonal-to-orthorhombic structural distortion at T$\rm{_{S}}$~=~130~K which is rapidly suppressed by $x \simeq$ 0.10 deep within the superconducting dome. The change in unit cell symmetry is followed by a previously unreported magnetoelastic distortion at 120~K. The temperature dependence of the thermal expansion coefficient $α_{V}$ reveals a rich phase diagram for SmFeAsO: (i) a global minimum at 125 K corresponds to the opening of a spin-density wave instability as measured by pump-probe femtosecond spectroscopy whilst (ii) a global maximum at 110 K corresponds to magnetic ordering of the Sm and Fe sublattices as measured by magnetic x-ray scattering. At much lower temperatures than T$\rm{_{N}}$, SmFeAsO exhibits a significant negative thermal expansion on the order of -40~ppm~$\cdot$~K$^{-1}$ in contrast to the behavior of other rare-earth oxypnictides such as PrFeAsO and the actinide oxypnictide NpFeAsO where the onset of $α<$ 0 only appears in the vicinity of magnetic ordering. Correlating this feature with the temperature and doping dependence of the resistivity and the unit cell parameters, we interpret the negative thermal expansion as being indicative of the possible condensation of itinerant electrons accompanying the opening of a SDW gap, consistent with transport measurements.

cond-mat.supr-con↗

arXiv2017

Converting topological insulators into topological metals within the tetradymite family

We report the electronic band structures and concomitant Fermi surfaces for a family of exfoliable tetradymite compounds with the formula $T_2$$Ch_2$$Pn$, obtained as a modification to the well-known topological insulator binaries Bi$_2$(Se,Te)$_3$ by replacing one chalcogen ($Ch$) with a pnictogen ($Pn$) and Bi with the tetravalent transition metals $T$ $=$ Ti, Zr, or Hf. This imbalances the electron count and results in layered metals characterized by relatively high carrier mobilities and bulk two-dimensional Fermi surfaces whose topography is well-described by first principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states. In contrast to Bi$_2$Se$_3$, where the surface Dirac bands are at the $Γ-$point, for (Zr,Hf)$_2$Te$_2$(P,As) there are Dirac cones of strong topological character around both the $\bar Γ$- and $\bar {M}$-points which are above and below the Fermi energy, respectively. For Ti$_2$Te$_2$P the surface state is predicted to exist only around the $\bar {M}$-point. In agreement with these predictions, the surface states that are located below the Fermi energy are observed by angle resolved photoemission spectroscopy measurements, revealing that they coexist with the bulk metallic state. Thus, this family of materials provides a foundation upon which to develop novel phenomena that exploit both the bulk and surface states (e.g., topological superconductivity).

cond-mat.str-el↗

arXiv2017

Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB$_6$

The search for a Fermi surface in the absence of a conventional Fermi liquid has thus far yielded very few potential candidates. Among promising materials are spin-frustrated Mott insulators near the insulator-metal transition, where theory predicts a Fermi surface associated with neutral low energy excitations. Here we reveal another route to experimentally realise a Fermi surface in the absence of a Fermi liquid by the experimental study of a Kondo insulator SmB$_6$ positioned close to the insulator-metal transition. We present experimental signatures down to low temperatures ($\ll 1$ K) associated with a Fermi surface in the bulk, including a sizeable linear specific heat coefficient, and on the application of a finite magnetic field, bulk magnetic quantum oscillations, finite quantum oscillatory entropy, and substantial enhancement in thermal conductivity well below the charge gap energy scale. Thus, the weight of evidence indicates that despite an extreme instance of Fermi liquid breakdown in Kondo insulating SmB$_6$, a Fermi surface arises from novel itinerant low energy excitations that couple to magnetic fields, but not weak DC electric fields.

cond-mat.str-el↗