arXiv subjects

Ana Akrap

Publications and source records attributed to Ana Akrap.

At least 19 recordsLinked to original sources

arXiv2026

Interlayer interactions reshape charge-density wave through electronic elasticity in 4H$_{\mathrm{b}}$-TaS$_2$

Incommensurate charge-density waves (CDWs) in layered quantum materials frequently exhibit widely varying ordering wave vectors, even among nominally identical samples, obscuring their intrinsic electronic properties. Here we identify an inherent origin of this variability through the electronic elasticity of an incommensurate CDW using the model heterostructure 4H$_{\mathrm{b}}$-TaS$_2$, composed of alternating commensurate CDW on 1T and incommensurate CDW on 1H layers. Low-temperature scanning tunneling microscopy, combined with Fourier and quasiparticle-interference analysis, exploits the lattice-pinned 1T CDW as an internal reference to resolve discrete compressive ($-2.3\%$) and tensile ($+3.2\%$) elastic states of the neighboring 1H CDW selected by the interlayer registry of the adjacent layers. Corresponding few-meV shifts of a flat band demonstrate that weak interlayer interactions reshape the low-energy electronic structure through the intrinsic elasticity of the incommensurate CDW. These findings establish electronic elasticity as a mechanism by which subtle interlayer interactions control correlated electronic states in van der Waals heterostructures.

cond-mat.str-el↗

arXiv2026

Landau level spectroscopy in current solid state physics

Landau level spectroscopy plays an important role in modern condensed-matter physics. In this technique, electrons in a solid are subjected to quantizing magnetic fields and probed experimentally, often through optical methods. Direct and detailed insights into the electronic properties of crystalline materials are obtained, particularly the properties related to their band structure. Landau level spectroscopy enables the precise extraction of key parameters such as effective mass, carrier density, mobility, and band gap, and serves as a powerful tool for studying interactions between electrons and other quasiparticles in solids. Over its more than seventy-year history, Landau level spectroscopy has been applied mainly to semiconductors and semimetals. Today, its scope also includes graphene-based systems, surface and bulk states in topological materials, and other emergent systems with a narrow or vanishing band gap. In this work, we review the fundamentals of Landau level spectroscopy and illustrate them with selected examples from the literature.

cond-mat.mtrl-sci↗

arXiv2025

Optical conductivity of layered topological semimetal TaNiTe$_5$

We present an infrared spectroscopy study of the layered topological semimetal TaNiTe$_5$, a material with a quasi-one-dimensional structure and strong in-plane anisotropy. Despite its structural features, infrared reflectivity and electronic transport measurements along the $a$ and $c$ crystallographic axes show metallic behavior without evidence of reduced dimensionality. Optical conductivity reveals an anisotropic but conventional metallic response with low scattering rates and a single sharp infrared-active phonon mode at $396$ cm$^{-1}$ ($49$ meV). Ab initio calculations closely match the experimental optical data and confirm a three-dimensional electronic structure. Our results demonstrate that TaNiTe$_5$ behaves as a three-dimensional anisotropic semimetal in its electronic and optical properties.

cond-mat.str-el↗

arXiv2024

Magneto-optical response of magnetic semiconductors EuCd2X2 (X= P, As, Sb)

In this study, we identify EuCd2X2 (for X = P, As, Sb) as a series of magnetic semiconductors. We examine how the band gap of the series responds to X changing from phosphorus (P), to arsenic (As), and finally antimony (Sb). We characterize the samples using electronic transport and magnetization measurements. Based on infrared spectroscopy, we find that the band gap reduces progressively from 1.23 eV in EuCd2P2, to 0.77 eV in EuCd2As2, and finally 0.52 eV in EuCd2Sb2. In a magnetic field, all three systems show a strong response and their band gaps decrease at 4 K. This decrease is non-monotonic as we change X. It is strongest in the phosphorous compound and weakest in the antimony compound. For all the three compositions, EuCd2X2 remains a semiconductor up to the highest magnetic field applied (16 T).

cond-mat.mtrl-sci↗

arXiv2023

EuCd$_2$As$_2$: a magnetic semiconductor

EuCd$_2$As$_2$ is now widely accepted as a topological semimetal in which a Weyl phase is induced by an external magnetic field. We challenge this view through firm experimental evidence using a combination of electronic transport, optical spectroscopy and excited-state photoemission spectroscopy. We show that the EuCd$_2$As$_2$ is in fact a semiconductor with a gap of 0.77 eV. We show that the externally applied magnetic field has a profound impact on the electronic band structure of this system. This is manifested by a huge decrease of the observed band gap, as large as 125~meV at 2~T, and consequently, by a giant redshift of the interband absorption edge. However, the semiconductor nature of the material remains preserved. EuCd$_2$As$_2$ is therefore a magnetic semiconductor rather than a Dirac or Weyl semimetal, as suggested by {\em ab initio} computations carried out within the local spin-density approximation.

cond-mat.mtrl-sci↗

arXiv2022

ATP-induced reconfiguration of the micro-viscoelasticity of cardiac and skeletal myosin solutions

We study the high-frequency micro-mechanical response of suspensions composed by cardiac and skeletal muscle myosin by optical trapping interferometry. We observe that in low ionic strength solutions upon the addition of magnesium adenosine triphosphate (\ch{MgATP^2-}), myosin suspensions radically change their micro-mechanics properties, generating a viscoelastic fluid characterized by a complex modulus similar to a suspension of worm-like micelles. This transduction of energy, from chemical to mechanical, may be related to the relaxed states of myosin, which regulate muscle contractility and can be involved in the etiology of many myopathies. Within an analogous generic mechanical response, cardiac and skeletal myosin suspensions provide different stress relaxation times, elastic modulus values, and characteristic lengths. These discrepancies probably rely on the dissimilar physiological functions of cardiac and skeletal muscle, on the different MgATPase hydrolysis rates of cardiac and skeletal myosins, and on the observed distinct cooperative behavior of their myosin heads in the super-relaxed state. \textit{In vitro} studies like these allow to understand the foundations of muscle cells mechanics on the micro-scale, and may contribute to the engineering of biological materials whose micro-mechanics can be activated by energy regulators.

cond-mat.soft↗

arXiv2022

Micro-mechanical response and power-law exponents from the longitudinal fluctuations of F-actin solutions

We investigate the local fluctuations of filamentous actin (F-actin), with focus on the skeletal thin filament, using single-particle optical trapping interferometry. This experimental technique allows us to detect the Brownian motion of a tracer bead immersed in a complex fluid with nanometric resolution at the microsecond time-scale. The mean square displacement, loss modulus, and velocity autocorrelation function (VAF) of the trapped microprobes in the fluid follow power-law behaviors, whose exponents can be determined in the short-time/high-frequency regime along several decades. We obtain 7/8 subdiffusive power-law exponents for polystyrene depleted microtracers at low optical trapping forces. Microrheologically, the elastic modulus of these suspensions is observed to be constant up to the limit of high frequencies, confirming the origin of this subdiffusive exponent on the local longitudinal fluctuations of the polymers. Deviations from this value are measured and discussed in relation to the characteristic lengths scales of these F-actin networks and probes' properties, and also in connection with the different power-law exponents detected in the VAFs. Finally, we observe that the thin filament, composed by tropomyosin (Tm) and troponin (Tn) coupled to F-actin in the presence of Ca$^{2+}$, returns exponent values less dispersed than F-actin alone, which we interpret as a micro-measurement of the filament stabilization.

cond-mat.soft↗

arXiv2022

Magnetic freeze-out and anomalous Hall effect in ZrTe$_5$

The ultra-quantum limit is achieved when a magnetic field confines an electron gas in its lowest spin-polarised Landau level. Here we show that in this limit, electron doped ZrTe$_5$ shows a metal-insulator transition followed by a sign change of the Hall and Seebeck effects at low temperature. We attribute this transition to a magnetic freeze-out of charge carriers on the ionised impurities. The reduction of the charge carrier density gives way to an anomalous Hall response of the spin-polarised electrons. This behaviour, at odds with the usual magnetic freeze-out scenario, occurs in this Dirac metal because of its tiny Fermi energy, extremely narrow band gap and a large $g$-factor. We discuss the different possible sources (intrinsic or extrinsic) for this anomalous Hall contribution.

cond-mat.mtrl-sci↗

arXiv2022

Characterization of two electronic subsystems in cuprates through optical conductivity

Understanding the physical properties of unconventional superconductors as well as of other correlated materials presents a formidable challenge. Their unusual evolution with doping, frequency, and temperature has frequently led to non-Fermi-liquid (non-FL) interpretations. Optical conductivity is a major challenge in this context. Here, the optical spectra of two archetypal cuprates, underdoped HgBa$_2$CuO$_{4+\delta }$ and optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta }$, are interpreted based on the standard Fermi liquid (FL) paradigm. At both dopings, perfect frequency-temperature FL scaling is found to be modified by the presence of a second, gapped electronic subsystem. This non-FL component emerges as a well-defined mid-infrared spectral feature after the FL contribution -- determined independently by transport -- is subtracted. Temperature, frequency, and doping evolution of the MIR feature identify a gapped rather than dissipative response. In contrast, the dissipative response is found to be relevant for pnictides and ruthenates. Such an unbiased FL/non-FL separation is extended across the cuprate phase diagram, capturing all the key features of the normal state and providing a natural explanation why the superfluid density is attenuated on the overdoped side. Thus, we obtain a unified interpretation of optical responses and transport measurements in all analyzed physical regimes and all analyzed compounds.

cond-mat.supr-con↗

arXiv2021

Addressing Shape and Extent of Weyl cones in TaAs by Landau level spectroscopy

Tantalum arsenide, TaAs, is a prime example of a topological semimetal with two types of Weyl nodes, W1 and W2, whose bulk signatures have proven elusive. We apply Landau level spectroscopy to crystals with multiple facets and identify -- among other low-energy excitations between parabolic bands -- the response of a cone extending over a wide energy range. Comparison with density functional theory studies allows us to associate this conical band with nearly isotropic W2 nodes. In contrast, W1 cones, which are more anisotropic and less extended in energy, appear to be buried too deep beneath the Fermi level. They cannot be accessed directly. Instead, the excitations in their vicinity give rise to an optical response typical of a narrow-gap semiconductor rather than a Weyl semimetal.

cond-mat.mes-hall↗

arXiv2021

Synthesis of Murunskite Single Crystals: A Bridge Between Cuprates and Pnictides

Numerous contemporary investigations in condensed matter physics are devoted to high temperature (high-$T_c$ ) cuprate superconductors. Despite its unique effulgence among research subjects, the enigma of the high-$T_c$ mechanism still persists. One way to advance its understanding is to discover and study new analogous systems. Here we begin a novel exploration of the natural mineral murunskite, K$_2$FeCu$_3$S$_4$, as an interpolation compound between cuprates and ferropnictides, the only known high-$T_c$ superconductors at ambient pressure. Because in-depth studies can be carried out only on single crystals, we have mastered the synthesis and growth of high quality specimens. Similar to the cuprate parent compounds, these show semiconducting behavior in resistivity and optical transmittance, and an antiferromagnetic ordering at 100 K. Spectroscopy (XPS) and calculations (DFT) concur that the sulfur 3$p$ orbitals are partially open, making them accessible for charge manipulation, which is a prerequisite for superconductivity in analogous layered structures. DFT indicates that the valence band is more cuprate-like, while the conduction band is more pnictide-like. With appropriate doping strategies, this parent compound promises exciting future developments.

cond-mat.mtrl-sci↗

arXiv2021

Carrier concentrations and optical conductivity of a band-inverted semimetal in two and three dimensions

Here we study the single-particle, electronic transport and optical properties of a gapped system described by a simple two-band Hamiltonian with inverted valence bands. We analyze its properties in the three-dimensional (3D) and the two-dimensional (2D) case. The insulating phase changes into a metallic phase when the band gap is set to zero. The metallic phase in the 3D case is characterized by a nodal surface. This nodal surface is equivalent to a nodal ring in two dimensions. Within a simple theoretical framework, we calculate the density of state, the total and effective charge carrier concentration, the Hall concentration and the Hall coefficient, for both 2D and 3D cases. The main result is that the three concentrations always differ from one another in the present model. These concentrations can then be used to resolve the nature of the electronic ground state. Similarly, the optical conductivity is calculated and discussed for the insulating phase. We show that there are no optical excitations in the metallic phase. Finally, we compare the calculated optical conductivity with the rule-of-thumb derivation using the joint density of states.

cond-mat.mes-hall↗

arXiv2021

Unidirectional Kondo scattering in layered NbS2

Crystalline defects can modify quantum interactions in solids, causing unintuitive, even favourable, properties such as quantum Hall effect or superconducting vortex pinning. Here we present another example of this notion - an unexpected unidirectional Kondo scattering in single crystals of 2H-NbS2. This manifests as a pronounced low-temperature enhancement in the out-of-plane resistivity and thermopower below 40 K, hidden for the in-plane charge transport. The anomaly can be suppressed by the c-axis-oriented magnetic field, but is unaffected by field applied along the planes. The magnetic moments originate from layers of 1T-NbS2, which inevitably form during the growth, undergoing a charge-density-wave reconstruction with each superlattice cell (David-star-shaped cluster of Nb atoms) hosting a localised spin. Our results demonstrate the unique and highly anisotropic response of a spontaneously formed Kondo lattice heterostructure, intercalated in a layered conductor.

cond-mat.str-el↗

arXiv2020

Structural phase transition and bandgap control through mechanical deformation in layered semiconductors 1T-ZrX2 (X = S, Se)

Applying elastic deformation can tune a material physical properties locally and reversibly. Spatially modulated lattice deformation can create a bandgap gradient, favouring photo-generated charge separation and collection in optoelectronic devices. These advantages are hindered by the maximum elastic strain that a material can withstand before breaking. Nanomaterials derived by exfoliating transition metal dichalcogenides TMDs are an ideal playground for elastic deformation, as they can sustain large elastic strains, up to a few percent. However, exfoliable TMDs with highly strain-tunable properties have proven challenging for researchers to identify. We investigated 1T-ZrS2 and 1T-ZrSe2, exfoliable semiconductors with large bandgaps. Under compressive deformation, both TMDs dramatically change their physical properties. 1T-ZrSe2 undergoes a reversible transformation into an exotic three-dimensional lattice, with a semiconductor-to-metal transition. In ZrS2, the irreversible transformation between two different layered structures is accompanied by a sudden 14 % bandgap reduction. These results establish that Zr-based TMDs are an optimal strain-tunable platform for spatially textured bandgaps, with a strong potential for novel optoelectronic devices and light harvesting.

cond-mat.mtrl-sci↗

arXiv2020

Distinguishing the gapped and Weyl semimetal scenario in ZrTe$_5$: insights from an effective two-band model

Here we study the static and dynamic transport properties of a low energy two-band model proposed previously in E. Martino et al. [PRL 122, 217402 (2019), arXiv:1905.00280], with an anisotropic in-plane linear momentum dependence, and a parabolic out-of-plane dispersion. The model is extended to include a negative band gap, which leads to the emergence of a Weyl semimetal (WSM) state, as opposed to the gapped semimetal (GSM) state when the band gap is positive. We calculate and compare the zero and finite frequency transport properties of the GSM and WSM cases. The $dc$ properties that are calculated for the GSM and WSM cases are Drude spectral weight, mobility and resistivity. We determine their dependence on the Fermi energy and crystal direction. The in- and out-of-plane optical conductivities are calculated in the limit of the vanishing interband relaxation rate for both semimetals. The main common features are an $\omega^{1/2}$ in-plane and $\omega^{3/2}$ out-of-plane frequency dependence of the optical conductivity. We seek particular features related to the charge transport that could unambiguously point to one ground state over the other, based on the comparison with the experiment. Differences between the WSM and GSM are in principle possible only at extremely low carrier concentrations, and at low temperatures.

cond-mat.mes-hall↗

arXiv2020

Optical conductivity of the type-II Weyl semimetal TaIrTe$_4$

TaIrTe$_4$ is an example of a candidate Weyl type-II semimetal with a minimal possible number of Weyl nodes. Four nodes are reported to exist a single plane in $k$-space. The existence of a conical dispersion linked to Weyl nodes has yet to be shown experimentally. Here we use optical spectroscopy as a probe of the band structure on a low-energy scale. Studying optical conductivity allows us to probe intraband and interband transitions with zero momentum. In TaIrTe$_4$, we observe a narrow Drude contribution and an interband conductivity that may be consistent with a tilted linear band dispersion up to 40~meV. The interband conductivity allows us to establish the effective parameters of the conical dispersion; effective velocity $v=1.1\cdot 10^{4}$~m/s and tilt $\gamma=0.37$. The transport data, Seebeck and Hall coefficients, are qualitatively consistent with conical features in the band structure. Quantitative disagreement may be linked to the multiband nature of TaIrTe$_4$.

cond-mat.mtrl-sci↗

arXiv2019

Probing intraband excitations in ZrTe$_5$: a high-pressure infrared and transport study

Zirconium pentatetelluride, ZrTe5, shows remarkable sensitivity to hydrostatic pressure. In this work we address the high-pressure transport and optical properties of this compound, on samples grown by flux and charge vapor transport. The high-pressure resistivity is measured up to 2 GPa, and the infrared transmission up to 9 GPa. The dc conductivity anisotropy is determined using a microstructured sample. Together, the transport and optical measurements allow us to discern band parameters with and without the hydrostatic pressure, in particular the Fermi level, and the effective mass in the less conducting, out-of-plane direction. The results are interpreted within a simple two-band model characterized by a Dirac-like, linear in-plane band dispersion, and a parabolic out-of-plane dispersion.

cond-mat.mtrl-sci↗

arXiv2019

Low-energy excitations in type-II Weyl semimetal Td-MoTe2 evidenced through optical conductivity

Molybdenum ditelluride, MoTe2, is a versatile material where the topological phase can be readily tuned by manipulating the associated structural phase transition. The fine details of the band structure of MoTe2, key to understanding its topological properties, have proven difficult to disentangle experientially due to the multi-band character of the material. Through experimental optical conductivity spectra, we detect two strong low-energy interband transitions. Both are linked to excitations between spin-orbit split bands. The lowest interband transition shows a strong thermal shift, pointing to a chemical potential that dramatically decreases with temperature. With the help of ab initio calculations and a simple two-band model, we give qualitative and quantitative explanation of the main features in the temperature-dependent optical spectra up to 400 meV.

cond-mat.mtrl-sci↗