arXiv subjects

Ashvini Vallipuram

Publications and source records attributed to Ashvini Vallipuram.

6 recordsLinked to original sources

arXiv2026

Observation of microscopic domain effects in the metal-insulator transition of thin-film NdNiO$_3$

Perovskite oxides display correlated electrical, magnetic, and thermal properties that can be further tuned in the thin-film limit, making them contenders for next-generation electronics. Measuring thermal transport in thin films is challenging, because traditional techniques are dominated by the substrate. Here, frequency-domain thermoreflectance (FDTR) of an epitaxial NdNiO$_3$ thin film reveals a sharp change in out-of-plane thermal conductivity across the metal-insulator transition. Complementary frequency-domain photoreflectance (FDPR) reveals a large change in ambipolar diffusivity of photoexcited carriers. While the in-plane electrical resistance shows large hysteresis, out-of-plane thermal and charge transport shows negligible hysteresis. We attribute this discrepancy to anisotropy in the percolation of nanoscale domains across the transition as the film thickness approaches the domain length scale. We establish FDTR and FDPR as sensitive probes of quantum material phase transitions and highlight NdNiO$_3$ for thermal control and memory applications.

cond-mat.mes-hall↗

arXiv2025

Thermal Hall conductivity in the strongest cuprate superconductor: Estimate of the mean free path in the trilayer cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8 + δ}$

The thermal Hall conductivity of the trilayer cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ (Hg1223) - the superconductor with the highest critical temperature $T_c$ at ambient pressure - was measured at temperatures down to 2 K for three dopings in the underdoped regime ($p$ = 0.09, 0.10, 0.11). By combining a previously introduced simple model and prior theoretical results, we derive a formula for the inverse mean free path, $1 / \ell$, which allows us to estimate the mean free path of $d$-wave quasiparticles in Hg1223 below $T_c$. We find that $1 / \ell$ grows as $T^3$, in agreement with the theoretical expectation for a clean $d$-wave superconductor. Measurements were also conducted on the single layer mercury-based cuprate HgBa$_2$CuO$_{6+δ}$ (Hg1201), revealing that the mean free path in this compound is roughly half that of its three-layered counterpart at the same doping ($p$ = 0.10). This observation is be attributed to the protective role of the outer planes in Hg1223, which results in a more pristine inner plane. We also report data in an ultraclean crystal of YBa$_2$Cu$_3$O$_y$ (YBCO) with full oxygen content $p$ = 0.18, believed to be the cleanest of any cuprate, and find that $\ell$ is not longer than in Hg1223.

cond-mat.supr-con↗

arXiv2025

Electronic mean free path of the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ from thermal Hall conductivity

We use thermal transport to access the electronic mean free path of $d$-wave quasiparticles in one of the most widely studied cuprate superconductors, Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212). We have measured the thermal conductivity $κ_{\rm xx}$ and the thermal Hall conductivity $κ_{\rm xy}$ of three single crystals across a range of dopings. In the overdoped and optimally-doped samples, a clear enhancement is observed in both $κ_{\rm xx}$ and $κ_{\rm xy}$ upon cooling below the critical temperature $T_{\rm c}$, due to a suppression of the inelastic electron-electron scattering as electrons condense into pairs. The underdoped sample shows no enhancement in either, pointing to a high degree of disorder in that sample. For the two highest dopings, the magnitude of the enhancement in $κ_{\rm xy}$ is controlled by the strength of the elastic impurity scattering. Using a prior model to estimate the mean free path from $κ_{\rm xy}$ data, we find that the mean free path in Bi2212 is approximately 7 times shorter than in YBa$_2$Cu$_3$O$_7$, considered to be one of the least disordered cuprates. We conclude that the thermal Hall technique is a good way to compare the mean free path of $d$-wave quasiparticles in various cuprate materials.

cond-mat.supr-con↗

arXiv2024

Role of magnetic ions in the thermal Hall effect of the paramagnetic insulator TmVO$_{4}$

In a growing number of materials, phonons have been found to generate a thermal Hall effect, but the underlying mechanism remains unclear. Inspired by previous studies that revealed the importance of Tb$^{3+}$ ions in generating the thermal Hall effect in a family of pyrochlores, we investigated the role of Tm$^{3+}$ ions in TmVO$_{4}$, a paramagnetic insulator with a different crystal structure. We observe a negative thermal Hall conductivity in TmVO$_{4}$ with a magnitude such that the Hall angle, $|κ_{xy}$/$κ_{xx}|$, is approximately 1 x 10$^{-3}$ at $H$ = 15 T and $T$ = 20 K, typical for a phonon-generated thermal Hall effect. In contrast to the negligible $κ_{xy}$ found in the nonmagnetic pyrochlore analog (where the Tb$^{3+}$ ions are replaced with Y$^{3+}$), we observe a negative $κ_{xy}$ in YVO$_{4}$ with a Hall angle of magnitude comparable to that of TmVO$_{4}$. This shows that the Tm$^{3+}$ ions are not essential for the thermal Hall effect in this family of materials. Interestingly, at an intermediate Y concentration of $x$ = 0.3 in Tm$_{1-x}$Y$_{x}$VO$_{4}$, $κ_{xy}$ was found to have a positive sign, pointing to the importance of impurities in the thermal Hall effect of phonons.

cond-mat.mtrl-sci↗

arXiv2023

Planar thermal Hall effect from phonons in a Kitaev candidate material

Kitaev materials are a promising platform for the realization of quantum spin liquid states. The thermal Hall effect has emerged as a potential probe of exotic excitations within such states. In the Kitaev candidate material $α$-RuCl$_{3}$, however, the thermal Hall conductivity $κ_{xy}$ has been attributed not only to exotic Majorana fermions or chiral magnons, but also to phonons. It has been shown theoretically that the former two types of heat carriers can generate a "planar" thermal Hall effect, whereby the magnetic field is parallel to the heat current, as observed experimentally, but it is unknown whether phonons also could. Here we show that a planar thermal Hall effect is present in another Kitaev candidate material, Na$_{2}$Co$_{2}$TeO$_{6}$. On the basis of a striking similarity between the temperature and field dependence of $κ_{xy}$ and that of the phonon-dominated thermal conductivity $κ_{xx}$, we argue that the planar thermal Hall effect in Na$_{2}$Co$_{2}$TeO$_{6}$ is generated by phonons. The phonon contributed planar $κ_{xy}$ also shows a strong sample dependence, which indicates an extrinsic origin of the mechanism. By conducting a complete study with different in-plane configurations of heat current $J$ and magnetic field $H$, $i.e.$ $H$ $\|$ $J$ and $H$ $\bot$ $J$, we observe a large difference in $κ_{xy}$ between these two configurations, which reveals that the direction of the heat current $J$ may play an important role in determining the planar thermal Hall effect. Our observation calls for a re-evaluation of the planar thermal Hall effect observed in $α$-RuCl$_{3}$.

cond-mat.str-el↗

arXiv2023

Planar parallel phonon Hall effect and local symmetry breaking

Y-kapellasite [Y3Cu9(OH)19Cl8] is a frustrated antiferromagnetic insulator which remains paramagnetic down to a remarkably low Néel temperature of about 2 K. Having studied this material in the paramagnetic regime, in which phonons are the only possible heat carriers, we report the observation of a planar parallel thermal Hall effect coming unambiguously from phonons. This is an advantage over the Kitaev quantum spin liquid candidates α-RuCl3 and Na2Co2TeO6 where in principle other heat carriers can be involved [1-4]. As it happens, Y-kapellasite undergoes a structural transition attributed to the positional freezing of a hydrogen atom below about 33 K. Above this transition, the global crystal symmetry forbids the existence of a planar parallel signal - the same situation as in Na2Co2TeO6 and cuprates [3-5]. This points to the notion of a local symmetry breaking at the root of the phonon Hall effect. In this context, the advantage of Y-kapellasite over Na2Co2TeO6 (with high levels of Na disorder and stacking faults) and cuprates (with high levels of disorder coming from dopants and oxygen vacancies) is its clean structure, where the only degree of freedom available for local symmetry breaking is this hydrogen atom randomly distributed over six equivalent positions above 33 K. This provides a specific and concrete case for the general idea of local symmetry breaking leading to the phonon Hall effect in a wide range of insulators.

cond-mat.str-el↗