Universität zu Köln

Sharma, Rohit ORCID: 0000-0001-9815-0733 (2025). Experimental Studies of Thermal Hall Effect in Correlated Insulating Oxides and Charge-Compensated Topological Insulators. PhD thesis, Universität zu Köln.

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Abstract

In this thesis, thermal transport measurements, with a focus on the thermal Hall effect (THE), are conducted on Bi-based 3D topological insulators and various titanate compounds. The studied titanates include pyrochlores Y₂₋ₓDyₓTi₂O₇ and perovskite EuTiO₃. To perform these measurements, new experimental setups are constructed and optimized to cover a broad temperature range from 0.25 K to 300 K. These setups comprise the homemade dipsticks Homer and Bart and a sample holder for the Heliox VL ³He insert. The Heliox VL, equipped with RuO₂ thermometers, enables measurements from 0.25 K to 10 K. In the Homer dipstick, Cernox thermometers are installed, covering the temperature range from 5 K to 100 K, while the Bart dipstick, using constantan-chromel thermocouples, spans 50 K to 300 K. Together, these setups provide an overlapping temperature regime, ensuring high sensitivity for detecting small temperature gradients and enabling reliable thermal Hall measurements. Measurements on pyrochlore titanates (Y₂Ti₂O₇, DyYTi₂O₇, and Dy₂Ti₂O₇) reveal phonon-dominated longitudinal thermal conductivities (κₓₓ) in all samples. The presence of Dy³⁺ magnetic ions introduces significant field-dependent effects on κₓₓ. THE measurements indicate a negative, linear field dependence of κₓᵧ across all samples. Contrary to earlier reports suggesting negligible thermal Hall ratios (κₓᵧ/κₓₓ) in Y₂Ti₂O₇, our results demonstrate finite κₓᵧ/κₓₓ values in Y₂Ti₂O₇, consistent with observations in other insulating materials with phononic THE. Similar κₓᵧ/κₓₓ values are found in Y₂Ti₂O₇ and DyYTi₂O₇ with slightly higher values in Dy₂Ti₂O₇, suggesting that magnetic impurities are not necessary for generating a phononic THE. These findings support an intrinsic origin of THE in Y₂Ti₂O₇ and a combination of intrinsic and extrinsic contributions in DyYTi₂O₇ and Dy₂Ti₂O₇. Thermal transport measurements on EuTiO₃ reveal glass-like κₓₓ behavior, attributed to strong spin-lattice coupling. In the paramagnetic phase, κₓᵧ exhibits a linear, negative field dependence, with κₓᵧ/κₓₓ ratios consistent with those observed in other insulators, where a phononic origin is suggested. In the antiferromagnetic phase, κₓᵧ displays non-linear behavior, with a positive signal emerging below 1.2 T. The thermal Hall resistivity, λᵧₓ, scales with magnetization at fields above 1.2 T, indicating an anomalous THE contribution. In the spin-flop phase, κₓᵧ/κₓₓ reaches exceptionally high values of approximately 4 × 10⁻³ at low magnetic fields around 0.3 T, suggesting that EuTiO₃ might hold potential for use in thermomagnetic devices operating at cryogenic temperatures. Measurements on well-compensated Bi₂₋ₓSbₓTe₃₋ᵧSeᵧ topological insulator samples reveal phonon-dominated κₓₓ and a linear field dependence of κₓᵧ. The observed κₓᵧ/κₓₓ ratios align with previous reports of phononic THE in insulators. While the estimated electronic contribution using the Wiedemann-Franz law, κₓᵧᵉˡ = σₓᵧ L₀T, matches the measured κₓᵧ at high temperatures, the observed negative κₓᵧ below 60 K is significantly larger than expected for electronic contributions. This supports the theoretical proposal that phononic THE can result from skew scattering of phonons on charge defects. In contrast, weakly-compensated topological insulator TlBi₀.₁₅Sb₀.₈₅Te₂ samples exhibit a pronounced phononic THE with non-linear κₓᵧ field dependence over a broad temperature range. This behavior is modeled effectively using a single-band model of hole-like carriers. Thermal and electrical transport measurements at high temperatures consistently confirm the dominant carrier type and mobility. However, at lower temperatures, mobile charge carriers localized within charge puddles are detected only through thermal measurements. A record κₓᵧ/κₓₓ ratio exceeding 2% is observed for non-magnetic materials. These results agree with the theoretical model describing phononic THE induced by charge puddles, capturing the non-monotonic field dependence and suggesting that carriers within these puddles may reach the high-field limit (μB ≫ 1).

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Sharma, Rohit rsharma.csr@gmail.com, sharma@ph2.uni-koeln.de orcid.org/0000-0001-9815-0733 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-754105
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Thermal Hall effect English Phonon thermal Hall effect English Correlated insulating Oxides English titanate pyrochlores English titanate perovskite English Charge-compensated topological insulators English Chiral phonons English Thermal conductivity English Heat transport English Thermoelectric English UNSPECIFIED English
Date of oral exam:	11 February 2025
Referee:	Name Academic Title Lorenz, Thomas Prof. Dr. Braden, Markus Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/75410