Finite-temperature violation of the anomalous transverse Wiedemann-Franz law

The Wiedemann-Franz (WF) law links the ratio of electronic charge and heat conductivity to fundamental constants. It has been tested in numerous solids, but the extent of its relevance to the anomalous transverse transport, which represents the topological nature of the wave function, remains an open question. Here we present a study of anomalous transverse response in the noncollinear antiferromagnet Mn$_{3}$Ge extended from room temperature down to sub-Kelvin temperature and find that the anomalous Lorenz ratio remains close to the Sommerfeld value up to 100 K, but not above. The finite-temperature violation of the WF correlation is caused by a mismatch between the thermal and electrical summations of the Berry curvature, rather than the inelastic scattering as observed in ordinary metals. This interpretation is backed by our theoretical calculations, which reveals a competition between the temperature and the Berry curvature distribution. The accuracy of the experiment is supported by the verification of the Bridgman relation between the anomalous Ettingshausen and Nernst effects. Our results identify the anomalous Lorenz ratio as an extremely sensitive probe of Berry spectrum near the chemical potential.Comment: 9 pages,6 figures, Supplemental Material include

Tuning the anomalous Nernst and Hall effects with shifting the chemical potential in Fe-doped and Ni-doped Co3_3Sn2_2S2_2

Co$_3$Sn$_2$S$_2$ is believed to be a magnetic Weyl semimetal. It displays large anomalous Hall, Nernst and thermal Hall effects with a remarkably large anomalous Hall angle. Here, we present a comprehensive study of how substituting Co by Fe or Ni affects the electrical and thermoelectric transport. We find that doping alters the amplitude of the anomalous transverse coefficients. The maximum decrease in the amplitude of the low-temperature anomalous Hall conductivity $\sigma^A_{ij}$ is twofold. Comparing our results with theoretical calculations of the Berry spectrum assuming a rigid shift of the Fermi level, we find that given the modest shift in the position of the chemical potential induced by doping, the experimentally observed variation occurs five times faster than expected. Doping affects the amplitude and the sign of the anomalous Nernst coefficient. Despite these drastic changes, the amplitude of the $\alpha^A_{ij}/\sigma^A_{ij}$ ratio at the Curie temperature remains close to $\approx 0.5 k_B/e$, in agreement with the scaling relationship observed across many topological magnets.Comment: 8 pages, 9 figure

Phase change behaviors of Zn-doped Ge2Sb2Te5 films

This work was financially supported by the Program for New Century Excellent Talents in University (Grant No. NCET-10-0976), the International Science & Technology Cooperation Program of China (Grant No. 2011DFA12040), the National Program on Key Basic Research Project (973 Program) (Grant No. 2012CB722703), the Natural Science Foundation of China (Grant Nos. 61008041 and 60978058), the Natural Science Foundation of Zhejiang Province, China (Grant No. Y1090996), the Natural Science Foundation of Ningbo City, China (Grant No. 2011A610092), the Program for Innovative Research Team of Ningbo city (Grant No. 2009B21007), and sponsored by K. C. Wong Magna Fund in Ningbo University

Enhanced thermal stability and electrical behavior of Zn-doped Sb2Te films for phase change memory application

Zn-doped Sb₂Te films are proposed to present the feasibility for phase-change memory application. Zn atoms are found to significantly increase crystallization temperature of Zn x (Sb₂Te)1−x films and be almost linearly with the wide range of Zn-doping concentration from x = 0 to 29.67 at.%. Crystalline resistances are enhanced by Zn-doping, while keeping the large amorphous/crystalline resistance ratio almost constant at ∼10⁵. Especially, the Zn 26.07 (Sb₂Te)73.93 and Zn 29.67 (Sb₂Te)70.33 films exhibit a larger resistance change, faster crystallization speed, and better thermal stability due to the formation of amorphous Zn-Sb and Zn-Te phases as well as uniform distribution of Sb₂Te crystalline grains

Anomalous transverse response of Co2_2MnGa and universality of the room-temperature αijA/σijA\alpha^A_{ij}/\sigma^A_{ij} ratio across topological magnets

The off-diagonal (electric, thermal and thermoelectric) transport coefficients of a solid can acquire an anomalous component due to the non-trivial topology of the Bloch waves. We present a study of the anomalous Hall (AHE), Nernst (ANE) and thermal Hall effects (ATHE) in the Heusler Weyl ferromagnet Co$_2$MnGa. The Anomalous Wiedemann-Franz law, linking electric and thermal responses, was found to be valid over the whole temperature window. This indicates that the AHE has an intrinsic origin and the Berry spectrum is smooth in the immediate vicinity of the Fermi level. From the ANE data, we extract the magnitude and temperature dependence of $\alpha^A_{ij}$ and put under scrutiny the $\alpha^A_{ij}/\sigma^A_{ij}$ ratio, which approaches k$_B$/e at room temperature. We show that in various topological magnets the room-temperature magnitude of this ratio is a sizeable fraction of k$_B$/e and argue that the two anomalous transverse coefficients depend on universal constants, the Berry curvature averaged over a window set by either the Fermi wavelength (for Hall) or the de Broglie thermal length (for Nernst). Since the ratio of the latter two is close to unity at room temperature, such a universal scaling finds a natural explanation in the intrinsic picture of anomalous transverse coefficients.Comment: 5 pages, 4 figures, supplemental material include

Nonlinear charge transport induced by gate voltage oscillation in few-layer MnBi2Te4

Nonlinear charge transport, including nonreciprocal longitudinal resistance and nonlinear Hall effect, has garnered significant attention due to its ability to explore inherent symmetries and topological properties of novel materials. An exciting recent progress along this direction is the discovery of significant nonreciprocal longitudinal resistance and nonlinear Hall effect in the intrinsic magnetic topological insulator MnBi2Te4 induced by the quantum metric dipole. Given the importance of this finding, the inconsistent response with charge density, and conflicting requirement of C3z symmetry, it is imperative to elucidate every detail that may impact the nonlinear transport measurement. In this study, we reveal an intriguing experimental factor that inevitably gives rise to sizable nonlinear transport signal in MnBi2Te4. We demonstrate that this effect stems from the gate voltage oscillation caused by the application of a large alternating current to the sample. Furthermore, we propose a methodology to significantly suppress this effect by individually grounding the voltage electrodes during the second-harmonic measurements. Our investigation emphasizes the critical importance of thoroughly assessing the impact of gate voltage oscillation before determining the intrinsic nature of nonlinear transport in all 2D material devices with an electrically connected operative gate electrode.Comment: 28 pages, 12 figure