More Like this

1. Field-induced resistance peak in a superconducting niobium thin film proximity coupled to a surface reconstructed SrTiO3

   https://doi.org/10.1038/s41535-020-0242-4  

   Singh, Akhilesh Kr. ; Kar, Uddipta ; Redell, Matthew D. ; Wu, Tsung-Chi ; Peng, Wei-Hsiang ; Das, Bipul ; Kumar, Satish ; Lee, Wei-Cheng ; Lee, Wei-Li ( July 2020 , npj Quantum Materials)

   Oxygen vacancy is known to play an important role for the physical properties in SrTiO₃(STO)-based systems. On the surface, rich structural reconstructions had been reported owing to the oxygen vacancies, giving rise to metallic surface states and unusual surface phonon modes. More recently, an intriguing phenomenon of a huge superconducting transition temperature enhancement was discovered in a monolayer FeSe on STO substrate, where the surface reconstructed STO (SR-STO) may play a role. In this work, SR-STO substrates were prepared via thermal annealing in ultra-high vacuum followed by low energy electron diffraction analyses on surface structures. Thin Nb films with different thicknesses (d) were then deposited on the SR-STO. The detailed studies of the magnetotransport and superconducting property in the Al(1 nm)/Nb(d)/SR-STO samples revealed a large positive magnetoresistance and a pronounced resistance peak near the onset of the resistive superconducting transition in the presence of an in-plane field. Remarkably, the amplitude of the resistance peak increases with increasing fields, reaching a value of nearly 57% of the normal state resistance at 9 T. Such resistance peaks were absent in the control samples of Al(1 nm)/Nb(d)/STO and Al(1 nm)/Nb(d)/SiO₂. Combining with DFT calculations for SR-STO, we attribute the resistance peak to the interface resistance from the proximity coupling of the superconducting niobium to the field-enhanced long-range magnetic order in SR-STO that arises from the spin-polarized in-gap states due to oxygen vacancies.

    

   more » « less
2. Electrical Detection and Magnetic Imaging of Stabilized Magnetic Skyrmions in Fe 1− x Co x Ge ( x < 0.1) Microplates

   https://doi.org/10.1002/adfm.201805418  

   Stolt, Matthew J. ; Schneider, Sebastian ; Mathur, Nitish ; Shearer, Melinda J. ; Rellinghaus, Bernd ; Nielsch, Kornelius ; Jin, Song ( February 2019 , Advanced Functional Materials)

   Magnetic skyrmions are topologically protected spin textures that are being investigated for their potential use in next generation magnetic storage devices. Here, magnetic skyrmions and other magnetic phases in Fe_1−xCo_xGe (x< 0.1) microplates (MPLs) newly synthesized via chemical vapor deposition are studied using both magnetic imaging and transport measurements. Lorentz transmission electron microscopy reveals a stabilized magnetic skyrmion phase near room temperature (≈280 K) and a quenched metastable skyrmion lattice via field cooling. Magnetoresistance (MR) measurements in three different configurations reveal a unique anomalous MR signal at temperatures below 200 K and two distinct field dependent magnetic transitions. The topological Hall effect (THE), known as the electronic signature of magnetic skyrmion phase, is detected for the first time in a Fe_1−xCo_xGe nanostructure, with a large and positive peak THE resistivity of ≈32 nΩ cm at 260 K. This large magnitude is attributed to both nanostructuring and decreased carrier concentrations due to Co alloying of the Fe_1−xCo_xGe MPL. A consistent magnetic phase diagram summarized from both the magnetic imaging and transport measurements shows that the magnetic skyrmions are stabilized in Fe_1−xCo_xGe MPLs compared to bulk materials. This study lays the foundation for future skyrmion‐based nanodevices in information storage technologies.

    

   more » « less
3. Electronic structure in a transition metal dipnictide TaAs 2

   https://doi.org/10.1088/1361-648X/ad04fc  

   Regmi, Sabin ; Huang, Cheng-Yi ; Khan, Mojammel A. ; Wang, Baokai ; Pradhan Sakhya, Anup ; Hosen, M. Mofazzel ; Thompson, Jesse ; Singh, Bahadur ; Denlinger, Jonathan D. ; Ishigami, Masahiro ; et al ( November 2023 , Journal of Physics: Condensed Matter)

   The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently,${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$, a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the ($\bar{2}01$) surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$are trivial bulk bands. The absence of symmetry-protected surface state on the ($\bar{2}01$) surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$.

    

   more » « less
4. Nonsaturating large magnetoresistance in semimetals

   https://doi.org/10.1073/pnas.1808747115  

   Leahy, Ian A. ; Lin, Yu-Ping ; Siegfried, Peter E. ; Treglia, Andrew C. ; Song, Justin C. W. ; Nandkishore, Rahul M. ; Lee, Minhyea ( October 2018 , Proceedings of the National Academy of Sciences)

   The rapidly expanding class of quantum materials known as topological semimetals (TSMs) displays unique transport properties, including a striking dependence of resistivity on applied magnetic field, that are of great interest for both scientific and technological reasons. So far, many possible sources of extraordinarily large nonsaturating magnetoresistance have been proposed. However, experimental signatures that can identify or discern the dominant mechanism and connect to available theories are scarce. Here we present the magnetic susceptibility (χ), the tangent of the Hall angle (${\tan \theta }_H$), along with magnetoresistance in four different nonmagnetic semimetals with high mobilities, NbP, TaP, NbSb₂, and TaSb₂, all of which exhibit nonsaturating large magnetoresistance (MR). We find that the distinctly different temperature dependences,$\chi \left(T\right)$, and the values of${\tan \theta }_H$in phosphides and antimonates serve as empirical criteria to sort the MR from different origins: NbP and TaP are uncompensated semimetals with linear dispersion, in which the nonsaturating magnetoresistance arises due to guiding center motion, while NbSb₂and TaSb₂are compensated semimetals, with a magnetoresistance emerging from nearly perfect charge compensation of two quadratic bands. Our results illustrate how a combination of magnetotransport and susceptibility measurements may be used to categorize the increasingly ubiquitous nonsaturating large magnetoresistance in TSMs.

    

   more » « less
5. Spin memory of the topological material under strong disorder

   https://doi.org/10.1038/s41535-020-0241-5  

   Korzhovska, Inna ; Deng, Haiming ; Zhao, Lukas ; Deshko, Yury ; Chen, Zhiyi ; Konczykowski, Marcin ; Zhao, Shihua ; Raoux, Simone ; Krusin-Elbaum, Lia ( June 2020 , npj Quantum Materials)

   Robustness to disorder is the defining property of any topological state. The ultimate disorder limits to topological protection are still unknown, although a number of theories predict that even in the amorphous state a quantized conductance might yet reemerge. Here we report that in strongly disordered thin films of the topological material Sb₂Te₃disorder-induced spin correlationsdominate transport of charge—they engender a spin memory phenomenon, generated by the nonequilibrium charge currents controlled by localized spins. We directly detect a glassy yet robust disorder-induced magnetic signal in filmsfree of extrinsic magnetic dopants, which becomes null in a lower-disorder crystalline state. This is where large isotropic negative magnetoresistance (MR)—a hallmark of spin memory—crosses over to positive MR, first with only one e²/hquantum conduction channel, in a weakly antilocalized diffusive transport regime with a 2D scaling characteristic of the topological state. A fresh perspective revealed by our findings is that spin memory effect sets a disorder threshold to the protected topological state. It also points to new possibilities of tuning spin-dependent charge transport by disorder engineering of topological materials.

    

   more » « less