Large magnetoresistance and nonzero Berry phase in the nodal-line semimetal MoO2

We performed calculations of the electronic band structure and the Fermi surface as well as measured the longitudinal resistivity rho(xx)(T,H), Hall resistivity rho(xy)(T,H), and quantum oscillations of the magnetization as a function of temperature at various magnetic fields for MoO2 with a monoclinic crystal structure. The band structure calculations show that MoO2 is a nodal-line semimetal when spin-orbit coupling is ignored. It was found that a large magnetoresistance reaching 5.03 x 10 4 % at 2 K and 9 T, its nearly quadratic field dependence, and a field-induced up-turn behavior of rho(xx)(T), the characteristics common for many topologically nontrivial as well as trivial semimetals, emerge also in MoO2. The observed properties are attributed to a perfect charge-carrier compensation, evidenced by both calculations relying on the Fermi surface topology and the Hall resistivity measurements. Both the observation of negative magnetoresistance for magnetic field along the current direction and the nonzero Berry phase in de Haas-van Alphen measurements indicate that pairs of Weyl points appear in MoO2, which may be due to the crystal symmetry breaking. These results highlight MoO2 as a platform for studying the topological properties of oxides.

Linear and quadratic magnetoresistance in the semimetal SiP2

Huancheng Chen, Quansheng Wu, Oleg Yazyev, Shengnan Zhang

Multiple mechanisms for extremely large magnetoresistance (XMR) found in many topologically nontrivial/trivial semimetals have been theoretically proposed, but experimentally it is unclear which mechanism is responsible in a particular sample. In this paper, by the combination of band structure calculations, numerical simulations of magnetoresistance (MR), Hall resistivity, and de Haas-van Alphen (dHvA) oscillation measurements, we studied the MR anisotropy of SiP2 which is verified to be a topologically trivial, incomplete compensation semimetal. It was found that as magnetic field H is applied along the a axis, the MR exhibits an unsaturated nearly linear H dependence, which was argued to arise from incomplete carriers compensation. For the H parallel to [101] orientation, an unsaturated nearly quadratic H dependence of MR up to 5.88 x 10(4) % (at 1.8 K, 31.2 T) and field-induced up-turn behavior in resistivity were observed, which was suggested due to the existence of hole open orbits extending along the k(x) direction. Good agreement of the experimental results with the simulations based on the calculated Fermi surface (FS) indicates that the topology of FS plays an important role in its MR.

AMER PHYSICAL SOC2020

Extremely large magnetoresistance in the "ordinary" metal ReO3

Huancheng Chen, Quansheng Wu, Oleg Yazyev, Shengnan Zhang

The extremely large magnetoresistance (XMR) observed in many topologically nontrivial and trivial semimetals has attracted much attention in relation to its underlying physical mechanism. In this paper, by combining the band structure and Fermi surface (FS) calculations with the Hall resistivity and de Haas-van Alphen (dHvA) oscillation measurements, we studied the anisotropy of magnetoresistance (MR) of ReO3 with a simple cubic structure, an "ordinary" nonmagnetic metal considered previously. We found that ReO3 exhibits almost all the characteristics of XMR semimetals: the nearly quadratic field dependence of MR, a field-induced upturn in resistivity followed by a plateau at low temperatures, and high mobilities of charge carriers. It was found that for magnetic field H applied along the c axis, the MR exhibits an unsaturated H-1.75 dependence, which was argued to arise from the complete carrier compensation supported by the Hall resistivity measurements. For H applied along the direction of 15 degrees relative to the c axis, an unsaturated H-1.90 dependence of MR up to (9.43 x 10(3))% at 10 K and 9 T was observed, which was explained by the existence of electron open orbits extending along the k(x) direction. Two mechanisms responsible for XMR observed usually in the semimetals occur also in the simple metal ReO3 due to its peculiar FS (two closed electron pockets and one open electron pocket), once again indicating that the details of FS geometrical configuration are a key factor for the observed XMR in materials.

AMER PHYSICAL SOC2021

Longueur de diffusion de spin et magnétisme de structures nanotubes de carbone / matériaux ferromagnétiques

Xavier Hoffer

The aim of this thesis work is to study the spin-dependent transport in multiwalled carbon nanotubes (MWNT). To do so, the electrical resistance of MWNT contacted between ferromagnetic electrodes has been measured, as a function of the contacts magnetic configuration and temperature. An original sample fabrication process to contact MWNT has been developed. The MWNT growth is achieved via chemical vapour deposition in the pores of an alumina template, with electrodeposited nickel or cobalt nanowires as catalyst. The pores are 1,5 µm, in length, and 40 nm in diameter. The second electrical contact is sputter deposited or evaporated over the membrane surface. This geometry allows us to contact MWNT between ferromagnetic electrodes, which magnetisation states versus the applied magnetic field are known. Furthermore, the electrical current flows perpendicularly to the plane of the contacts layers. Finally, it enables us to tune the length of the MWNT between the electrodes. The features of the measured spin-dependent magnetoresistance (SD-MR) signals cannot be correctly interpreted with common tunnel or giant magnetoresistance approaches. Every MWNT which is less than 300 nm long between nickel contacts destroys the spin polarisation of the current. MWNT longer than 300 nm featured spin-dependent magnetoresistance signals, with a small amplitude and depending on the direction of the current. One 500 nm long MWNT between cobalt contacts shows 25% SD-MR signals at 2,5 K, at zero applied current. This signal is caused by a thermopower effect, and not by a spin polarised current. Another signal, from a 700 nm long between cobalt contacts is more similar to usual signals shown by magnetic tunnel junctions. In order to understand spin-dependent transport in MWNT, it is therefore necessary to determine first the electrical transport mechanisms, independent of the spin. Nowadays, it is still poorly understood. Therefore, besides spin-dependent transport measurements, we measured the conductance versus the temperature and the bias voltage. At temperatures below 50 K, the conductance diminishes as the temperature and the bias voltage decrease. This effect, called Zero-Bias Anomaly (ZBA), is a consequence of disorder and electron-electron interactions in our systems. 46 samples out of 113 have shown power law scaling laws of the ZBA. From these scaling-laws, we get the power law coefficient α. Such scaling laws have been observed many times for carbon nanotubes. However, none of these studies has such a large spectra for the values of α as we have : for our samples, α ranges between 0 and 1,7. With samples contacted via cobalt cobalt electrodes, α is usually larger than nickel contacted samples. Therefore, a single parameter α enables us to describe and characterise the electrical transport in our samples. The large number of measured samples allows us to correlate this coefficient α with other experimental parameters, such as the MWNT length, or the metallic or structural nature of the contact electrodes. We also have established the linear relation : ln G0 ~ A · α, with G0 the extrapolated conductance at 1 K. To our knowledge, this relation has not yet been predicted nor observed. The physical interpretation of this relation is interpreted in the Coulomb blockade formalism. Finally, a link between the weak localisation amplitude et the value of α has been displayed. In this case, the weak localisation amplitude not only depends on α, but also on the metallic nature of the electrodes.

EPFL2004

Longueur de diffusion de spin et magnétisme de structures nanotubes de carbone / matériaux ferromagnétiques

Xavier Hoffer

EPFL2004