Haiwen Liu

Anisotropic magnetotransport and exotic longitudinal linear magnetoresistance in WTe2 crystals

Recently, the WTe2 semimetal, as a typical layered transition-metal dichalcogenide, attracted much attention due to an extremely large, non-saturating parabolic magnetoresistance in the perpendicular field. Here, we report a systematic study of the angular dependence of the magnetoresistance in a WTe2 single crystal. The significant anisotropic magnetotransport behavior in different magnetic field directions and violation of the Kohlers rule are observed. Unexpectedly, when the applied field and excitation current are both parallel to the tungsten chains of WTe2, an exotic large longitudinal linear magnetoresistance as high as 1200% at 15T and 2K is identified. These results imply that the WTe2 semimetal, due to its balanced hole and electron populations, seems to be the first material for which a large longitudinal linear magnetoresistance appears when the external magnetic field is parallel to the applied current. Finally, our work may stimulate studies of double-carrier correlated materials and the corresponding quantum physics.

Quantum Griffiths singularity of superconductor-metal transition in Ga thin films

Cooling to see the effects of disorder In sufficiently strong external magnetic fields, thin superconducting films typically become insulating. The presence of disorder can affect this phase transition. Theorists have proposed that disorder can cause the so-called Griffiths singularity, where the behavior of the system is determined by a small number of superconducting islands that form above the critical magnetic field. Xing et al. observed a signature of such a singularity in thin films of gallium by analyzing transport data taken at very low temperatures (see the Perspective by Markovic). In this regime, thermal fluctuations were not strong enough to homogenize the system, which allowed the rare islands to form. Science, this issue p. 542; see also p. 509 Systematic transport measurements at low temperatures and in magnetic fields indicate the divergence of the dynamical critical exponent. [Also see Perspective by Markovic] The Griffiths singularity in a phase transition, caused by disorder effects, was predicted more than 40 years ago. Its signature, the divergence of the dynamical critical exponent, is challenging to observe experimentally. We report the experimental observation of the quantum Griffiths singularity in a two-dimensional superconducting system. We measured the transport properties of atomically thin gallium films and found that the films undergo superconductor-metal transitions with increasing magnetic field. Approaching the zero-temperature quantum critical point, we observed divergence of the dynamical critical exponent, which is consistent with the Griffiths singularity behavior. We interpret the observed superconductor-metal quantum phase transition as the infinite-randomness critical point, where the properties of the system are controlled by rare large superconducting regions.

Crossover between Weak Antilocalization and Weak Localization of Bulk States in Ultrathin Bi2Se3 Films

We report transport studies on the 5u2005nm thick Bi2Se3 topological insulator films which are grown via molecular beam epitaxy technique. The angle-resolved photoemission spectroscopy data show that the Fermi level of the system lies in the bulk conduction band above the Dirac point, suggesting important contribution of bulk states to the transport results. In particular, the crossover from weak antilocalization to weak localization in the bulk states is observed in the parallel magnetic field measurements up to 50 Tesla. The measured magneto-resistance exhibits interesting anisotropy with respect to the orientation of parallel magnetic field B// and the current I, signifying intrinsic spin-orbit coupling in the Bi2Se3 films. Our work directly shows the crossover of quantum interference effect in the bulk states from weak antilocalization to weak localization. It presents an important step toward a better understanding of the existing three-dimensional topological insulators and the potential applications of nano-scale topological insulator devices.

Anisotropic Fermi surface and quantum limit transport in high mobility three-dimensional Dirac semimetal Cd3As2

Analyzing changes in resistivity is one component of condensed-matter physics research that has applications in the electronics industry. Now, researchers experimentally show that the resistivity of a Cd

Crossover from 3D to 2D Quantum Transport in Bi2Se3/In2Se3 Superlattices

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One-dimensional quantum channel in a graphene line defect

As

Topological Imbert-Fedorov shift in Weyl semimetals

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The Chiral Anomaly and Ultrahigh Mobility in Crystalline HfTe5

crystal can be modulated by the geometry of the materials Fermi surface.

Magnetic-tunnelling-induced Weyl node annihilation in TaP

The topological insulator/normal insulator (TI/NI) superlattices (SLs) with multiple Dirac channels are predicted to offer great opportunity to design novel materials and investigate new quantum phenomena. Here, we report first transport studies on the SLs composed of TI Bi2Se3 layers sandwiched by NI In2Se3 layers artificially grown by molecular beam epitaxy (MBE). The transport properties of two kinds of SL samples show convincing evidence that the transport dimensionality changes from three-dimensional (3D) to two-dimensional (2D) when decreasing the thickness of building block Bi2Se3 layers, corresponding to the crossover from coherent TI transport to separated TI channels. Our findings provide the possibility to realizing 3D surface states in TI/NI SLs.

Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe2

The magnetoelectric (ME) coupling of CoFeB/Ru/CoFeB synthetic antiferromagnetic (SAF) structure is studied at room-temperature when the SAF structure was deposited on the (011)- Pb(Mg1/3Nb2/3)O-3-PbTiO3 piezoelectric substrate. It was found that the magnetoelectric coupling showed butterfly like curve when measured in the absence of magnetic field but it showed hysteresis loop-like behavior as well as the exchange-bias like shift of the ME hysteresis under fixed magnetic fields of +250 Oe and +500 Oe. This exchange-bias like hysteretic ME coupling is attributed to the direct coupling of CoFeB layer with the substrate ferroelectric domain, the absence of magnetocrystalline anisotropy and the switching of 109 degrees ferroelastic domains of the substrate. We have also measured the magnetoelectric coupling of CoFe/Ru/CoFe SAF structure but observed no exchange-bias like hysteresis behavior. Our results establish another way of obtaining the non-volatile memory devices based on magnetoelectric systems