Universal Conductance Fluctuations of Topological Insulators

As an exotic quantum condensed matter, the topological insulator (TI) is a bulk-insulating material with a Dirac-type conducting surface state. Such dissipationless transport of topological surface states (TSSs) is protected by the time-reversal symmetry, which leads to the potential applications in spintronics and quantum computations. Understanding the topological symplectic transport of the Dirac fermions is a key issue to study and design the TI-based devices. In this review, we introduce the progress on the universal conductance fluctuation (UCF) of TSSs. Firstly, we report the two dimensional UCF phenomenon in TIs, and its topological nature is demonstrated based on the investigations of UCF by angle-varying, in-plane field tuning and scaling analysis. Secondly, we discuss the statistical symmetry of UCF in TIs. For a single TSS, the applied magnetic field will drive the system from a Gaussian symplectic ensemble into a Gaussian unitary ensemble. It results a 2^0.5 fold increase of the UCF amplitude. However, the experiment reveals a decreasing of the UCF amplitude of 2^0.5 times. This is contradictory to the theoretical prediction. Actually, there are two TSSs and they are coherently coupled to each other in TIs since the sample's thickness is shorter than its bulk dephasing length. This leads to a Gaussian orthogonal ensemble of the interface coupling system without an external field. In such situation, the UCF amplitude will decrease by 2^0.5 times with the field increasing. It is consistent with the experimental results. Finally, the other progress on UCFs is also discussed

Free-standing Graphene as Gold Standard

We demonstrate using graphene sheets as a novel mass standard in the scanning transmission electron microscopy (STEM) based mass spectrometry. Here, free-standing graphene sheets are investigated by STEM. The discrete number of graphene layers enables an accurate calibration of STEM intensity, as performed over an extended thickness and with single atomic layer sensitivity. This leads to the direct and accurate measurement of the electron mean free path. As an application, we gain the insight of carbon nanoparticles of complex structure

Weak antilocalization in Cd3As2 thin films

Recently, it has been theoretically predicted that Cd3As2 is a three dimensional Dirac material, a new topological phase discovered after topological insulators, which exhibits a linear energy dispersion in the bulk with massless Dirac fermions. Here, we report on the low-temperature magnetoresistance measurements on a ~50nm-thick Cd3As2 film. The weak antilocalization under perpendicular magnetic field is discussed based on the two-dimensional Hikami-Larkin-Nagaoka (HLN) theory. The electron-electron interaction is addressed as the source of the dephasing based on the temperature-dependent scaling behavior. The weak antilocalization can be also observed while the magnetic field is parallel to the electric field due to the strong interaction between the different conductance channels in this quasi-two-dimensional film

Diagnosing the magnetic field-tuned symmetry nature of the topological electrons by conductance fluctuations in bulk-insulating BiSbTeSe2 devices

We extract the quantum conductance fluctuations and study its magnetic field dependence in the gate-dependent transport of the topological electrons in bulk-insulating BiSbTeSe2 devices. While increasing the magnetic field from 0 to 12 Tesla, the fluctuation magnitudes are found reduced by a ratio of sqrt(2) and form a quantized step. The step is observed both in n-type and p-type transport. It is also confirmed in the nonlocal measurements. This essentially demonstrates the breaking of the time reversal symmetry of the three-dimensional Z2 topological insulators

Quantum oscillations and nontrivial transport in (Bi0.92In0.08)2Se3

Quantum phase transition in topological insulators has drawn heightened attention in condensed matter physics and future device applications. Here we report the magnetotransport properties of single crystalline (Bi0.92In0.08)2Se3. The average mobility of about 1000 cm2/Vs is obtained from the Lorentz law at the low field up to 50 K. The quantum oscillations rise at a field of about 5 T, revealing a high mobility of 1.4*10^4 cm2/Vs at 2 K. The topological Dirac fermions are evident by the nontrivial Berry phase in the Landau Fan diagram. The properties make the (Bi0.92In0.08)2Se3 a promising platform for the investigation of quantum phase transition in topological insulators.Comment: 8 pages, 3 figure

Non-topological Origin of the Planar Hall Effect in Type-II Dirac Semimetal NiTe2

Dirac and Weyl semimetals are new discovered topological nontrivial materials with the linear band dispersions around the Dirac/Weyl points. When applying non-orthogonal electric current and magnetic field, an exotic phenomenon called chiral anomaly arises and negative longitudinal resistance can be detected. Recently, a new phenomenon named planer Hall effect (PHE) is considered to be another indication of chiral anomaly which has been observed in many topological semimetals. However, it still remains a question that is the PHE only attributed to chiral anomaly? Here we demonstrate the PHE in a new-discovered type-II Dirac semimetal NiTe2 by low temperature transport. However, after detailed analysis, we conclude that the PHE results from the trivial orbital magnetoresistance. This work reveals that PHE is not a sufficient condition of chiral anomaly and one need to take special care of other non-topological contribution in such studies

Two-step splitting the expandable graphite for few-layer graphene

Few-layer graphene sheets are prepared by splitting the expanded graphites using a high-power sonication. Atomic-level quantitative scanning transmission electron microscopy (Q-STEM) is employed to carry out the efficient layer statisticsm, enabling global optimization of the experimental conditions. A two-step splitting mechanism is thus revealed, in which the mean layer number was firstly reduced to less than 20 by heating to 1100{\deg}C and then tuned to the few-layer region by a 5-minute 104W/litre sonication. Raman spectroscopic analysis confirms the above mechanism and demonstrates that the sheets are largely free of defects and oxides

Three-Dimensional Anisotropic Magnetoresistance in the Dirac Node-Line Material ZrSiSe

The family of materials defined as ZrSiX (X = S, Se, Te) has been established as Dirac node-line semimetals, and subsequent study is urgent to exploit the promising application of unusual magnetoresistance property. In this work, we systematically investigated the anisotropic magnetoresistance in the newly-discovered Dirac node-line material ZrSiSe. By applying a magnetic field of 3 T by a vector field, the three-dimensional (3D) magnetoresistance (MR) shows strong anisotropy. The MR ratio of maximum and minimum directions can reach 7 at 3 T and keeps increasing at the higher magnetic field. The anisotropic MR forms a butterfly-shaped curve, which indicates the quasi-2D electronic structures. This is further confirmed by the angular-dependent Shubnikov-de Haas (SdH) oscillations. The first-principles calculations establish the quasi-2D tubular-shaped Fermi surface near the X point in the Brillouin zone. Our findings shed light on the 3D mapping of MR and the potential applications in magnetic sensors based on ZrSiSe Dirac materials

Oscillating planar Hall response from the surface electrons in bulk crystal Sn doped Bi1.1Sb0.9Te2S

We report the low-temperature magneto-transport in the bulk-insulating single crystal of topological insulator Sn doped Bi1.1Sb0.9Te2S. The Shubnikov-de Haas oscillations appear with their reciprocal frequency proportional to cos/theta , demonstrating the dominant transport of topological surface states. While the magnetic field is rotating in the sample surface, the planar Hall effect arises with sizeable oscillations following a relation of cos/theta sin/theta . Its amplitude reaches the maximum at the lowest temperature and drops to nearly zero at the temperature higher than 100 K. All these evidences consolidate such planar Hall oscillations as a new golden criterion on the topological surface transport

Experimental observation of nanojets formed by heating the PbO-coated Pb clusters

In this article, we will present the first experimental observation of nanojets formed by heating PbO-coated Pb clusters, which has been predicted theoretically by M. Moseler and U Landman. During the heating, the hot liquid ejects through the broken orifice into vacuum, and forms a condensed trail in the tadpole shape as shown in the TEM micrographs. The temperature-variable Raman spectra indicates that the nanojet formation is closely related to the heating temperature and thus essentially to the internal pressure in the coated clusters. The pressure inside the shell, which rises from the inner core's melting and its confined volume expansion and then drops after the final explosion, dominates the whole nanojetting process