Eun Sang Choi

Enhancement of thermal and electrical properties of carbon nanotube polymer composites by magnetic field processing

We show that the thermal and electrical properties of single wall carbon nanotube (CNT)-polymer composites are significantly enhanced by magnetic alignment during processing. The electrical transport properties of the composites are mainly governed by the hopping conduction with localization lengths comparable to bundle diameters. The bundling of nanotubes during the composite processing is an important factor for electrical, and in particular, for thermal transport properties. Better CNT isolation will be needed to reach the theoretical thermal conductivity limit for CNT composites.

Physical properties of Hastelloy® C-276™ at cryogenic temperatures

In recent years, the Ni–Mo–Cr superalloy Hastelloy® C-276™ has been used as a substrate material for fabricating superconducting tapes such as YBCO and MgB2 coated conductors. With increasing piece length, these coated conductors are within reach of large scale commercial applications. However, data on the physical properties of Hastelloy C-276 at temperatures relevant for these applications are not yet available. In this work, physical properties including magnet succeptibility, specific heat, electrical resistivity, and the Seebeck coefficient are measured from 2to300K and thermal conductivity from 2to200K. Our results show that Hastelloy C-276 exhibits Curie paramagnetism between 4 and 300K with a Curie constant C=0.091K. A spin-glass-like behavior is observed below 3K. The electrical resistivity has a minimum at ∼12K, and shows a linear weak T dependence at higher temperatures. The specific heat Cp between 15 and 40K follows Cp=γT+AT3. Below ∼10K, an upturn in Cp∕T with decreasing T is interpreted by ...

Chiral Majorana fermion modes in a quantum anomalous Hall insulator–superconductor structure

A propagating Majorana mode Although Majorana fermions remain elusive as elementary particles, their solid-state analogs have been observed in hybrid semiconductor-superconductor nanowires. In a nanowire setting, the Majorana states are localized at the ends of the wire. He et al. built a two-dimensional heterostructure in which a one-dimensional Majorana mode is predicted to run along the sample edge (see the Perspective by Pribiag). The heterostructure consisted of a quantum anomalous Hall insulator (QAHI) bar contacted by a superconductor. The authors used an external magnetic field as a “knob” to tune into a regime where a Majorana mode was propagating along the edge of the QAHI bar covered by the superconductor. A signature of this propagation—half-quantized conductance—was then observed in transport experiments. Science, this issue p. 294; see also p. 252 Transport experiments showing half-integer quantized conductance indicate a propagating Majorana edge mode. Majorana fermion is a hypothetical particle that is its own antiparticle. We report transport measurements that suggest the existence of one-dimensional chiral Majorana fermion modes in the hybrid system of a quantum anomalous Hall insulator thin film coupled with a superconductor. As the external magnetic field is swept, half-integer quantized conductance plateaus are observed at the locations of magnetization reversals, giving a distinct signature of the Majorana fermion modes. This transport signature is reproducible over many magnetic field sweeps and appears at different temperatures. This finding may open up an avenue to control Majorana fermions for implementing robust topological quantum computing.

Electrical Detection of Spin-Polarized Surface States Conduction in (Bi0.53Sb0.47)2Te3 Topological Insulator

Strong spin-orbit interaction and time-reversal symmetry in topological insulators enable the spin-momentum locking for the helical surface states. To date, however, there has been little report of direct electrical spin injection/detection in topological insulator. In this Letter, we report the electrical detection of spin-polarized surface states conduction using a Co/Al2O3 ferromagnetic tunneling contact in which the compound topological insulator (Bi0.53Sb0.47)2Te3 was used to achieve low bulk carrier density. Resistance (voltage) hysteresis with the amplitude up to about 10 Ω was observed when sweeping the magnetic field to change the relative orientation between the Co electrode magnetization and the spin polarization of surface states. The two resistance states were reversible by changing the electric current direction, affirming the spin-momentum locking in the topological surface states. Angle-dependent measurement was also performed to further confirm that the abrupt change in the voltage (resistance) was associated with the magnetization switching of the Co electrode. The spin voltage amplitude was quantitatively analyzed to yield an effective spin polarization of 1.02% for the surface states conduction in (Bi0.53Sb0.47)2Te3. Our results show a direct evidence of spin polarization in the topological surface states conduction. It might open up great opportunities to explore energy-efficient spintronic devices based on topological insulators.

Competing weak localization and weak antilocalization in ultrathin topological insulators.

We demonstrate evidence of a surface gap opening in topological insulator (TI) thin films of (Bi(0.57)Sb(0.43))(2)Te(3) below six quintuple layers through transport and scanning tunneling spectroscopy measurements. By effective tuning the Fermi level via gate-voltage control, we unveil a striking competition between weak localization and weak antilocalization at low magnetic fields in nonmagnetic ultrathin films, possibly owing to the change of the net Berry phase. Furthermore, when the Fermi level is swept into the surface gap of ultrathin samples, the overall unitary behaviors are revealed at higher magnetic fields, which are in contrast to the pure WAL signals obtained in thicker films. Our findings show an exotic phenomenon characterizing the gapped TI surface states and point to the future realization of quantum spin Hall effect and dissipationless TI-based applications.

Anomalous Fermi surface in FeSe seen by Shubnikov–de Haas oscillation measurements

We have observed Shubnikov-de Haas oscillations in FeSe. The Fermi surface deviates significantly from predictions of band-structure calculations and most likely consists of one electron and one hole thin cylinder. The carrier density is in the order of 0.01 carriers/ Fe, an order-of-magnitude smaller than predicted. Effective Fermi energies as small as 3.6 meV are estimated. These findings call for elaborate theoretical investigations incorporating both electronic correlations and orbital ordering.

Electronic transport in carbon nanotube ropes and mats

We suggest that conduction in carbon nanotube ropes or mats is best understood in terms of a heterogeneous model, involving regions of metallic conduction together with hopping or tunnelling through small electrical barriers corresponding to defects of various types. Such a model gives a good account of the measured resistivity, in particular the crossover from nonmetallic to metallic sign for the resistivity temperature dependence seen in many samples, in analogy to our similar model for conducting polymers. In agreement with this model, the thermopower has been observed to be more metal-like than resistivity, but does show pronounced nonlinearities as a function of temperature. We investigate electron-phonon effects, density of states peaks, and semiconductor contributions as possible causes of these nonlinearities.

Pauli-limited upper critical field of Fe1+yTe1-xSex

In this work, we investigated the temperature dependence of the upper critical field {mu}{sub 0}H{sub c2}(T) of Fe1.02(3)Te0.61(4)Se0.39(4) and Fe1.05(3)Te0.89(2)Se0.11(2) single crystals by measuring the magnetotransport properties in stable dc magnetic fields up to 35 T. Both crystals show that {mu}{sub 0}H{sub c2}(T) in the ab plane and along the c-axis exhibit saturation at low temperatures. The anisotropy of {mu}{sub 0}H{sub c2}(T) decreases with decreasing temperature, becoming nearly isotropic when the temperature T {yields} 0. Furthermore, {mu}{sub 0}H{sub c2}(0) deviates from the conventional Werthamer-Helfand-Hohenberg theoretical prediction values for both field directions. Our analysis indicates that the spin-paramagnetic pair-breaking effect is responsible for the temperature-dependent behavior of {mu}{sub 0}H{sub c2}(T) in both field directions.

Eu5In2Sb6, Eu5In2−xZnxSb6: rare earth zintl phases with narrow band gaps

The new Zintl phase Eu5In2Sb6 was obtained from a direct element combination reaction in a sealed graphite tube at 870 °C and its structure was determined. It crystallizes in the orthorhombic space group Pbam(No. 55), with a unit cell of a = 12.510(3) A, b = 14.584(3) A, c = 4.6243(9) A, and Z = 2. Eu5In2Sb6 shows the Ca5Ga2As6–type structure and has a one-dimensional structure with infinite anionic double chains [In2Sb6]10− separated by Eu2+ ions. Each single chain is made of corner sharing InSb4 tetrahedra. Two such tetrahedral chains are bridged by a Sb2 group to form double chains. The compound satisfies the classical Zintl concept and is a narrow band gap semiconductor. The substitution of Zn for In atoms in Eu5In2Sb6 resulted in an increase in hole concentration and the material is more metallic. Polycrystalline ingots of Eu5In2Sb6 and Eu5In2−xZnxSb6 showed electrical conductivity of ∼36 S cm−1, ∼146 S cm−1 and Seebeck coefficient of ∼76 µV K−1and ∼51 µV K−1 at room temperature, respectively.

Metal-to-insulator switching in quantum anomalous Hall states

After decades of searching for the dissipationless transport in the absence of any external magnetic field, quantum anomalous Hall effect (QAHE) was recently achieved in magnetic topological insulator films. However, the universal phase diagram of QAHE and its relation with quantum Hall effect (QHE) remain to be investigated. Here, we report the experimental observation of the giant longitudinal resistance peak and zero Hall conductance plateau at the coercive field in the six quintuple-layer (Cr0.12Bi0.26Sb0.62)2Te3 film, and demonstrate the metal-to-insulator switching between two opposite QAHE plateau states up to 0.3 K. Moreover, the universal QAHE phase diagram is confirmed through the angle-dependent measurements. Our results address that the quantum phase transitions in both QAHE and QHE regimes are in the same universality class, yet the microscopic details are different. In addition, the realization of the QAHE insulating state unveils new ways to explore quantum phase-related physics and applications.