B. Y. Kang

Terahertz single conductance quantum and topological phase transitions in topological insulator Bi2Se3 ultrathin films

Strong spin-orbit interaction and time-reversal symmetry in topological insulators generate novel quantum states called topological surface states. Their study provides unique opportunities to explore exotic phenomena such as spin Hall effects and topological phase transitions, relevant to the development of quantum devices for spintronics and quantum computation. Although ultrahigh-vacuum surface probes can identify individual topological surface states, standard electrical and optical experiments have so far been hampered by the interference of bulk and quantum well states. Here, with terahertz time-domain spectroscopy of ultrathin Bi₂Se₃ films, we give evidence for topological phase transitions, a single conductance quantum per topological surface state, and a quantized terahertz absorbance of 2.9% (four times the fine structure constant). Our experiment demonstrates the feasibility to isolate, detect and manipulate topological surface states in the ambient at room temperature for future fundamental research on the novel physics of topological insulators and their practical applications.

Valence Characterization of Surface and Subsurface Region in SmB6

Samarium hexaboride (), which lies in the mixed valence regime in the Anderson model, has been predicted to possess topologically protected surface states. The intensive investigations on have brought up the long-standing questions about the discrepancy between the surface and bulk electronic properties in rare earth compounds in general. Here, we investigate and eventually clarify this discrepancy in the particular case of by the photoemission core-level spectra. We focus on the change in both Sm and B states depending on time, temperature, probing depth and surface termination on the cleaved (1 0 0) surface. Our spectra show that the unusual time-dependent change in the Sm valence occurs within a period of hours, which is not related to the adsorption of residual gases. Moreover, we observe a reduction of the surface feature in the B and Sm states on the same timescale accompanied by the formation of a subsurface region. Thus, it indicates the relatively slow charge redistribution between the surface and subsurface regions. Our findings demonstrate that the f states is strongly involved in the surface relaxation.

Valence characterisation of the subsurface region in

Samarium hexaboride (), which lies in the mixed valence regime in the Anderson model, has been predicted to possess topologically protected surface states. The intensive investigations on have brought up the long-standing questions about the discrepancy between the surface and bulk electronic properties in rare earth compounds in general. Here, we investigate and eventually clarify this discrepancy in the particular case of by the photoemission core-level spectra. We focus on the change in both Sm and B states depending on time, temperature, probing depth and surface termination on the cleaved (1 0 0) surface. Our spectra show that the unusual time-dependent change in the Sm valence occurs within a period of hours, which is not related to the adsorption of residual gases. Moreover, we observe a reduction of the surface feature in the B and Sm states on the same timescale accompanied by the formation of a subsurface region. Thus, it indicates the relatively slow charge redistribution between the surface and subsurface regions. Our findings demonstrate that the f states is strongly involved in the surface relaxation.

Magnetic anisotropy and magnon gap state of SmB4 single crystal

This paper investigates the magnetic and electronic properties of a single-crystal SmB4. Two magnetic phase transition temperatures at TN=25 K and Tm≈7 K were revealed by the temperature-dependent magnetization for an applied magnetic field perpendicular to the c-axis. Magnetic transition at TN=25 K is caused by the antiferromagnetic order with Sm3+ moments that lie on the ab-plain. However, the antiferromagnetic transition temperature was not observed in a temperature-dependent magnetization for an applied magnetic field along the c-axis. The second transition occurring at Tm≈7 K was observed for directions both parallel and perpendicular to the c-axis and was suppressed by applying magnetic fields. Interestingly, the second transition exhibited local maximum behavior, implying the second order phase transition. Furthermore, temperature-dependant resistivity showed a magnon gap feature below the second magnetic transition temperature T

Application of emitter-sample hybrid terahertz time-domain spectroscopy to investigate temperature-dependent optical constants of doped InAs.

We investigate temperature-dependent carrier dynamics of InAs crystal by using reflection-type terahertz time-domain spectroscopy, particularly with a recently developed emitter-sample hybrid structure. We successfully obtain the optical conductivity in a terahertz frequency of bulk InAs whose dc conductivity is in the range of 100-150  Ω-1 cm-1. We find that both real and imaginary parts of the optical conductivity can be fit well with the simple Drude model, and the free-carrier density and the scattering rate obtained from the fit are in good agreement with corresponding values obtained by using other techniques, such as the Hall measurement and the dc-resistivity measurement. These results clearly demonstrate that the proposed technique of adopting the emitter-sample hybrid structure can be exploited to determine temperature-dependent optical constants in a reflection geometry and hence to investigate electrodynamics of bulk metallic systems.

Chemical pressure and electron doping effects in SrPd2Ge2 single crystals

Sr(Pd1−xPtx)2Ge2, SrPd2(Ge1−xSix)2, and Sr(Pd1−xAgx)2Ge2 single crystals were synthesized using a self−flux method to examine the effects of chemical pressure and electron doping on the superconductivity of SrPd2Ge2 superconductors. The highest enhanced superconducting phase transition temperature (Tc) was 3.26 K, which was observed in Sr(Pd1−xPtx)2Ge2 at x = 0.0618. In contrast, the Tc of SrPd2(Ge1−xSix)2 remained nearly constant as the amount of dopant increased, and electron doping in Sr(Pd1−xAgx)2Ge2 suppressed the superconductivity. In addition, for x ≤ 0.0657, the superconducting upper critical field (Hc2) increased linearly with an increase in the value of x in Sr(Pd1−xPtx)2Ge2, while Hc2 remained nearly constant in SrPd2(Ge1−xSix)2. The results of the present study were compared with those of pnictide compounds with a ThCr2Si2−type structure.

Irreversible proliferation of magnetic moments at cleaved surfaces of the topological Kondo insulator SmB6

Author(s): He, H; Miao, L; Augustin, E; Chiu, J; Wexler, S; Breitweiser, SA; Kang, B; Cho, BK; Min, CH; Reinert, F; Chuang, YD; Denlinger, J; Wray, LA | Abstract:

Zero field magnetic phase transitions and anomalous low temperature upturn in resistivity of single crystalline α-TmAlB4

In this study, pure α-TmAlB4 (YCrB4 structure) single crystals were grown with no β-TmAlB4 (ThMoB4 structure) intergrowth, and zero magnetic field transitions were confirmed through specific heat capacity, magnetization, and electric resistivity measurements. The anomalous magnetic transition was found at approximately 6.2 K with long range antiferromagnetic transition at 5.6 K. The difference in field dependence between these two transitions indicates that they do not share a common magnetic origin. In addition, we investigated electrical resistivity down to 20 mK, and found upturn behavior at around 0.8 K. The low temperature upturn anomaly in resistivity was not found for other compounds of investigation for RAlB4 (R=rare earth elements), which suggests that an α-RAlB4 system has a significantly different ground state, compared to a β-RAlB4 system.

Understanding spin configuration in the geometrically frustrated magnet TbB4: A resonant soft X-ray scattering study

Abstract The frustrated magnet has been regarded as a system that could be a promising host material for the quantum spin liquid (QSL). However, it is difficult to determine the spin configuration and the corresponding mechanism in this system, because of its geometrical frustration (i.e., crystal structure and symmetry). Herein, we systematically investigate one of the geometrically frustrated magnets, the TbB4 compound. Using resonant soft x-ray scattering (RSXS), we explored its spin configuration, as well as Tbs quadrupole. Comprehensive evaluations of the temperature and photon energy/polarization dependences of the RSXS signals reveal the mechanism of spin reorientation upon cooling down, which is the sophisticated interplay between the Tb spin and the crystal symmetry rather than its orbit (quadrupole). Our results and their implications would further shed a light on the search for possible realization of QSL.