S. Jia

Princeton University, Princeton, NJ, United States

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Publications (67)170.24 Total impact

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    ABSTRACT: A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards this end, major progress was recently made with the introduction of highly resistive Bi$_2$Te$_2$Se, in which surface state conductance and quantum oscillations are observed at low temperatures. Nevertheless, an unresolved and pivotal question remains: while room temperature ARPES studies reveal clear evidence of TI surface states, their observation in transport experiments is limited to low temperatures. A better understanding of this surface state suppression at elevated temperatures is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this work, we simultaneously measure TI bulk and surface states via temperature dependent optical spectroscopy, in conjunction with transport and ARPES measurements. We find evidence of coherent surface state transport at low temperatures, and propose that phonon mediated coupling between bulk and surface states suppresses surface conductance as temperature rises.
    04/2014;
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    ABSTRACT: The advent of Dirac materials has made it possible to realize two-dimensional gases of relativistic fermions with unprecedented transport properties in condensed matter. Their photoconductive control with ultrafast light pulses is opening new perspectives for the transmission of current and information. Here we show that the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator can control an essential parameter for photoconductivity-the balance between excess electrons and holes in the Dirac cone. This can result in a strongly out of equilibrium gas of hot relativistic fermions, characterized by a surprisingly long lifetime of more than 50 ps, and a simultaneous transient shift of chemical potential by as much as 100 meV. The unique properties of this transient Dirac cone make it possible to tune with ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that is impossible with conventional optoelectronic materials.
    Nature Communications 01/2014; 5:3003. · 10.02 Impact Factor
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    ABSTRACT: Using circular dichroism-angle resolved photoemission spectroscopy (CD-ARPES), we report a study of the effect of angular momentum transfer between polarized photons and topological surface states on the surface of highly bulk insulating topological insulator Bi2Te2Se. The photoelectron dichroism is found to be strongly modulated by the frequency of the helical photons including a dramatic sign-flip. Our results suggest that the observed dichroism and its sign-flip are consequences of strong coupling between the photon field and the spin-orbit nature of the Dirac modes on the surface. Our studies reveal the intrinsic dichroic behavior of topological surface states and point toward the potential utility of bulk insulating topological insulators in device applications.
    Physical Review B 07/2013; · 3.77 Impact Factor
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    ABSTRACT: We report the direct observation in real space of the charge density wave (CDW) phase transition in pristine 2H-NbSe2 using atomic-resolution scanning tunneling microscopy (STM). We find that static CDW order is established in nanoscale regions in the vicinity of defects at temperatures that are several times the bulk transition temperature Tcdw. On lowering the temperature, the correlation length of these patches increases steadily until CDW order is established in all of space, demonstrating the crucial role played by defects in the physics of the transition region. The nanoscale CDW order has an energy and temperature-independent wavelength. Spectroscopic imaging measurements of the real-space phase of the CDW indicate that an energy gap in NbSe2 occurs at 0.7eV below the Fermi energy in the CDW phase, suggesting that strong electron-lattice interactions and not Fermi surface physics is the dominant cause for CDW formation in NbSe2.
    07/2013;
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    ABSTRACT: How does strong disorder affect the electronic states of complex electronic materials? This question is of relevance to many quantum materials such as the cuprates and pnictides, where interesting electronic phases like superconductivity only arise in strongly disordered samples. The study of these materials is complicated by the presence of multiple electronic phases, which obscures the interpretation of local spectroscopic measurements. To gain insight into this problem, we study 2H-NbSe2, a relatively simple material with a 2D charge density wave ground state. To tune the disorder in the sample, we use sulfur substitution to go from weak (in pristine NbSe2) to strong disorder (in NbSe2-xSx). We use variable-temperature scanning tunneling microscopy and spectroscopy to visualize the electronic structure in real space. Strong changes in the local electronic spectrum are observed with the introduction of disorder, with a pseudogap appearing in the local density of states. We also observe strong changes in the quasiparticle interference from spectroscopic images. We will discuss the interpretation of quasiparticle interference in the limit of strong disorder, and its relevance to existing measurements in the cuprates and pnictides.
    03/2013;
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    ABSTRACT: An unresolved question in experimental research on topological insulators (TI) is the suppression mechanism of a TI's surface state transport. While room temperature ARPES studies reveal clear evidence of surface states, their observation in transport measurements is limited to low temperatures. A better understanding of this suppression is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this talk, we report the temperature dependent optical properties of the topological insulator Bi2Te2Se (BTS), obtained by infrared spectroscopy and ellipsometry, probing surface and bulk states simultaneously. We see clear evidence of coherent surface state transport at low temperature and find that electron-phonon coupling causes the gradual suppression of surface state transport as temperature rises to 43K. In the bulk, electron-phonon coupling enables the emergence of an indirect band gap transition, which peaks at 43K, and is limited by thermal ionization of the bulk valance band above 43K. For comparison with other resistive TIs, we also discuss the optical properties to BiSbSe2Te.
    03/2013;
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    ABSTRACT: Topological insulators (TIs) are materials with high spin-orbit coupling that possess conductive helical surface states. In order to study the exotic properties of the TI surface states, it is favorable to work with TIs that have a low bulk conductivity and exhibit insulating behavior. Bi2Te2Se has been confirmed to have a high bulk resistivity, and it still shows Shubnikov-de Haas oscillations originating from the two-dimensional surface states. We report the observation of coherent Andreev tunneling into the surface states of Bi2Te2Se in high-temperature superconductor (Bi2Sr2CaCu2O8+δ)/Bi2Te2Se junctions fabricated by mechanical bonding method. The differential conductance measurements will be presented in various temperatures and magnetic fields. The characterization of the zero-bias conductance peak observed, suggests that we are tunneling into the surface states of the TI rather than the bulk states.
    03/2013;
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    ABSTRACT: An in-situ method to tune the chemical potential near the Dirac Point (DP) of a topological insulator (TI) would greatly facilitate several key experiments. However, in as-grown crystals of Bi-based TIs, the chemical potential μ lies high above the DP. Using liquid gating on 50-μm thick crystals of Bi2Te2Se, we demonstrate that μ can be tuned by a factor of 6 by observing changes to the Shubnikov-de Haas (SdH) period. A surprise is that the SdH amplitudes increase sharply with gating. Liquid gating allows the n=1 Landau level to be accessed, and the π-Berry phase to be determined with improved accuracy. We will discuss reversibility of liquid gating, and how we may distinguish the purely gating action from chemical reaction.
    03/2013;
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    ABSTRACT: We report the demonstration of hybrid high-Tc-superconductor-semiconductor tunnel junctions, enabling new interdisciplinary directions in condensed matter research. The devices were fabricated by our newly-developed mechanical bonding technique, resulting in high-Tc-semiconductor planar junctions acting as superconducting tunnel diodes. Tunneling-spectra characterization of the hybrid junctions of Bi2Sr2CaCu2O8+{\delta} combined with bulk GaAs, or a GaAs/AlGaAs quantum well, exhibits excess voltage and nonlinearity - in good agreement with theoretical predictions for a d-wave superconductor-normal material junction, and similar to spectra obtained in scanning tunneling microscopy. Additional junctions are demonstrated using Bi2Sr2CaCu2O8+{\delta} combined with graphite or Bi2Te3. Our results pave the way for new methods in unconventional superconductivity studies, novel materials and quantum technology applications.
    Physical Review X 01/2013; 2(4). · 6.71 Impact Factor
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    ABSTRACT: In Bi$_2$Te$_2$Se, the period of quantum oscillations arising from surface Dirac fermions can be increased 6-fold using ionic liquid gating. At large gate voltages, the Fermi energy reaches the $N$ = 1 Landau level in a 14-Tesla field. This enables the $\frac12$-shift predicted for the Dirac spectrum to be measured accurately. A surprising result is that liquid gating strongly enhances the surface mobility. By analyzing the Hall conductivity, we show that the enhancement occurs on only one surface. We present evidence that the gating process is fully reversible (hence consistent with band-bending by the $E$-field from the anion layer accumulated). In addition to the surface carriers, the experiment yields the mobility and density of the bulk carriers in the impurity band. By analyzing the charge accumulation vs. gate voltage, we also obtain estimates of the depletion width and the areal depletion capacitance $C_d/A$. The value of $C_d/A$ implies an enhanced electronic polarizability in the depletion region.
    Physical Review B 11/2012; 88(3). · 3.77 Impact Factor
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    ABSTRACT: CoSn is the prototype compound of the B35 structure, which has long been of interest due to its rarity and unusually low packing density. We report the synthesis and properties of the solid solution Co3Sn3−xGex for 0 ⩽ x ⩽ 2, in order to clarify the conditions necessary to stabilize such a phase. By taking advantage of the chemical differences between the two crystallographically inequivalent Sn sites in the structure, we observe ordered ternary phases, nominally Co3SnGe2 and Co3Sn2Ge. The electron count and unit cell configuration remain unchanged from CoSn; these observations thus help to clarify some ambiguities about the stabilizing forces in this crystal structure. All compounds are normal metals with room temperature conductivities ranging from 5 to 50 μΩ cm and are weakly paramagnetic.
    Journal of Alloys and Compounds 10/2012; 539:137-143. · 2.39 Impact Factor
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    ABSTRACT: Defects in the topological insulator Bi2Te2Se are studied by scanning tunneling microscopy. Small numbers of TeBi antisite defects are found and are postulated to be the origin of n-type carriers in this tetradymite composition near the n-to-p crossover. Based on this defect chemistry, we design an alternative method for obtaining resistive Bi2+xTe2−xSe samples, by the introduction of compensating p-type carriers through BiTe antisite defects induced by making the material slightly Bi rich. Our resistivity and Hall coefficient measurements of Bi2+xTe2−xSe crystals grown by the Bridgeman-Stockbarger method show that the carrier concentration at base temperature is significantly reduced from that of stoichiometric samples. Analysis of the measurements reveals the possible underlying chemical distribution along the boules during growth.
    Physical review. B, Condensed matter 10/2012; 86(16).
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    ABSTRACT: Interest in the superconducting proximity effect has been reinvigorated recently by novel optoelectronic applications as well as by the possible emergence of the elusive Majorana fermion at the interface between topological insulators and superconductors. Here we produce high-temperature superconductivity in Bi(2)Se(3) and Bi(2)Te(3) via proximity to Bi(2)Sr(2)CaCu(2)O(8+δ), to access higher temperature and energy scales for this phenomenon. This was achieved by a new mechanical bonding technique that we developed, enabling the fabrication of high-quality junctions between materials, unobtainable by conventional approaches. We observe proximity-induced superconductivity in Bi(2)Se(3) and Bi(2)Te(3) persisting up to at least 80 K-a temperature an order of magnitude higher than any previous observations. Moreover, the induced superconducting gap in our devices reaches values of 10 mV, significantly enhancing the relevant energy scales. Our results open new directions for fundamental studies in condensed matter physics and enable a wide range of applications in spintronics and quantum computing.
    Nature Communications 09/2012; 3:1056. · 10.02 Impact Factor
  • E. D. Mun, S. Jia
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    ABSTRACT: The thermoelectric power, S(T), of the heavy fermions YbT2Zn20 (T = Fe, Ru, Os, Ir, Rh, and Co) has been measured to shed further light on their strong electronic correlations. A large, negative, local minimum in S(T) with approximately −70 μV/K is found for all compounds. From the observed local minimum, the energy scales associated with both the Kondo temperature and the crystalline electric field splitting are deduced and compared to previous specific heat measurements. At low temperatures, a highly enhanced S(T)/T value is observed for all members, although S(T) does show a deviation from a purely linear temperature dependence, S(T) = αT, for T ≠ Fe members. In the zero-temperature limit, estimated by a simple linear extrapolation, the enhanced S(T)/T value strongly correlates with the electronic specific heat coefficient, C(T)/T.
    Physical review. B, Condensed matter 09/2012; 86(11).
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    ABSTRACT: Seebeck coefficients, electrical resistivities, thermal conductivities and figure of merit ZT of Bi2−xSbxTeSe2 crystals (x=0.8, 0.9, 1.0, 1.1, and 1.2) measured along the hexagonal basal plane are presented. The crystals gradually change from n- to p-type with increasing Sb content, with the crossover lying in the region between x=1.0 and 1.1. The crossover is accounted for by a simple (p–n) electron-hole compensation model, as supported by carrier concentrations determined from Hall measurements. ZT was found to be maximized near the crossover on the p-type side, with the high electrical resistance of the Se-rich crystals apparently the limiting factor in the performance. These materials may serve as a basis for future nanostructuring or doping studies.
    Solid State Communications 07/2012; 152(14):1208–1211. · 1.53 Impact Factor
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    ABSTRACT: It is predicted that electrons on the surface of a topological insulator can acquire a mass (massive Dirac fermion) by opening up a gap at the Dirac point when time-reversal symmetry is broken via the out-of-plane magnetization. We report photoemission studies on a series of topological insulator materials focusing on the spectral behavior in the vicinity of the Dirac node. Our results show that the spectral intensity is suppressed resulting in a "gap"-like feature in materials with or without any magnetic impurity or doping. The Zeeman gap in magnetically doped samples, expected to be rather small, is likely masked by the non-magnetic strong spectral weight suppression involving a large energy scale we report. The photoemission spectral weight suppression observed around the Dirac node thus cannot be taken as the sole evidence for a time-reversal symmetry breaking magnetic gap. We discuss a few possible extrinsic and kinematic origins of the Dirac point spectral weight suppression ("gap") observed in many commonly studied topological materials.
    06/2012;
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    ABSTRACT: We perform spin-resolved and spin-integrated angle-resolved photoemission spectroscopy measurements on a series of compositions in the BiTl(S1-xSex)2 system, focusing on x-values in the vicinity of the critical point for the topological phase transition (the band inversion composition). We observe quasi two dimensional (2D) states on the outer boundary of the bulk electronic bands in the trivial side (non-inverted regime) of the transition. Systematic spin-sensitive measurements reveal that the observed 2D states are spin-momentum locked, whose spin texture resembles the helical spin texture on the surface of a topological insulator. These anomalous states are observed to be only prominent near the critical point, thus are possibly related to strong precursor states of topological phase transition near the relaxed surface.
    04/2012;
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    ABSTRACT: Topological insulators are a class of semiconductors characterized by the presence of current-carrying helical surface states lying within the bulk gap. The surface states of these materials possess massless Dirac-like dispersion. Helical spin texture of the surface states leads to suppression of backscattering in these materials. Results of scanning tunneling spectroscopy study of Bi2Te2Se (BTS) topological insulator will be presented. Similar to previously studied Bi2Te3 and Bi2Se3 the new material shows a relatively large band gap and a simple surface band structure. High bulk resistivity and high surface electron mobility make it a compound of interest for potential applications. Differential conductance mapping with scanning tunneling microscope is used to visualize surface states of this novel highest-bulk-resistivity topological insulator. These experiments enable us to assess the variation of local density of states in this compound under different growth conditions and to correlate the findings with transport properties
    02/2012;
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    ABSTRACT: Recent scanning tunneling microscopy (STM) measurements have shown that crystal defects have a profound influence on the nature of the charge density wave (CDW) transition in the transition-metal dichalcogenides (TMD). In order to examine this effect systematically, we employ low temperature STM to visualize the CDW transition in intentionally doped TMD crystals. We use a newly designed homebuilt ultra-low-loss, variable temperature STM to perform measurements on crystal samples of NbSxSe2-x. We describe the effect of the sulphur atoms on both the local and global charge order in this material.
    02/2012;

Publication Stats

71 Citations
170.24 Total Impact Points

Institutions

  • 2009–2013
    • Princeton University
      • • Department of Chemistry
      • • Department of Physics
      Princeton, NJ, United States
  • 2012
    • University of Toronto
      • Department of Physics
      Toronto, Ontario, Canada
  • 2007–2011
    • Iowa State University
      • • Department of Physics and Astronomy
      • • Ames Laboratory
      Ames, IA, United States