arXiv subjects

Y. Xia

Publications and source records attributed to Y. Xia.

At least 37 records · Page 2Linked to original sources

arXiv2010

Single-Dirac-cone Z2 topological insulator phases in distorted Li2AgSb-class and related quantum critical Li-based spin-orbit compounds

We have extended our new materials class search for the experimental realization of Z2 topological insulators from binary [Bi2Se3-class, Xia et.al., Nature Phys. 5, 398 (2009)] and the ternary [Half-Heusler class, Lin et.al., arXiv:1003.0155v1 (2010); arXiv:1003.2615v1 (2010)] series to non-Heusler Li-based ternary intermetallic series Li2M'X ($M'$=Cu, Ag, and Au, $X$=Sb and Bi) with CuHg2Ti-type structure. We discovered that the distorted-Li2AgSb is a lightweight compound harboring a 3D topological insulator state with Z2=-1 although the groundstate lies near a critical point, whereas the related Li2CuSb-type compounds are topologically trivial. Non-Heusler ternary Li2M'X series (with a number of variant compounds) we identified here is a new platform for deriving novel stoichiometric compounds, artificial quantum-well/heterostructures, nano-wires, nano-ribbons and nanocrystals. We have grown some of these bulk materials (experimental results will be reported separately).

cond-mat.mes-hall↗

arXiv2010

A new platform for topological quantum phenomena : Topological Insulator states in thermoelectric Heusler-related ternary compounds

Topological insulators (TI) realize a novel state of quantum matter that are distinguished by topological invariants of bulk band structure rather than spontaneously broken symmetries. A number of exotic quantum phenomena have been predicted to exist in multiply-connected geometries which require an enormous amount of materials flexibility. We have extended our previous search for TI materials from binary (Bi2X3 series) to the thermoelectric ternary compounds. We discover that the distorted LuPtSb is the first ternary compound harboring a 3D topological insulator state. We also show that the half-Heusler LuPtSb-type series is a natural platform that hosts a range of candidate compounds, alloys and artificial heterostructures (quantum-wells). We also discovered several different paradigms of trivial and non-trivial topological ordering in this class, including a metallic nontrivial topological state in YAuPb. Some of these materials are grown (results will be reported separately).

cond-mat.mes-hall↗

arXiv2010

The development of ferromagnetism in the doped topological insulator Bi2-xMnxTe3

The development of ferromagnetism in Mn-doped Bi2Te3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy analysis shows that the Mn substitutes on the Bi sites, forming compounds of the type Bi2-xMnxTe3, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compositions Bi1.96Mn0.04Te3 and Bi1.91Mn0.09Te3 a second order ferromagnetic transition is observed, with Tc ~ 9-12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi2Te3 basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped Bi2Te3 crystals are p-type. Angle resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine Bi2Te3 are also present in ferromagnetic Mn-doped Bi2-xMnxTe3, and that the dispersion relations of the surface states are changed in a subtle fashion.

cond-mat.mtrl-sci↗

arXiv2010

Direct observation of spin-polarized surface states in the parent compound of a topological insulator using high-resolution spin-resolved-ARPES spectroscopy in a Mott-polarimetry mode

We report high-resolution spin-resolved photoemission spectroscopy (Spin-ARPES) measurements on the parent compound Sb of the recently discovered 3D topological insulator Bi1-xSbx [D. Hsieh et al., Nature 452, 970 (2008)]. By modulating the incident photon energy, we are able to map both the bulk and (111) surface band structure, from which we directly demonstrate that the surface bands are spin polarized by the spin-orbit interaction and connect the bulk valence and conduction bands in a topologically non-trivial way. A unique asymmetric Dirac surface state gives rise to a k-splitting of its spin polarized electronic channels. These results complement our previously published works on this materials class and re-confirm our discovery of topological insulator states in the Bi-Sb series.

cond-mat.mes-hall↗

arXiv2010

First observation of Spin-Momentum Helical Locking in Bi2Se3 and Bi2Te3, demonstration of Topological-Order at 300K and a realization of topological-transport-regime

Both the theoretical and experimental discovery of single-Dirac-cone topological-insulator-class was reported at arXiv:0812.2078 (2008) [Y. Xia et.al., Nature Physics 5, 398-402 (2009) http://www.nature.com/nphys/journal/v5/n6/full/nphys1294.html]. Here we report the first observation of Spin-Momentum Helical Locking and Spin-Vortex structures in Bi2Se3 and Bi2Te3, demonstrate the existence of Topological-Order at 300K and report a material realization of topological-transport-regime for helical Dirac fermions. Our results reveal a one-to-one spin-momentum locked Dirac structure in Bi2Se3 and Bi2Te3 that is nearly 100% spin-polarized, which exhibits a tunable topological fermion density in the vicinity of the Kramers' point and can be driven to the long-sought topological-transport-regime. The observed topological nodal Dirac ground state is found to be protected even up to room temperature (300 K). Our results pave the way for future transport based studies of topological insulators, and possible room temperature applications of protected spin-polarized edge channels we observe with spin-ARPES in spintronic technology. All of these new results are made possible due to the Spin-resolved-ARPES (Mott polarimetric) technique [http://www.nature.com/nature/journal/v460/n7259/full/nature08234.html ] .

cond-mat.mes-hall↗

arXiv2009

Observation of intertwined Fermi surface topology, orbital parity symmetries and electronic interactions in iron arsenide superconductors

We present a polarization and topology resolved study of the low energy band structure in optimally doped superconducting Ba0.6K0.4Fe2As2 using angle resolved photoemission spectroscopy. Polarization-contrasted measurements allow us to identify and trace all low energy bands expected in models, revealing unexpected symmetry breaking and a surprisingly intertwined Fermi surface topology of hole-like bands near the Brillouin zone center. Band structure correlations across the Gamma-M spin fluctuation wavevector are compared with the superconducting gap anisotropy which suggest a partial scenario for spin-mediated interband instability contributing to superconductivity in the hole doped regime.

cond-mat.str-el↗

arXiv2009

Observation of unconventional band topology in a superconducting doped topological insulator, Cux-Bi2Se3: Topological Superconductor or non-Abelian superconductor?

The Cu-doped topological insulator Bi$_2$Se$_3$ has recently been found to undergo a superconducting transition upon cooling, raising the possibilities that it is the first known "topological superconductor" or realizes a novel non-Abelian superconducting state. Its true nature depends critically on the bulk and surface state band topology. We present the first photoemission spectroscopy results where by examining the band topology at many different copper doping values we discover that the topologically protected spin-helical surface states remain well protected and separate from bulk Dirac bands at the Fermi level where Copper pairing occurs in the optimally doped topological insulator. The addition of copper is found to result in nonlinear electron doping and strong renormalization of the topological surface states. These highly unusual observations strongly suggest that superconductivity on the topological surface of Cu$_x$Bi$_2$Se$_3$ cannot be of any conventional type in account of the general topological theory. Characteristics of the three dimensional bulk Dirac band structure are reported for the first time with respect to the superconducting doping state and topological invariant properties which should help formulate a specific theory for this novel superconductor.

cond-mat.supr-con↗

arXiv2009

A topological Dirac insulator in a quantum spin Hall phase (experimental realization of a 3D Topological Insulator)

When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect \cite{Klitzing,Tsui} dictates that an electronic excitation gap is generated in the sample bulk, but metallic conduction is permitted at the boundary. Recent theoretical models suggest that certain bulk insulators with large spin-orbit interactions may also naturally support conducting topological boundary states in the extreme quantum limit, which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic fields. Bulk Bi{1-x}Sbx single crystals are predicted to be prime candidates for one such unusual Hall phase of matter known as the topological insulator. The hallmark of a topological insulator is the existence of metallic spin-textured surface states that are higher dimensional analogues of the edge states that characterize a quantum spin Hall insulator. Here, using incident-photon-energy-modulated angle-resolved photoemission spectroscopy, we report the direct observation of massive Dirac particles in the bulk of Bi0.9Sb0.1, locate the Kramers' points at the sample's boundary and provide a comprehensive mapping of the topological Dirac insulator's gapless surface modes. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the much sought exotic "topological metal". They also suggest that this material has potential application in developing next-generation quantum computing devices that may incorporate "light-like" bulk carriers and topologically protected spin-textured edge-surface currents. This work is a detailed version of [Hsieh et.al., NATURE 452, 970 (2008), {Submitted in November 2007}].

cond-mat.mes-hall↗

arXiv2009

Observation of topologically protected Dirac spin-textures and πBerry's phase in pure Antimony (Sb) and topological insulator BiSb

A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetry based classification of condensed matter. Exotic spin transport phenomena such as the dissipationless quantum spin Hall effect have been speculated to originate from a novel topological order whose identification requires a spin sensitive measurement. Using Spin-resolved-ARPES, we probe the spin degrees of freedom and demonstrate that topological quantum numbers are uniquely determined from spin-texture Berry Phase imaging measurements. Applying this method to pure antimony (Sb) and Bi-Sb, we identify the origin of its novel Topological Order and the negative value of the mirror Chern number. These results taken together constitute the first observation of surface electrons collectively carrying a topological Berry's phase and definite mirror Chern chirality in pure Antimony (Sb) which are the key electronic properties for realizing topological quantum computing via the interface Majorana fermion framework. This paper contains the details of the above mentioned previously reported (Science \textbf{323}, 919 (2009)) results.

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arXiv2009

Time-reversal-protected single-Dirac-cone topological-insulator states in Bi2Te3 and Sb2Te3: Topologically Spin-polarized Dirac fermions with pi Berry's Phase

We show that the strongly spin-orbit coupled materials Bi2Te3 and Sb2Te3 (first non-Bi topological insulator) and their derivatives belong to the Z2 (Time-Reversal-Protected, elastic backscattering suppressed) topological-insulator class. Using a combination of first-principles theoretical calculations and photoemission spectroscopy, we directly show that Bi2Te3 is a large spin-orbit-induced indirect bulk band gap (about 150 meV) semiconductor whose surface is characterized by a single topological spin-Dirac cone. The electronic structure of self-doped Sb2Te3 exhibits similar Z2 topological properties. We demonstrate that the dynamics of surface spin-only Dirac fermions can be controlled through systematic Mn doping, making these materials classes potentially suitable for exploring novel topological physics. We emphasize (theoretically and experimentally) that the Dirac node is well within the bulk-gap and not degenerate with the bulk valence band.

cond-mat.mes-hall↗

arXiv2009

Discovery (theoretical prediction and experimental observation) of a large-gap topological-insulator class with spin-polarized single-Dirac-cone on the surface

Recent theories and experiments have suggested that strong spin-orbit coupling effects in certain band insulators can give rise to a new phase of quantum matter, the so-called topological insulator, which can show macroscopic entanglement effects. Such systems feature two-dimensional surface states whose electrodynamic properties are described not by the conventional Maxwell equations but rather by an attached axion field, originally proposed to describe strongly interacting particles. It has been proposed that a topological insulator with a single spin-textured Dirac cone interfaced with a superconductor can form the most elementary unit for performing fault-tolerant quantum computation. Here we present an angle-resolved photoemission spectroscopy study and first-principle theoretical calculation-predictions that reveal the first observation of such a topological state of matter featuring a single-surface-Dirac-cone realized in the naturally occurring Bi$_2$Se$_3$ class of materials. Our results, supported by our theoretical predictions and calculations, demonstrate that undoped compound of this class of materials can serve as the parent matrix compound for the long-sought topological device where in-plane surface carrier transport would have a purely quantum topological origin. Our study further suggests that the undoped compound reached via n-to-p doping should show topological transport phenomena even at room temperature.

cond-mat.mes-hall↗

arXiv2009

Topological Control: Systematic control of topological insulator Dirac fermion density on the surface of Bi2Te3

Three dimensional (3D) topological insulators are quantum materials with a spin-orbit induced bulk insulating gap that exhibit quantum-Hall-like phenomena in the absence of applied magnetic fields. The proposed applications of topological insulators in device geometries rely on the ability to tune the chemical potential on their surfaces in the vicinity of the Dirac node. Here, we demonstrate a suite of surface control methods based on a combination of photo-doping and molecular-doping to systematically tune the Dirac fermion density on the topological (111) surface of Bi2Te3. Their efficacy is demonstrated via direct electronic structure topology measurements using high resolution angle-resolved photoemission spectroscopy (ARPES). These results open up new opportunities for probing topological behavior of Dirac electrons on the Bi2Te3 surface. At least one of the methods demonstrated here can be successfully applied to other topological insulators such as the Bi{1-x}Sb{x}, Sb2Te3 and Bi2Se3 which will be shown elsewhere. More importantly, our methods of topological surface state manipulation demonstrated here are highly suitable for future spectroscopic studies of topological phenomena which will complement the transport results gained from the traditional electrical gating techniques.

cond-mat.mes-hall↗

arXiv2009

First observation of spin-helical Dirac fermions and topological phases in undoped and doped Bi2Te3 demonstrated by spin-ARPES spectroscopy

Electron systems that possess light-like dispersion relations or the conical Dirac spectrum, such as graphene and bismuth, have recently been shown to harbor unusual collective states in high magnetic fields. Such states are possible because their light-like electrons come in spin pairs that are chiral,which means that their direction of propagation is tied to a quantity called pseudospin that describes their location in the crystal lattice. An emerging direction in quantum materials research is the manipulation of atomic spin-orbit coupling to simulate the effect of a spin dependent magnetic field,in attempt to realize novel spin phases of matter. This effect has been proposed to realize systems consisting of unpaired Dirac cones that are helical, meaning their direction of propagation is tied to the electron spin itself, which are forbidden to exist in graphene or bismuth. The experimental existence of topological order can not be determined without spin-resolved measurements. Here we report a spin-and angle-resolved photoemission study of the hexagonal surface of the Bi2Te3 and Bi{2-x}MnxTe3 series, which is found to exhibit a single helical Dirac cone that is fully spin-polarized. Our observations of a gap in the bulk spin-degenerate band and a spin-resolved surface Dirac node close to the chemical potential show that the low energy dynamics of Bi2Te3 is dominated by the unpaired spin-helical Dirac modes. Our spin-texture measurements prove the existence of a rare topological phase in this materials class for the first time, and suggest its suitability for novel 2D Dirac spin device applications beyond the chiral variety or traditional graphene.

cond-mat.mes-hall↗

arXiv2009

p-type Bi2Se3 for topological insulator and low temperature thermoelectric applications

The growth and elementary properties of p-type Bi2Se3 single crystals are reported. Based on a hypothesis about the defect chemistry of Bi2Se3, the p-type behavior has been induced through low level substitutions (1 percent or less) of Ca for Bi. Scanning tunneling microscopy is employed to image the defects and establish their charge. Tunneling and angle resolved photoemission spectra show that the Fermi level has been lowered into the valence band by about 400 meV in Bi1.98Ca0.02Se3 relative to the n-type material. p-type single crystals with ab plane Seebeck coefficients of +180 microVK-1 at room temperature are reported. These crystals show a giant anomalous peak in the Seebeck coefficient at low temperatures, reaching +120 microVK-1 at 7 K, giving them a high thermoelectric power factor at low temperatures. In addition to its interesting thermoelectric properties, p-type Bi2Se3 is of substantial interest for studies of technologies and phenomena proposed for topological insulators.

cond-mat.mtrl-sci↗

arXiv2009

First direct observation of Spin-textures in Topological Insulators : Spin-resolved ARPES as a probe of topological quantum spin Hall effect and Berry's phase

A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetry based classification of condensed matter. Exotic spin transport phenomena such as the dissipationless quantum spin Hall effect have been speculated to originate from a novel topological order whose identification requires a spin sensitive measurement, which does not exist to this date in any system (neither in Hg(Cd)Te quantum wells nor in the topological insulator BiSb). Using Mott polarimetry, we probe the spin degrees of freedom of these quantum spin Hall states and demonstrate that topological quantum numbers are uniquely determined from spin texture imaging measurements. Applying this method to the Bi{1-x}Sb{x} series, we identify the origin of its novel order and unusual chiral properties. These results taken together constitute the first observation of surface electrons collectively carrying a geometrical quantum (Berry's) phase and definite chirality (mirror Chern number, n_M =-1), which are the key electronic properties for realizing topological computing bits with intrinsic spin Hall-like topological phenomena. Our spin-resolved results not only provides the first clear proof of a topological insulating state in nature but also demonstrate the utility of spin-resolved ARPES technique in measuring the quantum spin Hall phases of matter.

cond-mat.mes-hall↗

arXiv2009

A topological Dirac insulator in a quantum spin Hall phase : Experimental observation of first strong topological insulator

When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect \cite{Klitzing,Tsui} dictates that an electronic excitation gap is generated in the sample bulk, but metallic conduction is permitted at the boundary. Recent theoretical models suggest that certain bulk insulators with large spin-orbit interactions may also naturally support conducting topological boundary states in the extreme quantum limit, which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic field. Bulk Bi$_{1-x}$Sb$_x$ single crystals are expected to be prime candidates for one such unusual Hall phase of matter known as the topological insulator. The hallmark of a topological insulator is the existence of metallic surface states that are higher dimensional analogues of the edge states that characterize a spin Hall insulator. In addition to its interesting boundary states, the bulk of Bi$_{1-x}$Sb$_x$ is predicted to exhibit three-dimensional Dirac particles, another topic of heightened current interest. Here, using incident-photon-energy-modulated (IPEM-ARPES), we report the first direct observation of massive Dirac particles in the bulk of Bi$_{0.9}$Sb$_{0.1}$, locate the Kramers' points at the sample's boundary and provide a comprehensive mapping of the topological Dirac insulator's gapless surface modes. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the much sought exotic "topological metal". They also suggest that this material has potential application in developing next-generation quantum computing devices.

cond-mat.mes-hall↗

arXiv2009

Fermi surface topology and low-lying quasiparticle structure of magnetically ordered Fe1+xTe

We report the first photoemission study of Fe1+xTe - the host compound of the newly discovered iron-chalcogenide superconductors. Our results reveal a pair of nearly electron- hole compensated Fermi pockets, strong Fermi velocity renormalization and an absence of a spin-density-wave gap. A shadow hole pocket is observed at the "X"-point of the Brillouin zone which is consistent with a long-range ordered magneto-structural groundstate. No signature of Fermi surface nesting instability associated with Q= pi(1/2, 1/2) is observed. Our results collectively reveal that the Fe1+xTe series is dramatically different from the undoped phases of the high Tc pnictides and likely harbor unusual mechanism for superconductivity and quantum magnetic order.

cond-mat.supr-con↗

arXiv2008

Experimental determination of the microscopic origin of magnetism in parent iron pnictides

Like high Tc cuprates, the newly discovered iron based superconductors lie in close proximity to a magnetically ordered parent phase. However, while the magnetic order in parent cuprates is known to derive from a spin-spin local superexchange interaction, a plethora of experiments including neutron scattering have so far been unable to conclusively resolve whether a local moment Heisenberg description applies in parent iron based compounds, or whether magnetism arises from a collective SDW order instability. These two alternatives can in principle be distinguished by measuring the low energy momentum-resolved bulk-representative electronic structure of the magnetically ordered phase. Using a combination of polarization dependent ARPES and STM, we have isolated the complete low-lying bulk representative electronic structure of magnetic SrFe2As2 with d-orbital symmetry specificity for the first time. Our results show that while multiple bands with different iron d-orbital character indeed contribute to charge transport, only one pair of bands with opposite mirror symmetries microscopically exhibit an itinerant SDW instability with energy scales on the order of 50 meV. The orbital resolved band topology below T_SDW point uniquely to a nesting driven band hybridization mechanism of the observed antiferromagnetism in the iron pnictides, and is consistent with an unusual anisotropic nodal-density-wave state. In addition, these results place strong constraints on many theories of pnictide superconductivity that require a strict local moment magnetism starting point.

cond-mat.supr-con↗