arXiv subjects

D. Qian

Publications and source records attributed to D. Qian.

At least 19 recordsLinked to original sources

arXiv2018

Magnetic Properties of the low dimensional BaM$_2$Si$_2$O$_7$(M= Cu, Co, Mn) system

We performed susceptibility, magnetization, specific heat, and single crystal neutron diffraction measurements on single crystalline BaMn$_2$Si$_2$O$_7$. Based on the results, we revisited its spin structure with a more accurate solution and constructed a magnetic phase diagram with applied field along the $b$-axis, which contains a spin flop transition around 6 T. We also used susceptibility, magnetization, and specific heat results confirmed the ferrimagnetic-like magnetism in polycrystalline BaCo$_2$Si$_2$O$_7$. Furthermore, we performed LSDA + U calculations for the BaM$_2$Si$_2$O$_7$ (M = Cu, Co, and Mn) system. Our discussions based on the comparison among the obtained magnetic exchange interactions suggest the different structures and electronic configurations are the reasons for the different magnetic properties among the system members.

cond-mat.str-el↗

arXiv2018

Magnetoelectric and Raman spectroscopic studies of single-crystalline MnCr2O4

MnCr2O4 that exhibits spin frustration and complex spiral spin order is of great interest from both fundamental as well as application-oriented perspectives. Unlike CoCr2O4 whose ground state presents the coexistence of commensurate spiral spin order (CSSO) and ferroelectric order, MnCr2O4 shows no multiferroicity. One reason is that the spiral spin order is highly sensitive to the oxygen concentration in MnCr2O4. Here, we have successfully grown high-quality single-crystalline MnCr2O4 by the chemical vapor transport method. We observe a new first-order magnetic transition from the incommensurate spiral spin order (ICSSO) at 19.4 K to the CSSO at 17.4 K. This magnetic transition is verified by magnetization, specific heat, and magnetoelectric measurements, which also confirm that the ground state exhibits the coexistence of the CSSO and magnetoelectricity below 17.4 K. Interestingly, the temperature evolution of Raman spectra between 5.4 and 300 K suggests that the structure remains the same. We also find that the phase-transition temperature of the CSSO decreases as applied magnetic field increases up to 45 kOe.

cond-mat.mtrl-sci↗

arXiv2017

Antiferromagnetic order in epitaxial FeSe films on SrTiO3

Single monolayer FeSe film grown on Nb-doped SrTiO$_3$(001) substrate shows the highest superconducting transition temperature (T$_C$ $\sim$ 100 K) among the iron-based superconductors (iron-pnictide), while T$_C$ of bulk FeSe is only $\sim$ 8 K. Antiferromagnetic spin fluctuations were believed to be crucial in iron-pnictides, which has inspired several proposals to understand the FeSe/SrTiO$_3$ system. Although bulk FeSe does not show the antiferromagnetic order, calculations suggest that the parent FeSe/SrTiO$_3$ films are AFM. Experimentally, due to lacking of direct probe, the magnetic state of FeSe/SrTiO$_3$ films remains mysterious. Here, we report the direct evidences of the antiferromagnetic order in the parent FeSe/SrTiO$_3$ films by the magnetic exchange bias effect measurements. The phase transition temperature is $\geq$ 140 K for single monolayer film. The AFM order disappears after electron doping.

cond-mat.supr-con↗

arXiv2016

Electronic structure of Ba(Zn0.875Mn0.125)2As2 studied by angle-resolved photoemission spectroscopy

Electronic structure of single crystalline Ba(Zn$_{0.875}$Mn$_{0.125}$)$_{2}$As$_{2}$, parent compound of the recently founded high-temperature ferromagnetic semiconductor, was studied by high-resolution photoemission spectroscopy (ARPES). Through systematically photon energy and polarization dependent measurements, the energy bands along the out-of-plane and in-plane directions were experimentally determined. Except the localized states of Mn, the measured band dispersions agree very well with the first-principle calculations of undoped BaZn$_{2}$As$_{2}$. A new feature related to Mn 3d states was identified at the binding energies of about -1.6 eV besides the previously observed feature at about -3.3 eV. We suggest that the hybridization between Mn and As orbitals strongly enhanced the density of states around -1.6 eV. Although our resolution is much better compared with previous soft X-ray photoemission experiments, no clear hybridization gap between Mn 3d states and the valence bands proposed by previous model calculations was detected.

cond-mat.mtrl-sci↗

arXiv2014

Spin Order by Quantum Frustration in Triangular Lattice Mott Insulator NaCrO2 : A Neutron Scattering Study

We report high resolution neutron scattering measurements on the triangular lattice Mott insulator Na$_x$CrO$_2$ ($x$=1) which has recently been shown to exhibit an unusually broad fluctuating crossover regime extending far below the onset of spin freezing ($T_c\sim$41K). Our results show that below some crossover temperature ($T\sim0.75T_c$) a small incommensuration develops which helps resolve the spin frustration and drives three-dimensional magnetic order supporting coherent spin wave modes. This incommensuration assisted dimensional crossover suggests that inter-layer frustration is responsible for stabilizing the rare 2D correlated phase above 0.75$T_c$. In contrast to the host compound of 2D cobaltate superconductor such as Na$_x$CoO$_2$ ($ξ_c>50$Å), no magnetic long-range order is observed down to 1.5K ($ξ_c<16$Å).

cond-mat.str-el↗

arXiv2012

A topological crystalline insulator (TCI) phase via topological phase transition and crystalline mirror symmetry

A Z2 topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction driven band inversion. For example, the topological phase in the Bi-Sb system is due to an odd number of band inversions. A related spin-orbit system, the (Pb/Sn)Te class, has been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry protected topological crystalline insulator phase in the (Pb/Sn)Te class of materials which has been theoretically predicted to exist in its non-alloyed version. Our experimental results show that at a finite-Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states (as opposed to odd as observed in Bi-Sb alloy or Bi2Se3) revealing mirror protected topological order distinct from that observed in Bi-Sb or Bi2Se3. Our observation of the spin-polarized Dirac surface states in the inverted (Pb/Sn)Te and their absence in the non-inverted compounds related via a topological phase transition (spin-orbit induced band inversion type) provide the experimental groundwork for opening the research on novel topological order in future quantum devices.

cond-mat.mes-hall↗

arXiv2012

Deviating band symmetries and many-body interactions in a model hole doped iron pnictide superconductor

We present a polarization resolved study of the low energy band structure in the optimally doped iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (T$_c$=37K) using angle resolved photoemission spectroscopy. Polarization-contrasted measurements are used to identify and trace all three low energy hole-like bands predicted by local density approximation (LDA) calculations. The photoemitted electrons reveal an inconsistency with LDA-predicted symmetries along the $Γ$-X high symmetry momentum axis, due to unexpectedly strong rotational anisotropy in electron kinetics. We evaluate many-body effects such as Mott-Hubbard interactions that are likely to underlie the anomaly, and discuss how the observed deviations from LDA band structure affect the energetics of iron pnictide Cooper pairing in the hole doped regime.

cond-mat.supr-con↗

arXiv2012

Observation of Topological Crystalline Insulator phase in the lead tin chalcogenide Pb1-xSnxTe material class

We perform systematic angle-resolved photoemission spectroscopic measurements on the lead tin telluride Pb1-xSnxTe pseudobinary alloy system. We show that the (001) crystalline surface, which is a crystalline surface symmetric about the (110) mirror planes of the Pb1-xSnxTe crystal, pos- sesses four metallic surface states within its surface Brillouin zone. Our systematic Fermi surface and band topology measurements show that the observed Dirac-like surface states lie on the symmetric momentum-space cuts. We further show that upon going to higher electron binding energies, the surface states' isoenergetic countours in close vicinity of each X point are observed to hybridize with each other, leading to a Fermi surface fractionalization and the Lifshitz transition. In addition, systematic incident photon energy dependent measurements are performed, which enable us to un- ambiguously identify the surface states from the bulk bands. These systematic measurements of the surface and bulk electronic structure on Pb1-xSnxTe, supported by our first principles calculation results, for the first time, show that the Pb1-xSnxTe system belongs to the topological crystalline insulator phase due to the four band inversions at the L points in its Brillouin zone, which has been recently theoretically predicted.

cond-mat.mes-hall↗

arXiv2011

Topological Phase Transition and Texture Inversion in a Tunable Topological Insulator

The recently discovered three dimensional or bulk topological insulators are expected to exhibit exotic quantum phenomena. It is believed that a trivial insulator can be twisted into a topological state by modulating the spin-orbit interaction or the crystal lattice via odd number of band inversions, driving the system through a topological quantum phase transition. By directly measuring the topological invariants (for the method to directly measure Fu-Kane {$ν_0$}, see Hsieh \textit{et.al.,} Science 323, 919 (2009) at http://www.sciencemag.org/content/323/5916/919.abstract) we report the observation of a phase transition in a tunable spin-orbit system BiTl(S{1-d}Se{d})2 (which is an analog of the most studied topological insulator Bi2Se3, see Xia \textit{et.al.,} Nature Phys. 5, 398 (2009) at http://www.nature.com/nphys/journal/v5/n6/full/nphys1294.html and Spin-Momentum locking at http://www.nature.com/nature/journal/v460/n7259/full/nature08234.html) where the topological insulator state formation is visualized for the first time. In the topological state, vortex-like polarization states are observed to exhibit 3D vectorial textures, which collectively feature a chirality transition of its topological spin-textures as the spin-momentum locked (Z2 topologically ordered) electrons on the surface go through the zero carrier density point. Such phase transition and texture chirality inversion can be the physical basis for observing \textit{fractional charge} (e/2) and other related fractional topological phenomena.

cond-mat.mes-hall↗

arXiv2011

Direct observation of spin-polarized surface states in the parent compound of topological insulator Bi-Sb using spin-resolved-ARPES in a 3D Mott-polarimetry spin mode

We report high-resolution spin-resolved photoemission spectroscopy (Spin-ARPES) measurements on the parent compound Sb of the first discovered 3D topological insulator Bi{1-x}Sb{x} [D. Hsieh et al., Nature 452, 970 (2008) Submitted 2007]. 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 first bulk (3D) topological insulator - topological order in bulk solids. [Invited article for NJP-IOP Focus issue on "Topological Insulators"]

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.

cond-mat.mes-hall↗

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↗