A novel experimental approach to topological quantum phenomena: Traditionally spectroscopic methods have been used to characterize electronic behavior in quantum matter whereas initial discoveries originated from transport methods.
Our works in 3D topological insulators suggest that spectroscopic methods such as ARPES can be utilized to discover novel topological quantum phenomena Science, Nature, Nature, Science Previously topological quantum phenomena quantum Hall like effects were being probed mainly with transport methods pioneered by von Klitzing Following our demonstration of application of spin-ARPES, there are world-wide efforts to apply this technique and its derivatives to may tr?n b?t c?m tay and study novel topological quantum phenomena in condensed matter systems.
Traditionally spectroscopic methods have been used to characterize electronic or spin behavior in quantum matter whereas initial discoveries originated from non-spectroscopic methods. My work focuses on the theme which I often like to call spectroscopy for discovering new states of quantum matter. Three dimensional topological insulators 3D-TI originally called "Topological Insulators" to may tr?n b?t c?m tay them from 2D quantum Hall type effects and insulators are the first example of topological order in the bulk solids there is no genuine quantum Hall effect in three dimensions.
They feature a "protected" metallic Dirac-like surface state 2DEG or planar "topological metal" where electron's spin and momentum are locked to each other and possess half the degrees of freedom may tr?n b?t c?m tay in an ordinary electron Fermi gas. Strong spin-orbit coupling leads to an insulating bulk and the surface states may tr?n b?t c?m tay protected by time reversal symmetry and belong to the Z2 class see theory by Kane-Mele'05; Fu-Kane-Mele'07, Moore-Balents'07; precursor theory of TR-breaking 2D topological insulator by Haldane in In experiments, 3D Topological Insulators are an example of non-quantum-Hall-like topological matter experimentally discovered and reported around the same time, in parallel, as the spin Hall edge-states in Hg Cd Te, in Experimentally, these two are unrelated.
Since IQH state is a 2D topological insulator, spin quantum Hall effect is also may tr?n b?t c?m tay 2D topological insulator but time-reversal invariant protected by Z2 invariant. On the other hand, the 3D Topological Insulators are a new and distinct state of matter which cannot be reduced to multiple copies of IQH and there is no spin Hall effect in 3D the term "Topological Insulators" was originally used exclusively for the novel and unprecedented 3D state since there is no spin Hall like effect there.
The 3D state is thus an example of non-quantum-Hall-like topological matter and the first realization of topologically ordered bulk solid in nature Physics World Experimentally, 3D TI did not arise from quantum spin Hall effect. Additionally, transport measurements cannot provide a proof of Z2 topology either Physics World All of the 2D topological insulator examples IQH, FQH, QSH including the fractional one FQH involving Coulomb interaction are understood in the standard picture of quantized electron orbits in a spin-independent or spin-dependent magnetic field, the 3D topological insulator defies such description and is a novel type of topological order which cannot be reduced to multiple copies of quantum-Hall-like states.
In fact, the 3D topological insulator exists not only in zero magnetic field, they also differ from the 2D variety in three very may tr?n b?t c?m tay aspects: The non-quantum-Hall-like novel character and the extremely rich physics surface 2DEG, helical fermion gas of the 3D state has led to a world-wide research interest in this topic in general.
Also the novel experimental approach in studying topological quantum phenomena demonstrated by us are now being used by many other groups world-wide studying many other topological materials and phenomena. Topological Insulators 3DTI is new and unprecedented and cannot be reduced to multiple copies of quantum Hall or spin Hall like states.
Most topological states of matter are realized in two or lower dimensions quantum Hall states, quantum spin Hall effect, non-Fermi liquid chains and wires, quantum spin-liquids etc. Unlike all others, neither strong electron-electron interactions necessary for quantum spin liquidshigh magnetic fields and low temperatures necessary for quantum Hall statesnor low dimensionality needed for quantum Hall states, spin quantum Hall states QSHEand non-Fermi liquid spin chains are needed for the 3D topological insulator.
The theoretical and experimental discovery of the 3D TIs - the first example of topological order in bulk solids - has generated much experimental and theoretical efforts to understand and utilize all aspects of these quantum phenomena and the materials that exhibit them. The method to probe 2D topological-order is may tr?n b?t c?m tay with charge transport pioneered by Von Klitzing in the swhich either measures quantized transverse conductance plateaus in IQH systems or longitudinal conductance in quantum spin Hall QSH systems.
In a 3D topological insulator, the boundary itself supports a two dimensional electron gas 2DEG and transport is not Z2 topologically quantized hence cannot directly probe the topological invariants? In this paper, we review the birth of momentum- and spin-resolved spectroscopy as a new experimental approach and as a directly boundary sensitive method to study and prove topological-order in three-dimensions via the direct measurements of the topological invariants?
Experimentally demonstrated, a three dimensional topological insulator 3D-TI features a protected two-dimensional electron gas on its surface. The high magnetic fields, low temperatures or low dimensionality are not necessary may tr?n b?t c?m tay retaining the topological protection or topological order of a macroscopic 2DEG on the surface of a topological insulator.
Transport measurements that have been the key to probe topological order in conventional quantum Hall like systems cannot even in theory! Science, later further expanded: Sciencesee below for details. Topological Z2 Order is more directly manifested in the spin and momentum correlated motion of electrons on the surface. This leads to a new type of 2DEG where the electron's spin and linear momentum are one-to-one locked. Such a 2DEG only carries half of the total degrees of freedom of a conventional 2DEG and in the vicinity of the Kramers' point takes the form of a half Dirac gas: Natureand further details in related materials Phys.
Spin-Momentum locking and pi-Berry's phase lead to the absence of may tr?n b?t c?m tay backscattering on the surfaces. By combining spin-ARPES and may tr?n b?t c?m tay collaborationwe have demonstrated such absence of elastic backscattering: The advantages of large band gap and simple spin-polarized Dirac cone topology of these spin-orbit insulators led to our observation of topological quantum phenomena at room temperatures without magnetic fields and without high purity semiconductors Discovery of Single-Dirac-Cone TSS Bi2Se3 as a TI class: An exciting progress along this line is that the Bi2Se3 class of 3D TIs can be turned in to superconductors to form the host material for Majorana Fermions Nature Physics 6, Also both integer and fractional quantum Hall effects have been reported in this Bi2Se3 class of materials by other groups indicating the high mobility of the topological surface state.
These developments have unleashed a world-wide experimental effort to understand all aspects of electrical and spin properties leading to a nearly graphene-like revolution in physics Physics World Topological protection and degree of robustness: The Dirac cone materials are probed via the modification of surface potential: How robust the topological properties of a Topological Insulator surface are investigated Nature Physics 7, 32 This paper reported preliminary results regarding magntism, for a full detail see, Magnetic Topological Insulators: The effect of time-reversal symmetry leads to unconventional spin textures on the surface of a topological insulator.
Nature Physics 8, for Magnetic symmetry breaking, for details see Phys. B A novel experimental approach to topological quantum phenomena: Superconducting topological insulators can serve as Majorana platforms. Observation of topological order in a superconducting doped topological insulator demonstrates a platform for realizing Majorana fermions. Some of the doped topological insulators that superconduct at low temperatures may also turn out to be topological superconductors proposed in may tr?n b?t c?m tay based on our experimental observations and spin-ARPES results Nature Physics 6, and additional details in Phys.
B Bulk Insulating Topological Insulators: Also see works by other groups Phys. B and Phys. Working with Hsin Lin and others we demonstrated that first-principles based adiabatic continuation approach is a very powerful and efficient tool for constructing topological phase diagrams and locating non-trivial topological insulator materials.
Applied to real materials our results demonstrated the efficacy of adiabatic continuation as a may tr?n b?t c?m tay tool for exploring topologically nontrivial alloying systems and for identifying new topological insulators even when the may tr?n b?t c?m tay lattice does not possess inversion symmetry, and the approaches based on parity analysis of Fu-Kane are not viable.
Nature Materials 9, and more recently at Phys. B Topological Phase transition and Texture inversion: It is believed that a trivial insulator can be twisted into a topological state by modulating the spin-orbit interaction driving the system through a topological quantum phase transition.
We reported the observation of such a phase transition in a tunable spin-orbit system where the topological state formation is visualized.
In the topological state, vortex-like polarization states are observed to exhibit 3D vectorial textures, which collectively feature a chirality transition as the spin-momentum locked electrons on the surface go through the zero carrier density point.
Scienceand in a related system, see Phys. Z2 topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction-driven band inversion.
The topological phase in the Bi1? We experimentally investigated the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb1? Our observation of the spin-polarized Dirac surface states in the inverted Pb1? Symmetry-broken three-dimensional 3D topological Dirac semimetal systems with strong spin-orbit coupling can host many exotic Hall-like phenomena and Weyl fermion quantum transport.
Using high-resolution angle-resolved photoemission spectroscopy, we performed systematic electronic structure studies on Cd 3 As 2which has been predicted to be the parent material, from which many unusual topological phases can be derived. We observe a highly linear bulk band may tr?n b?t c?m tay to form a 3D dispersive Dirac cone projected at the Brillouin zone centre by studying the -cleaved surface. Remarkably, an unusually high in-plane Fermi velocity up to 1.
Our discovery of the Dirac-like bulk topological semimetal phase in Cd3As2 opens the door for exploring higher dimensional spin-orbit Dirac physics in a real material. The 3D Dirac semimetals can be realized from topo. Near the critical point a 3D Dirac version of Graphene is realized which was probed in our Science paper more recently elaborated at our Nat. Scienceand more recently in other materials such as Cd3As2 see below Nature Commun.
The advent of topological insulators has made it possible may tr?n b?t c?m tay realize two-dimensional spin polarized gases of relativistic fermions with unprecedented properties in condensed matter. Their photoconductive control with ultrafast light pulses is of interest in optoelectronics. In collaboration with M. Marsi group Paris we probed the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator Bi2Se3 and related materials.
Currently, we are probing other exotic aspects of may tr?n b?t c?m tay response of bulk insulating TI materials preprint and correlated electron systems. Understanding the spin behaviour of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nanodevices.
Using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we reported tunnelling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition and separately in magnetically doped thin-films.
Topological States can also arise in correlated electron systems such as Kondo or Mixed-Valence Insulators. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, we probed the surface electronic structure of the anomalous conductivity regime sub 6K may tr?n b?t c?m tay SmB6. We observe that the bulk bands exhibit a Kondo gap of 15 meV and identify in-gap low-lying states within a 4 meV window of the Fermi level on the -surface of this material.
These states disappear as temperature is raised above 15K in correspondence with the complete disappearance of the 2D conductivity channels in SmB6. Our bulk and surface measurements carried out in the transport-anomaly-temperature regime T 10K are consistent with the first-principle predicted Fermi surface topology of a topological Kondo insulator phase in this material. B 79, ; Y. T Experimental Discoveries: Physics 3, 62 Three-Dimensional Topological Insulators.
Phys 2, 55 Phys 82, Weyl may tr?n b?t c?m tay, Fermi arcs and chiral anomaly S. Group Members Research Einstein tried it hard We are interested in both quantum-many-body emergence and topological weakly or strongly interacting emergence. We search for or work on natural or artificially designed Novel Phases of Matter and Emergent New Particles We are currently designing novel next-generation ultrafast spectrometers for exploring topological may tr?n b?t c?m tay of novel quantum phenomena which is not possible with current generation of ultrafast techniques according to some of our theoretical predictions but relevant for what we call "topotronics".
We are part of the Princeton's Quantum Engineering and Engineering-Physics and often employ design, engineering and advanced-instrumentation approaches to our research on quantum matter. Light Source helped to spawn a revolution in materials research" Scientific American:
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