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Madhab Neupane

Publications and source records attributed to Madhab Neupane.

At least 91 records · Page 5Linked to original sources
arXiv2015

Discovery of the topological surface state in a noncentrosymmetric superconductor BiPd

Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due to the possibility of being a topological superconductor. Here, we report a systematic high-resolution angle resolved photoemission spectroscopy (ARPES) study of the normal state electronic properties of BiPd. Our experimental results show the presence of a surface state at higher binding energy with the location of Dirac point at around 700 meV below the Fermi level. The detailed photon energy and temperature dependent measurements complemented by our first-principles calculations provide further evidence for the presence of the topological surface state at high binding energy. The absence of topological surface states near the Fermi level negates the possibility of the topological superconducting behavior in the surface of this material. Our first direct experimental discovery of a topological surface state in BiPd provides novel information that will guide the future search for topological superconductivity in noncentrosymmetric materials.

cond-mat.mes-hall↗
arXiv2015

Two distinct topological phases in the mixed valence compound YbB6 and its differences from SmB6

We discuss the evolution of topological states and their orbital textures in the mixed valence compounds SmB6 and YbB6 within the framework of the generalized gradient approximation plus onsite Coulomb interaction (GGA+U) scheme for a wide range of values of U. In SmB6, the topological Kondo insulator (TKI) gap is found to be insensitive to the value of U, but in sharp contrast, Kondo physics in isostructural YbB6 displays a surprising sensitivity to U. In particular, as U is increased in YbB6, the correlated TKI state in the weak-coupling regime transforms into a d-p-type topological insulator phase with a band inversion between Yb-5d and B-2p orbitals in the intermediate coupling range, without closing the insulating energy gap throughout this process. Our theoretical predictions related to the TKI and non-TKI phases in SmB6 and YbB6 are in substantial accord with recent angle-resolved photoemission spectroscopy (ARPES) experiments.

cond-mat.mtrl-sci↗
arXiv2015

Unconventional transformation of spin Dirac phase across a topological quantum phase transition

The topology of a topological material can be encoded in its surface states. These surface states can only be removed by a bulk topological quantum phase transition into a trivial phase. Here we use photoemission spectroscopy to image the formation of protected surface states in a topological insulator as we chemically tune the system through a topological transition. Surprisingly, we discover an exotic spin-momentum locked, gapped surface state in the trivial phase that shares many important properties with the actual topological surface state in anticipation of the change of topology. Using a spin-resolved measurement, we show that apart from a surface band-gap these states develop spin textures similar to the topological surface states well-before the transition. Our results offer a general paradigm for understanding how surface states in topological phases arise and are suggestive for future realizing Weyl arcs, condensed matter supersymmetry and other fascinating phenomena in the vicinity of topological quantum criticality.

cond-mat.mes-hall↗
arXiv2015

A new type of Weyl semimetal with quadratic double Weyl fermions in SrSi2

We identify a Weyl semimetal state in an inversion breaking, stoichiometric compound strontium silicide, SrSi$_2$, with many new and novel properties that are distinct from the TaAs family. We theoretically show that SrSi$_2$ is a Weyl semimetal even without spin-orbit coupling and that, after the inclusion of spin-orbit coupling, two Weyl fermions stick together forming an exotic double Weyl fermion with quadratic dispersions and a higher chiral topological charge of 2. Moreover, we find that the Weyl nodes with opposite charges are located at different energies due to the absence of mirror symmetry in SrSi$_2$, leading to a unique topological quantum response that an external magnetic field can induce a dissipationless current. Our systematic results not only identify a much-needed robust Weyl semimetal candidate but also open the door to new topological Weyl physics that is not possible in the TaAs family of materials.

cond-mat.mes-hall↗
arXiv2015

Observation of Fermi Arc Surface States in a Topological Metal: A New Type of 2D Electron Gas beyond Z2 Topological Insulators

In a topological insulator, it is the electrons on the surface or edge that carry the signature of topology. Recently, a novel topological state has been proposed in metals or semimetals (gapless) whose band-structure is similar to that of a three-dimensional analog of graphene. However, to this date the signature of its topology remains an open question. We report the experimental discovery of a pair of polarized Fermi arc surface state modes in the form of a new type of two-dimensional polarized electron gas on the surfaces of Dirac semimetals. These Fermi arc surface states (FASS) are observed to connect across an even number of bulk band gapless nodes and found to have their spin uniquely locked to their momentum. We show that these states are distinctly different from the topological surface states (TSS) seen in all known topological insulators. Our observed exotic two-dimensional states not only uncover the novel topology of gapless Dirac metals (such as sodium tribismuth Na$_3$Bi) but also opens new research frontiers for the utilization of topological Fermi arc electron gases for a wide range of fundamental physics and spintronic studies.

cond-mat.mes-hall↗
arXiv2015

Topological Insulators, Topological Crystalline Insulators, Topological Semimetals and Topological Kondo Insulators

In this Book Chapter (invited) we briefly review the basic concepts defining topological insulators and focus on elaborating on the key experimental results that revealed and established their symmetry protected (SPT) topological nature. We then present key experimental results that demonstrate magnetism, Kondo insulation, mirror chirality or topological crystalline order and superconductivity in spin-orbit topological insulator settings and how these new (bulk insulating) phases of matter arise through topological quantum phase transitions from Bloch band insulators via topological or Dirac semimetals at the critical point.

cond-mat.mes-hall↗
arXiv2015

Observation of a topological 3D Dirac semimetal phase in high-mobility Cd3As2

Experimental identification of three-dimensional (3D) Dirac semimetals in solid state systems is critical for realizing exotic topological phenomena and quantum transport such as the Weyl phases, high temperature linear quantum magnetoresistance and topological magnetic phases. Using high resolution angle-resolved photoemission spectroscopy, we performed systematic electronic structure studies on well-known compound Cd3As2. For the first time, we observe a highly linear bulk Dirac cone located at the Brillouin zone center projected onto the (001) surface which is consistent with a 3D Dirac semimetal phase in Cd3As2. Remarkably, an unusually high Dirac Fermion velocity up to 10.2 \textrmÅ{\cdot}$eV (1.5 \times 10^{6} ms^-1) is seen in samples where the mobility far exceeds 40,000 cm^2/V.s suggesting that Cd3As2 can be a promising candidate as a hypercone analog of graphene in many device-applications which can also incorporate topological quantum phenomena in a large gap setting. Our experimental identification of this novel topological 3D Dirac semimetal phase, distinct from a 3D topological insulator phase discovered previously, paves the way for exploring higher dimensional relativistic physics in bulk transport and for realizing novel Fermionic matter such as a Fermi arc nodal metal.

cond-mat.mes-hall↗
arXiv2015

Non-Kondo-like Electronic Structure in the Correlated Rare-Earth Hexaboride YbB$_6$

We present angle-resolved photoemission studies on the rare-earth hexaboride YbB$_6$, which has recently been predicted to be a topological Kondo insulator. Our data do not agree with the prediction and instead show that YbB$_6$ exhibits a novel topological insulator state in the absence of a Kondo mechanism. We find that the Fermi level electronic structure of YbB$_6$ has three 2D Dirac cone like surface states enclosing the Kramers' points, while the f-orbital which would be relevant for the Kondo mechanism is $\sim1$ eV below the Fermi level. Our first-principles calculation shows that the topological state which we observe in YbB$_6$ is due to an inversion between Yb $d$ and B $p$ bands. These experimental and theoretical results provide a new approach for realizing novel correlated topological insulator states in rare-earth materials.

cond-mat.str-el↗
arXiv2015

Surface Versus Bulk Dirac States Tuning in a Three-Dimensional Topological Dirac Semimetal

Recently, crystalline-symmetry-protected three-dimensional (3D) bulk Dirac semimetal phase has been experimentally identified in a stoichiometric high-mobility compound, Cd3As2. The Dirac state observed in Cd3As2 has been attributed to originate mostly from the bulk state while calculations show that the bulk and surface states overlap over the entire Dirac dispersion energy range. In this study, we unambiguously reveal doping induced evolution of the ground state of surface and bulk electron dynamics in a 3D Dirac semimetal. We develop a systematic technique to isolate the surface and bulk states in Cd3As2, by simultaneously utilizing angle-resolved photoemission spectroscopy (ARPES) and in-situ surface deposition. Our experimental results provide a method for tuning the chemical potential as well as to observe surface states degenerate with bulk states, which will be useful for future applications of 3D Dirac semimetal.

cond-mat.mes-hall↗
arXiv2014

Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate Na2IrO3

We report high-resolution angle-resolved photoemission spectroscopy measurements on the honeycomb iridate Na2IrO3. Our measurements reveal the existence of a metallic surface band feature crossing the Fermi level with nearly linear dispersion and an estimated surface carrier density of 3.2 $\times$ 10$^{13}$ cm$^{-2}$, which has not been theoretically predicted or experimentally observed, and provides the first evidence for metallic behavior on the boundary of this material, whereas the bulk bands exhibit a robust insulating gap. We further show the lack of theoretically predicted Dirac cones at the $\overline{M}$ points of the surface Brillouin zone, which confirms the absence of a stacked quantum spin Hall phase in this material. Our data indicates that the surface ground state of this material is exotic and metallic, unlike as predicted in theory, and establishes Na2IrO3 as a rare example of a strongly correlated spin-orbit insulator with surface metallicity.

cond-mat.str-el↗
arXiv2014

Momentum space imaging of Cooper pairing in a half-Dirac-gas topological superconductor (a helical 2D topological superconductor)

Superconductivity in Dirac electrons has recently been proposed as a new platform between novel concepts in high-energy and condensed matter physics. It has been proposed that supersymmetry and exotic quasiparticles, both of which remain elusive in particle physics, may be realized as emergent particles in superconducting Dirac electron systems. Using artificially fabricated topological insulator-superconductor heterostructures, we present direct spectroscopic evidence for the existence of Cooper pairing in a half Dirac gas 2D topological superconductor. Our studies reveal that superconductivity in a helical Dirac gas is distinctly different from that of in an ordinary two-dimensional superconductor while considering the spin degrees of freedom of electrons. We further show that the pairing of Dirac electrons can be suppressed by time-reversal symmetry breaking impurities removing the distinction. Our demonstration and momentum-space imaging of Cooper pairing in a half Dirac gas and its magnetic behavior taken together serve as a critically important 2D topological superconductor platform for future testing of novel fundamental physics predictions such as emergent supersymmetry and quantum criticality in topological systems.

cond-mat.supr-con↗
arXiv2014

Observation of monolayer valence band spin-orbit effect and induced quantum well states (QWS) in MoX2

Transition metal dichalcogenides have attracted much attention recently due to their potential applications in spintronics and photonics as a result of the indirect to direct band gap transition and the emergence of the spin-valley coupling phenomenon upon moving from the bulk to monolayer limit. Here, we report high-resolution angle-resolved photoemission spectroscopy on MoSe2 (molybdenum diselenide) single crystals and monolayer films of MoS2 grown on Highly Ordered Pyrolytic Graphite substrate. Our experimental results, for the first time, resolve the two distinct bands at the Brillouin zone corner of the bulk MoSe2, and provide evidence for the critically important spin-orbit split bands of the monolayer MoS2. Moreover, by depositing potassium on cleaved surfaces of these materials, the process through which quantum well states form on the surfaces of transition metal dichalcogenides is systematically imaged. We present a theoretical model to account for the observed spin-orbit splitting and the rich spectrum of the quantum well states observed in our experiments. Our findings taken together provide important insights into future applications of transition metal dichalcogenides in nanoelectronics, spintronics, and photonics devices as they critically depend on the spin-orbit physics of these materials.

cond-mat.mes-hall↗
arXiv2014

Observation of Quantum-Tunneling Modulated Spin Texture in Ultrathin Topological Insulator Bi2Se3 Films

Understanding the spin-texture behavior of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nano-devices. Here by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunneling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We observe strongly binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultra-thin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunneling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. We present a theoretical model which captures this delicate relationship between quantum tunneling and Fermi surface spin polarization. Our high-resolution spin-based spectroscopic results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nano-device.

cond-mat.mes-hall↗
arXiv2014

Optical evidence of surface state suppression in Bi based topological insulators

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.

cond-mat.mtrl-sci↗
arXiv2014

Search for superconducting proximity effect in a topological insulator and high temperature superconductor heterostructure Bi2Se3/Bi2Sr2CaCu2O8+

Understanding the superconducting proximity effect on the surface of a topological insulator is of critical importance in realizing topological superconductivity and Majorana fermions in solid state settings. We fabricate delicate heterostructure samples between topological insulator (TI) Bi2Se3 thin film and high temperature superconductor Bi2Sr2CaCu2O8+ (Tc ~ 91 K). Using angle-resolved photoemission spectroscopy, we probe the electronic structure and the possible existence of superconducting gap on the top surface of Bi2Se3 thin films. Our systematic data show no significant proximity-induced superconducting gap in the topological surface states with sample temperature down to 10 K (<<91 K) and with a confidence level of sub 5 meV, much smaller than what have been claimed previously, which indicates the near absence of the proximity-induced superconductivity on the TI surface. Our momentum space imaging provides evidence for the coexistence of two crystalline phases in Bi2Se3/Bi2Sr2CaCu2O8+ samples, which we argue to be due to the strong mismatch of lattice crystalline symmetries. Our data identify the major contributors in reducing the proximity-induced superconducting gap below the meV range, including the lack of momentum space overlap between the Bi2Se3 and Bi2Sr2CaCu2O8+ Fermi surfaces, the strong mismatch of lattice crystalline symmetries and superconducting pairing symmetries, as well as the very short superconducting coherence length in Bi2Sr2CaCu2O8+. Our ARPES studies not only provide critical momentum space insights into the Bi2Se3/Bi2Sr2CaCu2O8+ heterostructure, but also set an upper bound on the proximity induced gap for realizing a Majorana fermion condition in this system, which may be further destabilized by the d-wave nodes dominated in a cuprate superconductor.

cond-mat.str-el↗
arXiv2014

Spin correlated electronic state on the surface of a spin-orbit Mott system (Layered Iridates)

Novel phases of two dimensional electron systems resulting from new surface or interface modified electronic structures have generated significant interest in material science. We utilize photoemission spectroscopy to show that the near-surface electronic structure of a bulk insulating iridate Sr$_3$Ir$_2$O$_7$ lying near metal-Mott insulator transition exhibit weak metallicity signified by finite electronic spectral weight at the Fermi level. The surface electrons exhibit a unique spin structure resulting from an interplay of spin-orbit, Coulomb interaction and surface quantum magnetism, distinct from a topological insulator state. Our results suggest the experimental realization of a novel quasi two dimensional interacting electron surface ground state, opening the door for exotic quantum entanglement and transport phenomena in iridate-based oxide devices.

cond-mat.str-el↗