/Tag: Bi2Se3

Effect of Sr doping on structure, morphology and transport properties of Bi2Se3 epitaxial thin films. (arXiv:1811.04442v1 [cond-mat.mtrl-sci])

2018-11-13T02:29:21+00:00November 13th, 2018|Categories: Publications|Tags: |

We report molecular beam epitaxy growth of Sr-doped Bi2Se3 films on (111) BaF2 substrate, aimed to realize unusual superconducting properties inherent to SrBi2Se3 single crystals. Despite wide range of the compositions, we do not achieve superconductivity. To explore the reason for that we study structural, morphological and electronic properties of the films and compare them to the corresponding properties of the single crystals. The dependence of the c-lattice constant in the films on Sr content appears to be more than an order of magnitude stronger than in the crystals. Correspondingly, all other properties also differ substantially, indicating that Sr atoms get different positions in lattices. We argue that these structural discrepancies come from essential differences in growth conditions. Our research calls for more detailed structural studies and novel growth approaches for design of superconducting SrxBi2Se3 thin films.

Published in: "arXiv Material Science".

Nematic Superconductivity in Cu1.5(PbSe)5(Bi2Se3)6. (arXiv:1811.00805v1 [cond-mat.supr-con])

2018-11-05T02:29:21+00:00November 5th, 2018|Categories: Publications|Tags: |

After the discovery of nematic topological superconductivity in CuxBi2Se3, carrier-doped topological insulators are established as a fertile ground for topological superconductors. The superconductor Cu1.5(PbSe)5(Bi2Se3)6 (CPSBS) contains Bi2Se3 blocks as a constitutional unit, but its superconducting gap appears to have nodes [S. Sasaki et al., Phys. Rev. B 90, 220504 (2014)], which is in contrast to the fully-opened gap in CuxBi2Se3 and the relation between the two superconductors remained an open question. Here we report our observation of clear two-fold symmetry in the in-plane magnetic-field-direction dependencies of the upper critical field and of the specific heat of CPSBS, which indicates that essentially the same nematic state is realized in CPSBS. This establishes CPSBS as a new variant of a nematic topological superconductor with preserved nodes.

Published in: "arXiv Material Science".

Self-assembled Bismuth Selenide (Bi2Se3) quantum dots grown by molecular beam epitaxy. (arXiv:1810.05081v1 [cond-mat.mtrl-sci])

2018-10-12T02:29:36+00:00October 12th, 2018|Categories: Publications|Tags: |

We report the growth of self-assembled Bi2Se3 quantum dots (QDs) by molecular beam epitaxy on GaAs substrates using the droplet epitaxy technique. The QD formation occurs after anneal of Bismuth droplets under Selenium flux. Characterization by atomic force microscopy, scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy and X-ray reflectance spectroscopy is presented. The quantum dots are crystalline, with hexagonal shape, and have average dimensions of 12 nm height (12 quintuple layers) and 46 nm width, and a density of $8.5 cdot 10^9 cm^{-2}$. This droplet growth technique provides a means to produce topological insulator QDs in a reproducible and controllable way, providing convenient access to a promising quantum material with singular spin properties.

Published in: "arXiv Material Science".

Axion Insulator State in a Ferromagnet/Topological Insulator/Antiferromagnet Heterostructure. (arXiv:1809.09265v1 [cond-mat.mtrl-sci])

2018-09-26T02:29:21+00:00September 26th, 2018|Categories: Publications|Tags: , |

Experimental realization of the axion insulator that exhibits the intriguing topological magnetoelectric effect has remained a challenge since it was theoretically predicted in three-dimensional (3D) topological insulators (TIs) a decade ago. One of the primary experimental difficulties is to break time-reversal symmetry with a special configuration of magnetization that points inwards or outwards over the whole surface of 3D TIs. Here, we propose that axion insulator state can show up in a ferromagnetic insulator/3D-TI/antiferromagnet insulator heterostructure. Importantly, its appearance only depends on the magnetization of the ferromagnetic insulator. Therefore, the axion insulator state in such heterostructure can be more easily observed by experiments. Remarkably, it is exemplified that the ferromagnetic insulator MnBi2Se4/Bi2Se3/antiferromagnetic insulator Mn2Bi2Se5 heterostructure manifests axion insulator state indeed. Our work thus sheds new light onto the exploration of axion insulator state in the heterostructures formed by magnetic insulators and 3D TIs.

Published in: "arXiv Material Science".

Topological insulator Bi2Se3 films on rare earth iron garnets and their high-quality interfaces. (arXiv:1809.04513v1 [cond-mat.mtrl-sci])

2018-09-13T02:29:29+00:00September 13th, 2018|Categories: Publications|Tags: , |

The integration of quantum materials like topological insulators (TIs) with magnetic insulators (MIs) has important technological implications for spintronics and quantum computing. Here we report excellent crystallinity of c-axis oriented epitaxial TI films Bi2Se3 grown on MI films, a rare earth iron garnet (ReIG), such as thulium iron garnet (Tm3Fe5O12, TmIG) by molecular beam epitaxy (MBE) with a Se-buffered low-temperature (SBLT) growth technique. We demonstrated a streaky reflection high-energy electron diffraction pattern starting from the very first quintuple layer of Bi2Se3, indicating the high-quality interface between TmIG and Bi2Se3, a prerequisite for studying interfacial exchange coupling effects. The strong interfacial exchange interaction was manifested by observations of anomalous Hall effect in the Bi2Se3/TmIG bilayer and a shift of ferromagnetic resonance field of TmIG induced by Bi2Se3. We have reproducibly grown high-quality Bi2Se3/ReIG and interfaces using this new TI growth method, which may be applied to grow other types of van der Waals (vdW) hetero-structures.

Published in: "arXiv Material Science".

Finite momentum Cooper pairing in three-dimensional topological insulator Josephson junctions

2018-08-28T10:36:19+00:00August 28th, 2018|Categories: Publications|Tags: |

Finite momentum Cooper pairing in three-dimensional topological insulator Josephson junctionsFinite momentum Cooper pairing in three-dimensional topological insulator Josephson junctions, Published online: 28 August 2018; doi:10.1038/s41467-018-05993-wUnconventional superconductivity may emerge from the interplay between strong spin–orbit coupling and magnetism. Here, Chen et al. report an anomalous Fraunhofer pattern in three-dimensional topological insulator Bi2Se3 and attribute it as a signature of finite momentum Cooper pairing.

Published in: "Nature Communications".

Definition of Polaritonic Fluctuations in Natural Hyperbolic Media: Applications to Hexagonal Boron Nitride, Bismuth Selenide and the Spontaneous Emission Sum Rule. (arXiv:1702.01862v3 [physics.optics] UPDATED)

2018-08-27T00:30:18+00:00August 27th, 2018|Categories: Publications|Tags: , |

The discovery of photonic hyperbolic dispersion surfaces in certain van der Waals bonded solids, such as hexagonal boron nitride and bismuth selenide (a topological insulator), offers intriguing possibilities for creating strongly modified light-matter interactions. However, open problems exist in quantifying electromagnetic field fluctuations in these media, complicating typical approaches for modeling photonic characteristics. Here, we address this issue by linking the identifying traits of hyperbolic response to a coupling between longitudinal and transverse fields that can not occur in isotropic media. This description allows us to calculate the influence of hyperbolic response on electromagnetic fluctuations without explicitly imposing a characteristic size (model of nonlocality), leading to formally bounded expressions so long as material absorption is included. We then apply this framework to two exemplary areas: the optical sum rule for modified spontaneous emission enhancement in a general uniaxial medium, and thermal electromagnetic field fluctuations in hexagonal boron nitride and bismuth selenide. We find that while the sum rule is satisfied, it does not constrain the enhancement of light-matter interactions in either case. We also show that both hexagonal boron nitride and bismuth selenide possess broad spectral regions where the magnitude of electromagnetic field fluctuations are over 120 times larger, and over 800 times larger along specific angular directions, than they are in vacuum.

Published : "arXiv Mesoscale and Nanoscale Physics".

Revealing Optical Transitions and Carrier Recombination Dynamics within the Bulk Band Structure of Bi2Se3. (arXiv:1805.09381v2 [cond-mat.mes-hall] UPDATED)

2018-08-21T04:30:19+00:00August 21st, 2018|Categories: Publications|Tags: , |

Bismuth selenide (Bi2Se3) is a prototypical three-dimensional topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements in a thick exfoliated nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3-1.2 eV). Theoretical modeling of the reflectivity spectral lineshapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding of how such states interact dynamically with the topologically protected Dirac surface states.

Published : "arXiv Mesoscale and Nanoscale Physics".

Switchable quantized conductance in topological insulators revealed by the Shockley-Ramo theorem. (arXiv:1807.11851v1 [cond-mat.mes-hall])

2018-08-01T04:30:29+00:00August 1st, 2018|Categories: Publications|Tags: |

Crystals with symmetry-protected topological order, such as topological insulators, promise coherent spin and charge transport phenomena even in the presence of disorder at room temperature. Still, a major obstacle towards an application of topological surface states in integrated circuits is a clear, reliable, and straightforward read-out independent of a prevailing charge carrier density in the bulk. Here, we demonstrate how to image and read-out the local conductance of helical surface modes in the prototypical topological insulators Bi2Se3 and BiSbTe3. We apply the so-called Shockley-Ramo theorem to design an optoelectronic probe circuit for the gapless surface states, and surprisingly find a precise conductance quantization at 1e2/h. The unprecedented response is a clear signature of local spin-polarized transport, and it can be switched on and off via an electrostatic field effect. The macroscopic, global read-out scheme is based on the displacement current resistivity, and it does not require coherent transport between electrodes.6 It provides a generalizable platform for studying further non-trivial gapless systems such as Weyl-semimetals and quantum spin-Hall insulators.

Published : "arXiv Mesoscale and Nanoscale Physics".

Numerical analysis of surface and edge states in slabs, stripes, rods and surface steps of topological insulators. (arXiv:1807.09068v1 [cond-mat.mes-hall])

2018-07-25T04:30:19+00:00July 25th, 2018|Categories: Publications|Tags: , |

By numerically solving the effective continuous model of a topological insulator with parameters corresponding to the band structure of the topological insulator Bi2Se3 , we analyze possible appearance of one-dimensional states in various geometries. Massless Dirac fermions are found at the edges of thin ribbons with surface oriented not only along the van der Waals gap but also in the perpendicular direction. Thick rods and slabs with surface steps host massive modes localized on surface faces. We argue that the modes are massive and their origin is due to the difference in the Dirac point energy of adjacent faces. The absence of one-dimensional states near edges of a large rectangular rod and surface steps is demonstrated.

Published : "arXiv Mesoscale and Nanoscale Physics".

Suppression of transport spin-polarization of surface states with emergence of ferromagnetism in Mn-doped Bi2Se3. (arXiv:1807.07066v1 [cond-mat.mes-hall])

2018-07-20T00:30:12+00:00July 20th, 2018|Categories: Publications|Tags: |

The surface states of topological insulators (TI) are protected by time reversal symmetry and they display intrinsic spin helicity where the momentum of the charge carriers decides their spin states. As a consequence, a current injected through the surface states becomes spin polarized and this transport spin-polarization leads to a proportionate suppression of Andreev reflection in superconductor/TI junctions. Here we show that upon doping Bi2Se3 with Mn, the transport spin-polarization is seen to be monotonically suppressed. The parent compound Bi2Se3 is found to exhibit a transport spin-polarization of about 63% whereas crystals with 10% Mn doping show transport spin-polarization of about 48%. This suppression is accompanied by an increasing ferromagnetic order of the crystals with Mn doping. Scanning tunneling spectroscopy shows that the topological protection of the surface states reduce due to Mn doping. The net measured transport spin-polarization is due to a competition of this effect with the increased magnetization on Mn doping. The present results provide important insights for the choice of magnetic topological insulators for spintronic applications.

Published : "arXiv Mesoscale and Nanoscale Physics".

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