Rm Bi Research Articles

Investigation of the Gyrotropy in the Spectral Region of the Absorption "Shoulder" of the Oxide Crystal Bi12SiO20

This paper presents the research of the gyrotropy of a pure Bi$$_{12}$$SiO$$_{20}$$ (BSO) and doped Bi$$_{12}$$SiO$$_{20}$$:Al (BSO:Al) crystals in the spectral region of ``absorption shoulder'' using measurements of the spectra of the natural optical rotation, the magneto-optical rotation and the optical spectroscopy. It is shown that the difference in the absorption of the BSO and BSO:Al crystals is related to the presence of two types of defects in the crystals BSO:Al – impurities $$\rm Al_{Si}^{3+}$$ in $$\rm[Al_{Si}^{3+}O_4]$$ and $$\rm Bi_{Si}^{3+}$$ in $$\rm[Bi_{Si}^{3+}O_4]$$. A ``red-shift'' of the optical rotation spectrum of BSO:Al crystals has also been observed relative to the optical rotation spectrum of BSO crystals.

Spin texture in a bilayer high-temperature cuprate superconductor

We investigate the possibility of spin texture in the bilayer cuprate superconductor $\rm Bi_2Sr_2CaCu_2O_{8+\delta}$ using cluster dynamical mean field theory (CDMFT). The one-band Hubbard model with a small interlayer hopping and a Rashba spin-orbit coupling is used to describe the material. The $d$-wave order parameter is not much affected by the presence of the Rashba coupling, but a small triplet component appears. We find a spin texture circulating in the same direction around $\mathbf{k}=(0,0)$ and $\mathbf{k}=(\pi,\pi)$ and stable against the superconducting phase. The amplitude of the spin structure, however, is strongly affected by the pseudogap phenomenon, more so than the spectral function itself.

Gate-controlled spin extraction from topological insulator surfaces

Spin-momentum locking, a key property of the surface states of three-dimensional topological insulators (3DTIs), provides a new avenue for spintronics applications. One consequence of spin-momentum locking is the induction of surface spin accumulations due to applied electric fields. In this work, we investigate the extraction of such electrically-induced spins from their host TI material into adjoining conventional, hence topologically trivial, materials that are commonly used in electronics devices. We focus on effective Hamiltonians for bismuth-based 3DTI materials in the ${\rm Bi}_2{\rm Se}_3$ family, and numerically explore the geometries for extracting current-induced spins from a TI surface. In particular, we consider a device geometry in which a side pocket is attached to various faces of a 3DTI quantum wire and show that it is possible to create current-induced spin accumulations in these topologically trivial side pockets. We further study how such spin extraction depends on geometry and material parameters, and find that electron-hole degrees of freedom can be utilized to control the polarization of the extracted spins by an applied gate voltage.

Volatile and permanent optical gratings recorded in Bi2TeO5photorefractive crystal under high cw intensity

We report the recording of optical gratings on photorefractive $ {{\rm Bi}_2}{{\rm TeO}_5} $Bi2TeO5 crystals using $ \lambda = 532\,\,{\rm nm} $λ=532nm wavelength light. We studied the behavior of this material under high light intensity and found the presence of fast and slow gratings, both of photorefractive nature and exhibiting quite significant light intensity dependence for the $ 1 - 13\,\,{\rm kW}/{{\rm m}^2} $1-13kW/m2 range. A permanent grating was found after the complete erasure of fast and slow holograms recorded at room temperature. The experimental results show that the diffraction efficiency of the permanent grating increases with the recorded light intensity. The permanent grating performance as an optical Bragg filter was characterized by measuring the angular selectivity approximately 1.0 mrad. We also show that the diffraction efficiency of the permanent grating is quite dependent on the direction of light polarization.

Ripple State in the Frustrated Honeycomb-Lattice Antiferromagnet.

We discover a new type of multiple-q state, a "ripple state," in a frustrated honeycomb-lattice Heisenberg antiferromagnet under magnetic fields. The ground state has an infinite ringlike degeneracy in the wave vector space, exhibiting a cooperative paramagnetic state, a "ring-liquid" state. We elucidate that the system exhibits the ripple state as a new low-temperature thermodynamic phase via a second-order phase transition from the ring-liquid state, keeping the ringlike spin structure factor. The spin texture in real space looks like a "water ripple" and can induce a giant electric polarization vortex. A possible relationship to the honeycomb-lattice compound, Bi_{3}Mn_{4}O_{12}(NO_{3}), is discussed.

New Nuclear Magnetic Moment of Bi209 : Resolving the Bismuth Hyperfine Puzzle

A recent measurement of the hyperfine splitting in the ground state of Li-like ^{208}Bi^{80+} has established a "hyperfine puzzle"-the experimental result exhibits a 7σ deviation from the theoretical prediction [J. Ullmann etal., Nat. Commun. 8, 15484 (2017)NCAOBW2041-172310.1038/ncomms15484; J. P. Karr, Nat. Phys. 13, 533 (2017)NPAHAX1745-247310.1038/nphys4159]. We provide evidence that the discrepancy is caused by an inaccurate value of the tabulated nuclear magnetic moment (μ_{I}) of ^{209}Bi. We perform relativistic density functional theory and relativistic coupled cluster calculations of the shielding constant that should be used to extract the value of μ_{I}(^{209}Bi) and combine it with nuclear magnetic resonance measurements of Bi(NO_{3})_{3} in nitric acid solutions and of the hexafluoridobismuthate(V) BiF_{6}^{-} ion in acetonitrile. The result clearly reveals that μ_{I}(^{209}Bi) is much smaller than the tabulated value used previously. Applying the new magnetic moment shifts the theoretical prediction into agreement with experiment and resolves the hyperfine puzzle.

Superconducting correlations induced by charge ordering in cuprate superconductors and Fermi-arc formation

We have developed a generalized electronic phase separation model of high-temperature cuprate superconductors that links the two distinct energy scales of the superconducting (SC) and pseudogap (PG) phases via a charge-density-wave (CDW) state. We show that simulated electronic-density modulations resembling the charge order (CO) modulations detected in cuprates intertwine the SC and charge orders by localizing charge and providing the energy scale for a spatially periodic SC attractive potential. Bulk superconductivity is achieved with the inclusion of Josephson coupling between nanoscale domains of intertwined fluctuating CDW and SC orders, and local SC phase fluctuations give rise to the Fermi arcs along the nodal directions of the SC gap. We demonstrate the validity of the model by reproducing the hole-doping dependence of the PG onset temperature $T^*$, and the SC transition temperature $T_c$ of ${\rm YBa_2Cu_3O_y}$ and ${\rm Bi_{2-y}Pb_ySr_{2-z}La_zCuO_{6+\delta}}$. The results show that the periodicity of the CDW order is controlled by the PG energy scale, and the hole-doping dependence of the SC energy gap is controlled by the charge ordering free energy.

Graphene-Bi2Te3 Heterostructure as Broadband Saturable Absorber for Ultra-Short Pulse Generation in Er-Doped and Yb-Doped Fiber Lasers

A new type of broadband saturable absorber made of graphene-Bi2Te3 heterostructure was reported, which synergistically combines light–matter interaction in graphene with that in a small bandgap semiconductor material Bi2Te3 to achieve an improved broadband nonlinear optical response. The graphene-Bi2Te3 heterostructure films were grown by chemical vapor deposition with 15% ${\rm Bi}_{2}{\rm Te}_{3}$ coverage on graphene, in which most of the Bi2Te3 nanoplatelets are less than 30 nm thick. It is interesting to find that the heterostructure thin film shows broadband saturable absorption property. At the communication band (around 1560 nm), the saturable intensity and modulation depth are measured to be 4.95 MW/cm2 and 18.98%, respectively. While around 1067 nm, the corresponding saturable intensity and modulation depth are experimentally measured to be 2.61 MW/cm2 and 23.11%, respectively. By incorporating this optical saturable absorber inside either an Er-doped or Yb-doped fiber laser, we are able to generate ultra-short pulse with very stable operation at 1565.6 and 1049.1 nm. Our experimental results clearly demonstrate that the graphene-Bi 2Te3 heterostructure can be a promising broadband nonlinear optical material for broadband ultra-fast laser photonics.

Electronic and transport properties of the Mn-doped topological insulatorBi2Te3: A first-principles study

We present a first-principles study of the electronic, magnetic, and transport properties of the topological insulator Bi$_{2}$Te$_{3}$ doped with Mn atoms in substitutional (Mn$_{\rm Bi}$) and interstitial van der Waals gap positions (Mn$_{\rm i}$), which act as acceptors and donors, respectively. The effect of native Bi$_{\rm Te}$- and Te$_{\rm Bi}$-antisite defects and their influence on calculated electronic transport properties is also investigated. We have studied four models representing typical cases, namely (i) Bi$_{2}$Te$_{3}$ with and without native defects, (ii) Mn$_{\rm Bi}$ defects with and without native defects, (iii) the same but for Mn$_{\rm i}$ defects, and (iv) the combined presence of Mn$_{\rm Bi}$ and Mn$_{\rm i}$. It was found that lattice relaxations around Mn$_{\rm Bi}$ defects play an important role for both magnetic and transport properties. The resistivity is strongly influenced by the amount of carriers, their type, and by the relative positions of the Mn-impurity energy levels and the Fermi energy. Our results indicate strategies to tune bulk resistivities, and also help to uncover the location of Mn atoms in measured samples. Calculations indicate that at least two of the considered defects have to be present simultaneously in order to explain the experimental observations, and the role of interstitials may be more important than expected.

On Airy Solutions of the Second Painlevé Equation

In this paper we discuss Airy solutions of the second Painlev\'e equation (\mbox{\rm P$_{\rm II}$}) and two related equations, the Painlev\'e XXXIV equation ($\mbox{\rm P}_{34}$) and the Jimbo-Miwa-Okamoto $\sigma$ form of \mbox{\rm P$_{\rm II}$}\ (\mbox{\rm S$_{\rm II}$}), are discussed. It is shown that solutions which depend only on the Airy function $\mathop{\rm Ai}\nolimits(z)$ have a completely difference structure to those which involve a linear combination of the Airy functions $\mathop{\rm Ai}\nolimits(z)$ and $\mathop{\rm Bi}\nolimits(z)$. For all three equations, the special solutions which depend only on $\mathop{\rm Ai}\nolimits(t)$ are \textit{tronqu\'ee} solutions, i.e.\ they have no poles in a sector of the complex plane. Further for both $\mbox{\rm P}_{34}$\ and \mbox{\rm S$_{\rm II}$}, it is shown that amongst these \textit{tronqu\'ee} solutions there is a family of solutions which have no poles on the real axis.

Doping dependence of the critical current inBi2Sr2CaCu2O8+δ

The doping dependence of the critical current density, $J_{c}$, was measured in a series of high quality c-axis oriented ${\rm Bi_2Sr_2CaCu_2O_{8+\delta}}$ thin films. For each doping level we measured the temperature dependence of $J_{c}$. We find that all samples have the same temperature dependence, but the critical current at zero temperature, $J_{c}(0)$, has a non-trivial doping dependence with a maximum at a unique doping level in the overdoped side of the superconducting dome.

Substitution Mechanism of Mn and Fe Ions in <inline-formula> <tex-math notation="TeX">${\rm Bi}_{4}{\rm Ti}_{3}{\rm O}_{12}$ </tex-math></inline-formula>

Single-phased polycrystalline Mn- and Fe-doped ${\rm Bi}_{4}{\rm Ti}_{3}{\rm O}_{12}$ were fabricated using a solid-state reaction technique, doping with various concentrations of Mn and Fe ions. Substitution mechanism of Mn and Fe ions in ${\rm Bi}_{4}{\rm Ti}_{3}{\rm O}_{12}$ were investigated with X-ray absorption near-edge structure (XANES) measurements and first-principles calculations. The valence states of the Mn and Fe ions are 4+ and 3+, respectively, inferred from the ${\rm L}_{2,3}$ -edge XANES profiles. From the K-edge XANES analysis, it is determined that Mn and Fe ions are substituted at one of the Ti sites, i.e., Ti(2a) or Ti(4e) sites. Our first-principles total electronic energy calculations suggest that Mn ions are likely to substitute at Ti(2a) sites rather than at Ti(4e) sites, whereas the opposite is true for Fe substitution. Taken together, these results give a clear description of the locations and charge states of the Mn and Fe dopants in ${\rm Bi}_{4}{\rm Ti}_{3}{\rm O}_{12}$ .

Zero-energy bound states in tunneling conductance spectra at the interface of ans-wave superconductor and a topological insulator in NbN/Bi2Se3/Au thin-film junctions

Measurements of conductance spectra in a superconductor - topological insulator - normal metal thin film junctions of NbN-$\rm Bi_2Se_3$-Au are reported. Junctions with ex-situ and in-situ prepared $\rm NbN-Bi_2Se_3$ interfaces were studied. At low temperatures, all the ex-situ junctions showed coherence peaks in their conductance spectra, but imbedded robust zero bias conductance peaks were observed only in junctions with a metallic or a metal to insulator transition below $\rm T_c$ of the NbN electrode. The in-situ junctions which had about two orders of magnitude lower resistance at low temperatures, generally showed flat conductance spectra at low bias, with no coherence or broad Andreev peaks, since the critical current of the NbN electrode was reached first, at voltage bias below the energy gap of the superconductor. A weak zero bias conductance peak however, was observed in one of these junctions. We conclude that significant tunneling barriers, as in the ex-situ prepared junctions, are essential for the observation of coherence peaks and the zero energy bound states. The later seem to originate in the $\rm Bi_2Se_3$-NbN interface, as they are absent in reference Au-NbN junctions without the topological layer sandwiched in between.

Pairing Mechanism for the High-TC Superconductivity: Symmetries and Thermodynamic Properties

The pairing mechanism for the high- superconductors based on the electron-phonon (EPH) and electron-electron-phonon (EEPH) interactions has been presented. On the fold mean-field level, it has been proven, that the obtained s-wave model supplements the predictions based on the BCS van Hove scenario. In particular: (i) For strong EEPH coupling and the energy gap () is very weak temperature dependent; up to the critical temperature extends into the anomalous normal state to the Nernst temperature. (ii) The model explains well the experimental dependence of the ratio on doping for the reported superconductors in the terms of the few fundamental parameters. In the presented paper, the properties of the d-wave superconducting state in the two-dimensional system have been also studied. The obtained results, like for s-wave, have shown the energy gap amplitude crossover from the BCS to non-BCS behavior, as the value of the EEPH potential increases. However, for the energy gap amplitude extends into the anomalous normal state to the pseudogap temperature. Finally, it has been presented that the anisotropic model explains the dependence of the ratio on doping for the considered superconductors.

Characterization of ${\rm Bi}_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{8+\delta}$ Stacks Fabricated by Acid Treatment Process

Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8+δ</sub> (Bi-2212) stacks of intrinsic Josephson junctions were fabricated by an acid-treatment process from inside a single crystal. The surrounding acid-treated product is a transparent material, BiOCl. During the process, an optical adhesive (NOA-61) was used to fix a piece of the crystal on a glass substrate and to coat the surface of the sample. The current-voltage characteristics of the stacks at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> = 77 K exhibited large hystereses and multiple branches with voltage spacings of approximately 2-3 mV. Under an external microwave field in the frequency range of 2-18 GHz, clear Shapiro steps were observed at voltages with multiples of ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</i> /2 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</i> ) <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rf</sub> . In addition, the thermal cycling properties of the critical current ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) were investigated. No apparent degradation of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> was observed during repeated thermal cycling between 300 and 77 K.

Numerical Investigation of the Quench Behavior of ${\rm Bi}_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{x}$ Wire

The quench behavior of Bi2Sr2CaCu2Ox (Bi2212) wire is investigated through numerical simulations. This work is part of the U.S. Very High Field Superconducting Magnet Collaboration (VHFSMC). Numerical simulations are carried out using a one-dimensional computational model of thermal transport in Bi2212 composite wires. A quench is simulated by introducing heat in a section of the wire, and the voltage and temperature are monitored as function of time and position. The quench energy, normal zone propagation velocity, and spatial distribution of temperature are calculated for varying transport current and applied magnetic field. The relevance of these simulations in defining criteria for experimental measurements is discussed.

Terahertz Hall measurements on optimally doped single-crystalBi2Sr2CaCu2O8+x

The infrared Hall angle in optimally doped single crystal $\rm Bi_2 Sr_2 Ca Cu_2 O_{8+x}$ was measured from 3.05 to 21.75 meV as a continuous function of temperature from 25 to 300\,K. In the normal state, the temperature dependence of the real part of the cotangent of the infrared Hall angle obeys the same power law as dc measurements. The measured Hall frequency $\rm \omega_H$ is significantly larger than the expected value based upon ARPES data analyzed in terms of the relaxation time approximation. This discrepancy as well as the temperature dependence of $\rm Re(\cot{\theta_H})$ and $\omega_H$ is well described by a Fermi liquid theory in which current vertex corrections produced by electron-magnon scattering are included.

Collective Modes of a Helical Liquid

We study low energy collective modes and transport properties of the "helical metal" on the surface of a topological insulator. At low energies, electrical transport and spin dynamics at the surface are exactly related by an operator identity equating the electric current to the in-plane components of the spin degrees of freedom. From this relation it follows that an undamped spin wave always accompanies the sound mode in the helical metal-thus it is possible to "hear" the sound of spins. In the presence of long range Coulomb interactions, the surface plasmon mode is also coupled to the spin wave, giving rise to a hybridized "spin-plasmon" mode. We make quantitative predictions for the spin-plasmon in Bi2Se3, and discuss its detection in a spin-grating experiment.

Origin of the high-energy kink in the photoemission spectrum of the high-temperature superconductorBi2Sr2CaCu2O8

The high-energy kink or the waterfall effect seen in the photoemission spectra of the cuprates is suggestive of the coupling of the quasiparticles to a high energy bosonic mode with implications for the mechanism of superconductivity. Recent experiments however indicate that this effect may be an artifact produced entirely by the matrix element effects, i.e. by the way the photoemitted electron couples to the incident photons in the emission process. In order to address this issue directly, we have carried out realistic computations of the photo-intensity in ${\rm Bi_2Sr_2CaCu_2O_8}$ (Bi2212) where the effects of the matrix element are included together with those of the corrections to the self-energy resulting from electronic excitations. Our results demonstrate that while the photoemission matrix element plays an important role in shaping the spectra, the waterfall effect is a clear signature of the presence of strong coupling of quasiparticles to electronic excitations.

Investigation of the Over Current Characteristics of HTS Tapes Considering the Application for HTS Power Devices

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> High temperature superconductor (HTS) is developed for applications in HTS power devices such as power transmission cable, transformers, superconducting fault current limiter (SFCL), motor, and generators. HTS power devices applied in power systems might undergo over current, such as over load, fault conditions, and unbalanced current. The electrical characteristic analyses of <formula formulatype="inline"><tex>${\rm Bi}_{2}{\rm Ca}_{2}{\rm Sr}_{2}{\rm Cu}_{3}{\rm Ox}$</tex></formula> (Bi-2223) HTS tapes and <formula formulatype="inline"><tex>${\rm YBa}_{2}{\rm Cu}_{3}{\rm Ox}$</tex></formula> (YBCO) tapes are very important for the application of HTS power devices. In this study, the transient phenomenon of HTS tapes was analyzed when the DC over current as well as the AC over current flow. Also, the experimental results of breakdown cycle, variation of temperature and resistance according to fault current in HTS tapes were analyzed. These results would be very significant data and give good information in order to design and fabricate HTS power devices. </para>