arXiv subjects

M. S. Bahramy

Publications and source records attributed to M. S. Bahramy.

48 records · Page 3Linked to original sources

arXiv2014

Control of a two-dimensional electron gas on SrTiO3(111) by atomic oxygen

We report on the formation of a two-dimensional electron gas (2DEG) at the bare surface of (111) oriented SrTiO3. Angle resolved photoemission experiments reveal highly itinerant carriers with a 6-fold symmetric Fermi surface and strongly anisotropic effective masses. The electronic structure of the 2DEG is in good agreement with self-consistent tight-binding supercell calculations that incorporate a confinement potential due to surface band bending. We further demonstrate that alternate exposure of the surface to ultraviolet light and atomic oxygen allows tuning of the carrier density and the complete suppression of the 2DEG.

cond-mat.str-el↗

arXiv2014

Mechanism of charge transfer/disproportionation in LnCu3Fe4O12 (Ln: Lanthanides)

The Fe-Cu intersite charge transfer and Fe charge disproportionation are interesting phenomena observed in some LnCu3Fe4O12 (Ln: Lanthanides) compounds containing light and heavy Ln atoms, respectively. We show that a change in the spin state is responsible for the intersite charge transfer in the light Ln compounds. At the high spin state, such systems prefer an unusual Cu-d^8 configuration, whereas at the low spin state they retreat to the normal Cu-d^9 configuration through a charge transfer from Fe to Cu-3d_{xy} orbital. We find that the strength of the crystal field splitting and the relative energy ordering between Cu-3d_{xy} and Fe-3d states are the key parameters, determining the intersite charge transfer (charge disproportionation) in light (heavy) Ln compounds. It is further proposed that the size of Ln affects the onsite interaction strength of Cu-3d states, leading to a strong modification of the Cu-L_3 edge spectrum, as observed by the X-ray absorption spectroscopy.

cond-mat.str-el↗

arXiv2012

Superconductivity in Cu$_{x}$IrTe$_{2}$ induced by interlayer hybridization

The change in the electronic structure of layered Cu$_{x}$IrTe$_{2}$ has been characterized by transport and spectroscopic measurements, combined with first-principles calculations. The Cu-intercalation suppresses the monoclinic distortion, giving rise to the stabilization of the trigonal phase with superconductivity. Thermopower and Hall resistivity measurements suggest the multiband nature with hole and electron carriers for this system, which is masked by the predominance of the hole carriers enhanced by the interlayer hybridization in the trigonal phase. Rather than the instability of Ir $d$ band, a subtle balance between the interlayer and intralayer Te-Te hybridizations is proposed as a main factor dominating the structural transition and the superconductivity.

cond-mat.supr-con↗

arXiv2012

Strongly spin-orbit coupled two-dimensional electron gas emerging near the surface of polar semiconductors

We investigate the two-dimensional (2D) highly spin-polarized electron accumulation layers commonly appearing near the surface of n-type polar semiconductors BiTeX (X = I, Br, and Cl) by angular-resolved photoemission spectroscopy. Due to the polarity and the strong spin-orbit interaction built in the bulk atomic configurations, the quantized conduction-band subbands show giant Rashba-type spin-splitting. The characteristic 2D confinement effect is clearly observed also in the valence-bands down to the binding energy of 4 eV. The X-dependent Rashba spin-orbit coupling is directly estimated from the observed spin-split subbands, which roughly scales with the inverse of the band-gap size in BiTeX.

cond-mat.mtrl-sci↗

arXiv2012

Mechanisms of enhanced orbital dia- and paramagnetism: Application to the Rashba semiconductor BiTeI

We study the magnetic susceptibility of a layered semiconductor BiTeI with giant Rashba spin splitting both theoretically and experimentally to explore its orbital magnetism. Apart from the core contributions, a large temperature-dependent diamagnetic susceptibility is observed when the Fermi energy E_F is near the crossing point of the conduction bands, while the susceptibility turns to be paramagnetic when E_F is away from it. These features are consistent with first-principles calculations, which also predict an enhanced orbital magnetic susceptibility with both positive and negative signs as a function of E_F due to band (anti)crossings. Based on these observations, we propose two mechanisms for an enhanced paramagnetic orbital susceptibility.

cond-mat.mtrl-sci↗

arXiv2012

Emergent quantum confinement at topological insulator surfaces

Bismuth-chalchogenides are model examples of three-dimensional topological insulators. Their ideal bulk-truncated surface hosts a single spin-helical surface state, which is the simplest possible surface electronic structure allowed by their non-trivial $\mathbb{Z}_2$ topology. They are therefore widely regarded ideal templates to realize the predicted exotic phenomena and applications of this topological surface state. However, real surfaces of such compounds, even if kept in ultra-high vacuum, rapidly develop a much more complex electronic structure whose origin and properties have proved controversial. Here, we demonstrate that a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states. In combination with circular dichroism in angle-resolved photoemission (ARPES) experiments, we further uncover a rich three-dimensional spin texture of this surface electronic system, resulting from the non-trivial topology of the bulk band structure. Moreover, our study reveals how the full surface-bulk connectivity in topological insulators is modified by quantum confinement.

cond-mat.mtrl-sci↗

arXiv2012

Giant magneto-optical response in non-magnetic semiconductor BiTeI driven by bulk Rashba spin splitting

We study the magneto-optical (MO) response of polar semiconductor BiTeI with giant bulk Rashba spin splitting at various carrier densities. Despite being non-magnetic, the material is found to yield a huge MO activity in the infrared region under moderate magnetic fields (<3 T). By comparison with first-principles calculations, we show that such an enhanced MO response is mainly due to the intraband transitions between the Rashba-split bulk conduction bands in BiTeI, which give rise to distinct novel features and systematic doping dependence of the MO spectra. We further predict an even more pronounced enhancement in the low-energy MO response and dc Hall effect near the crossing (Dirac) point of the conduction bands.

cond-mat.str-el↗

arXiv2012

Three-dimensional bulk band dispersion in polar BiTeI with giant Rashba-type spin splitting

In layered polar semiconductor BiTeI, giant Rashba-type spin-split band dispersions show up due to the crystal structure asymmetry and the strong spin-orbit interaction. Here we investigate the 3-dimensional (3D) bulk band structures of BiTeI using the bulk-sensitive $hν$-dependent soft x-ray angle resolved photoemission spectroscopy (SX-ARPES). The obtained band structure is shown to be well reproducible by the first-principles calculations, with huge spin splittings of ${\sim}300$ meV at the conduction-band-minimum and valence-band-maximum located in the $k_z=π/c$ plane. It provides the first direct experimental evidence of the 3D Rashba-type spin splitting in a bulk compound.

cond-mat.mtrl-sci↗

arXiv2012

Emergence of non-centrosymmetric topological insulating phase in BiTeI under pressure

The spin-orbit interaction affects the electronic structure of solids in various ways. Topological insulators are one example where the spin-orbit interaction leads the bulk bands to have a non-trivial topology, observable as gapless surface or edge states. Another example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. It is of particular importance to know how these two effects, i.e. the non-trivial topology of electronic states and Rashba spin splitting, interplay with each other. Here we show, through sophisticated first-principles calculations, that BiTeI, a giant bulk Rashba semiconductor, turns into a topological insulator under a reasonable pressure. This material is shown to exhibit several unique features such as, a highly pressure-tunable giant Rashba spin splitting, an unusual pressure-induced quantum phase transition, and more importantly the formation of strikingly different Dirac surface states at opposite sides of the material.

cond-mat.mtrl-sci↗

arXiv2011

Subband structure of a two-dimensional electron gas formed at the polar surface of the strong spin-orbit perovskite KTaO$_3$

We demonstrate the formation of a two-dimensional electron gas (2DEG) at the $(100)$ surface of the $5d$ transition-metal oxide KTaO$_3$. From angle-resolved photoemission, we find that quantum confinement lifts the orbital degeneracy of the bulk band structure and leads to a 2DEG composed of ladders of subband states of both light and heavy carriers. Despite the strong spin-orbit coupling, our measurements provide a direct upper bound for potential Rashba spin splitting of only $Δ{k}_\parallel\sim\0.02$ Å$^{-1}$ at the Fermi level. The polar nature of the KTaO$_3(100)$ surface appears to help mediate formation of the 2DEG as compared to non-polar SrTiO$_3(100)$.

cond-mat.str-el↗

arXiv2011

Origin of giant bulk Rashba splitting: Application to BiTeI

We theoretically propose the necessary conditions for realization of giant Rashba splitting in bulk systems. In addition to (i) the large atomic spin-orbit interaction in an inversion-asymmetric system, the following two conditions are further required; (ii) a narrow band gap, and (iii) the presence of top valence and bottom conduction bands of symmetrically the same character. As a representative example, using the first principles calculations, the recently discovered giant bulk Rashba splitting system BiTeI is shown to fully fulfill all these three conditions. Of particular importance, by predicting the correct crystal structure of BiTeI, different from what has been believed thus far, the third criterion is demonstrated to be met by a negative crystal field splitting of the top valence bands.

cond-mat.mtrl-sci↗

arXiv2006

Calculations of spin induced transport in ferromagnets

Based on first-principles density functional calculations, a general approach for determining and analyzing the degree of spin polarization (P) in ferromagnets is presented. The approach employs the so-called tetrahedron method to evaluate the Fermi surface integrations of P in both ballistic and diffusive regimes. The validity of the method is examined by comparing the calculated P values for Fe and Ni with the experiment. The method is shown to yield highly accurate results with minimal computational effort. Within our approach, it is also possible to systematically analyze the contributions of various types of electronic states to the spin induced transport. As a case study, the transport properties of the soft-ferromagnet CeMnNi4 are investigated in order to explain the origin of the existing difference between the experimental and theoretical values of P in this intermetallic compound.

cond-mat.mtrl-sci↗