Ferroelectric Rashba semiconductors as a novel class of multifunctional materials

Picozzi, Silvia

doi:10.3389/fphy.2014.00010

Cited by 175 publications

(179 citation statements)

References 30 publications

(47 reference statements)

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“…The well-resolved Rashba-split bulk bands allow us to derive the momentum splitting k R ≈ 0.13Å −1 , an energy splitting E R ≈ 190 meV, and Rashba parameter α R along the Z-A-direction to be around 4.2 eVÅ. Such a giant Rashba splitting even surpasses systems such as Bi/Ag(111) [28] and BiTeI [7,8] and is in excellent agreement with theoretical predictions [14]. A particularly interesting feature for potential spintronic applications is that the band structure is gapped at E F , despite the fact that GeTe appears to have always p-type metallic transport properties due to Ge vacancies [29].…”

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confidence: 99%

“…Although these materials exhibit a very large spin splitting, they lack an important property concerning functionalization, namely, the possibility to switch or tune the spin texture. This limitation can be overcome in a new class of functional materials displaying Rashba splitting coupled to ferroelectricity, the so-called ferroelectric Rashba semiconductors (FERS) [13,14].Recent photoemission experiments on α-GeTe-the stable rhombohedral room temperature configuration of the GeTe phase change material [15]-indicate that this system is a hallmark candidate for entanglement of the ferroelectric and spin-orbit order [16,17]. Due to the giant Rashba splitting spin injection from magnetic systems into GeTe appears viable in order to achieve spin-to-charge conversion [18].…”

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Disentangling bulk and surface Rashba effects in ferroelectricα-GeTe

Volfová²,

et al. 2016

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Macroscopic ferroelectric order in α-GeTe with its noncentrosymmetric lattice structure leads to a giant Rashba spin splitting in the bulk bands due to strong spin-orbit interaction. Direct measurements of the bulk band structure using soft x-ray angle-resolved photoemission (ARPES) reveals the three-dimensional electronic structure with spindle torus shape. By combining high-resolution and spin-resolved ARPES as well as photoemission calculations, the bulk electronic structure is disentangled from the two-dimensional surface electronic structure by means of surface capping, which quenches the complex surface electronic structure. This unravels the bulk Rashba-split states in the ferroelectric Rashba α-GeTe(111) semiconductor exhibiting a giant spin splitting with Rashba parameter α R around 4.2 eVÅ, the highest of so-far known materials. DOI: 10.1103/PhysRevB.94.205111 In spintronics an important goal is to be able to control the spin of the electron in solids without applying magnetic fields [1,2]. The most promising mechanism is based on the Rashba effect [3] and the subsequent spin precession induced in such systems [4]. While most research has previously focused on 2D electron systems [5,6], recently a threedimensional (3D) form of such Rashba effect was found in a series of bismuth tellurohalides BiTeX (X = I, Br, or Cl) [7][8][9][10][11][12]. Although these materials exhibit a very large spin splitting, they lack an important property concerning functionalization, namely, the possibility to switch or tune the spin texture. This limitation can be overcome in a new class of functional materials displaying Rashba splitting coupled to ferroelectricity, the so-called ferroelectric Rashba semiconductors (FERS) [13,14].Recent photoemission experiments on α-GeTe-the stable rhombohedral room temperature configuration of the GeTe phase change material [15]-indicate that this system is a hallmark candidate for entanglement of the ferroelectric and spin-orbit order [16,17]. Due to the giant Rashba splitting spin injection from magnetic systems into GeTe appears viable in order to achieve spin-to-charge conversion [18]. Therefore, ferroelectric [13] or multiferroic [19] Rashba semiconductors bring new multifunctional assets for spintronic devices. A crucial issue for the understanding of FERS is to disentangle the Rashba effect in the bulk caused by the bulk ferroelectric lattice distortion and surface effects arising from particular surface terminations and/or possible band bendings. This represents a major challenge for surface sensitive techniques such as angle-resolved photoemission (ARPES). For this reason up to now ARPES measurements on α-GeTe surfaces performed in the surface-sensitive UV regime have been dominated by surface effects [16,17] and clear information of the three-dimensional bulk electronic structure and its spin texture has not been obtained.

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Disentangling bulk and surface Rashba effects in ferroelectricα-GeTe

Volfová²,

et al. 2016

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“…Firstly, polar distortions and ferroelectric responses in the halide perovskite have been clearly observed in experiments (9,10). On the other hand, GeTe inevitably generates Ge vacancies to give considerable bulk conductivity, which in turn hinders the polarization switching (11). Secondly, unlike the multiple band edges and strong hexagonal warping in GeTe (7,8), the band edge states of the halide perovskites lie at a single point in the Brillouin zone with nearly isotropic Rashba bands.…”

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Switchable S = 1/2 and J = 1/2 Rashba bands in ferroelectric halide perovskites

Kim

Im²,

Freeman

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

276

306

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The Rashba effect is spin degeneracy lift originated from spinorbit coupling under inversion symmetry breaking and has been intensively studied for spintronics applications. However, easily implementable methods and corresponding materials for directional controls of Rashba splitting are still lacking. Here, we propose organic-inorganic hybrid metal halide perovskites as 3D Rashba systems driven by bulk ferroelectricity. In these materials, it is shown that the helical direction of the angular momentum texture in the Rashba band can be controlled by external electric fields via ferroelectric switching. Our tight-binding analysis and first-principles calculations indicate that S = 1=2 and J = 1=2 Rashba bands directly coupled to ferroelectric polarization emerge at the valence and conduction band edges, respectively. The coexistence of two contrasting Rashba bands having different compositions of the spin and orbital angular momentum is a distinctive feature of these materials. With recent experimental evidence for the ferroelectric response, the halide perovskites will be, to our knowledge, the first practical realization of the ferroelectric-coupled Rashba effect, suggesting novel applications to spintronic devices.electronic structure | density functional theory | effective Hamiltonian T he Rashba effect has been widely investigated in 2D surfaces, interfaces, quantum wells, and 3D bulk systems (1-6). The essential requisite for the Rashba effect is that the spin degeneracy is lifted by the inversion symmetry-breaking (ISB) field in the presence of the spin-orbit coupling (SOC). To date, major concerns have focused on enlarging Rashba strength characterized by the Rashba coefficient α R (3, 5, 6). The controllability in the direction of the ISB field, on the other hand, has not been seriously considered. In a surface or an interface, the potential gradient generated by the structural inversion asymmetry results in the loss of controllability; the field direction is mainly fixed according to the preformed surface or interface configuration. The situation is similar in recently discovered 3D Rashba material BiTeI (6), because this material has the compositional ISB field between Te and I layers.Controlling the ISB field and ultimately Rashba-type band splitting can be achieved by using a novel ferroelectric Rashba material (7,8). In a ferroelectric system, the bulk polarization controlled by external electric fields generates the ISB field. Therefore, the ferroelectric polarization directly couples to the spin splitting and the helical spin texture in the ferroelectric Rashba material, enabling the helicity reversal via the ferroelectric switching. Recent theoretical study suggested GeTe as a possible candidate for this mechanism, but the direct measurement of the ferroelectric polarization and switching is still missing due to the sizable conductivity of bulk GeTe (7,8).We consider organic-inorganic hybrid metal halide perovskites as promising ferroelectric Rashba materials. The general formula for this materia...

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“…It lacks inversion symmetry on either side of the topological phase transition. The strong spinorbit interaction, together with the breaking of inversion symmetry, result in a Rashba-type splitting [39][40][41] of the conduction and valence bands in both topological insulator (TI) and normal insulator (NI) phases. At the topological phase transition, the band gap closes between the lowest energy Rashba conduction band and the highest energy Rashba valence band, in the vicinity of the A point in the Brillouin zone ( Fig.…”

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Enhancement of the Bulk Photovoltaic Effect in Topological Insulators

Tan

Rappe

2016

Phys. Rev. Lett.

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We investigate the shift current bulk photovoltaic response of materials close to a band inversion topological phase transition. We find that the bulk photocurrent reverses direction across the band inversion transition, and that its magnitude is enhanced in the vicinity of the phase transition. These results are demonstrated with first principles DFT calculations of BiTeI and CsPbI3 under hydrostatic pressure, and explained with an analytical model, suggesting that this phenomenon remains robust across disparate material systems.The field of topological insulators [1,2] has proven to be a fruitful avenue of research in condensed matter physics in recent years. The symmetry protected surface states of topological insulators [3][4][5] give rise to novel and useful phenomena such as dissipationless transport [6,7]. Even though band topology is a ground state property, defined on the occupied valence bands of an insulator [8], the excited state properties should be affected by topological considerations as well. There is a growing body of research investigating the optical properties of topological insulators[9] using photoemission spectroscopy [3,4], Raman spectroscopy [10][11][12][13], nonlinear optics [14][15][16][17][18][19], and photocurrent generation [20][21][22].While the presence or absence of topological surface states has thus far being the main experimental route for detecting topological phase transitions, in this work we propose an optical, non-surface approach for detecting topological phase transitions in the bulk. This is a direct approach which does not rely on the quality or ease of detection of surface states. We consider the influence of band topology on the bulk photovoltaic effect (BPVE, also known as the photogalvanic effect) [23,24], which is the generation of photocurrents in the bulk of a singlephase material.The bulk photovoltaic effect is a second-order optical effect which gives current densities, J r = σ rst E s E t , as a quadratic function of the applied electric fields. This dictates that the BPVE can only be observed in materials with broken inversion symmetry. Measurements of appreciable BPVE have been reported for many ferroelectric materials [25][26][27][28]. In the prototypical ferroelectric BaTiO 3 , experiments [29,30] and first-principles calculations [31] show that the primary mechanism for BPVE is the shift current [32]. In this mechanism, carriers are excited into a current-carrying superposition of excited states. Due to its dominant effect in the bulk photovoltaic response, we will focus on the shift current in this paper.Because optical excitations probe both the valence and conduction band wavefunctions, we expect the band inversion process, which is the interchange of the conduction and valence band characters, to result in dramatic changes in the finite-frequency response functions of the system. We show that band inversion reverses the direction of the shift current. The magnitude of the shift cur- rent is enhanced in the vicinity of the band inversion transition....

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Ferroelectric Rashba semiconductors as a novel class of multifunctional materials

Cited by 175 publications

References 30 publications

Disentangling bulk and surface Rashba effects in ferroelectricα-GeTe

Disentangling bulk and surface Rashba effects in ferroelectricα-GeTe

Switchable S = 1/2 and J = 1/2 Rashba bands in ferroelectric halide perovskites

Enhancement of the Bulk Photovoltaic Effect in Topological Insulators

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