Emergent photovoltage on SmB6 surface upon bulk-gap evolution revealed by pump-and-probe photoemission spectroscopy

Ishida, Y.; Otsu, Toshio; Shimada, Toshihiro; Okawa, Mario; Kobayashi, Yasunori; Iga, Fumitoshi; Takabatake, Toshiro; Shin, Shik

doi:10.1038/srep08160

Cited by 34 publications

(26 citation statements)

References 52 publications

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“…SPV is observed when a charge gap exists in bulk and a surface band bending develops in the surface region, a case often realized on semiconductor surfaces [27,28]. SPV is also reported in recent TrPES studies on a Kondo insulator SmB 6 [29] and a bulk insulating topological insulator Bi 2 Te 2 Se [30]. The carrier dynamics at T < T * (Figs.…”

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confidence: 80%

Hybridization gap formation in the Kondo insulatorYbB12observed using time-resolved photoemission spectroscopy

et al. 2015

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A detailed low-energy electronic structure of a Kondo insulator YbB12 was revealed by a synergetic combination of ultrahigh-resolution laser photoemission spectroscopy (PES) and time-resolved PES. The former confirmed a 25-meV pseudogap corresponding to the Kondo temperature of this material, and more importantly, it revealed that a 15-meV gap and a Kondo-peak feature developed below a crossover temperature T * ∼ 110 K. In harmony with this, the latter discovered a very long recombination time exceeding 100 ps below ∼T * . This is a clear manifestation of photoexcited carriers due to the bottleneck in the recovery dynamics, which is interpreted as a developing hybridization gap of a hard gap. [4,5]. According to infrared spectroscopy studies [6], the Drude response is sharply decreased below ∼80 K and an indirect gap of ∼15 meV opens at 8 K. Inelastic neutron scattering studies reported the spin gap of 15 meV in the insulating state [7,8]. This crossover is frequently explained by a narrow-gap formation due to hybridization between the conduction band electrons and the localized f electrons (c-f hybridization) within the scheme of the periodic Anderson model. However, there is increasing evidence that some metallic states persist within the gap of many Kondo insulators, making elusive whether the low-temperature (T ) gap is a real charge gap or a pseudogap. For example, the resistivity is still as low as ∼1 Ω cm at low T in high-quality single crystals of some Kondo insulators, such as YbB 12 [3] and SmB 6 [9]. Photoemission spectroscopy (PES) studies [10][11][12][13][14] reported a pseudogap feature (<20 meV) that can be interpreted as a hybridization gap, but the finite spectral weight still remained around the Fermi level E F . In addition, tunneling spectra in YbB 12 exhibit that a finite state persists at the zero-bias voltage [15,16].In this Rapid Communication, to settle the above controversy, we investigate the electronic states within the * Present address: Department of Physics, University of Tokyo, Kashiwa, Chiba 277-8561, Japan pseudogap as well as its evolution on cooling by an unprecedented energy resolution set to 1 meV using ultrahigh-resolution laser PES [17]. Furthermore, we employed time-resolved PES (TrPES) with a pump-probe method and investigate the nonequilibrium electron dynamics of YbB 12 at various temperatures. The recovery time from photoexcited nonequilibrium states is the measure of a charge gap: If there is a gap as in insulators, semiconductors, or nodeless superconductors, the electronic recovery time will become exceedingly long in comparison with a typical metal [18]. Such behavior was reported in past TrPES studies on typical semiconducting materials, such as GaAs and Si [19]. Since TrPES provides direct information of the electron-hole recombination, it is particularly appropriate to investigate such dynamics.Single crystals of YbB 12 were grown by the floating zone method [3]. In both ultra-high-resolution laser PES and TrPES measurements, the samples were fractured in situ...

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Hybridization gap formation in the Kondo insulatorYbB12observed using time-resolved photoemission spectroscopy

et al. 2015

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“…Dirac materials are also usable as wide-band mode lockers that create ultrashort laser pulses of any color, owing to an ability to absorb light over a broad range of wavelengths [10][11][12]; Broad-band lasing may also be realized [12][13][14][15]. Investigations of the optical responses of Dirac fermions have opened pathways to take control of their charge [16][17][18], spin [19][20][21] and topological properties [22,23] by light.…”

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“…When founding the functions of optically nonequilibrated Dirac materials [10][11][12][13][14][15][16][17][18][19][20][21][22][23], it becomes important to understand how the energy of light is transferred to matter and converted thereafter. In fact, there is still no counterpart for the well-established 2-, 3-, or 4-level rate equations that nicely describes the population inversions in conventional lasers.…”

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Revealing the ultrafast light-to-matter energy conversion before heat diffusion in a layered Dirac semimetal

et al. 2016

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There is still no general consensus on how one can describe the out-of-equilibrium phenomena in matter induced by an ultrashort light pulse. We investigate the pulse-induced dynamics in a layered Dirac semimetal SrMnBi2 by pump-and-probe photoemission spectroscopy. At 1 ps, the electronic recovery slowed upon increasing the pump power. Such a bottleneck-type slowing is expected in a two-temperature model (TTM) scheme, although opposite trends have been observed to date in graphite and in cuprates. Subsequently, an unconventional power-law cooling took place at ∼100 ps, indicating that spatial heat diffusion is still ill defined at ∼100 ps. We identify that the successive dynamics before the emergence of heat diffusion is a canonical realization of a TTM scheme. Criteria for the applicability of the scheme is also provided.

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“…Nowadays, the investigations are mostly done at the pump fluence p of >10 µJ/cm 2 based on Ti:sapphire pulsed lasers 28 operating at 10 0 -10 4 kHz repetition. These include: ultrafast perturbation/melt of charge-density waves accompanying coherent oscillations [29][30][31][32][33][34] ; ultrafast modification 35 and phase transition 36,37 of correlated insulators; investigations into graphene 38,39 , graphitic materials [40][41][42][43][44] , semiconductors 45 , and cuprate superconductors [46][47][48][49] ; disclosure of novel states 50,51 , dynamics [52][53][54] , and functions 55,56 in topological insulators; and others 35 . One of the natural pathways to understand the farfrom-equilibrium phenomena is to approach them from the near-equilibrium, or mildly non-equilibrated states induced by a weak pump pulse.…”

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High repetition pump-and-probe photoemission spectroscopy based on a compact fiber laser system

Ishida

Otsu

Ozawa

et al. 2016

Review of Scientific Instruments

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The paper describes a time-resolved photoemission (TRPES) apparatus equipped with a Yb-doped fiber laser system delivering 1.2-eV pump and 5.9-eV probe pulses at the repetition rate of 95 MHz. Time and energy resolutions are 11.3 meV and ∼310 fs, respectively; the latter is estimated by performing TRPES on a highly oriented pyrolytic graphite (HOPG). The high repetition rate is suited for achieving high signal-to-noise ratio in TRPES spectra, thereby facilitating investigations of ultrafast electronic dynamics in the low pump fluence (p) region. TRPES of polycrystalline bismuth (Bi) at p as low as 30 nJ/mm 2 is demonstrated. The laser source is compact and is docked to an existing TRPES apparatus based on a 250-kHz Ti:sapphire laser system. The 95-MHz system is less prone to space-charge broadening effects compared to the 250-kHz system, which we explicitly show in a systematic probe-power dependency of the Fermi cutoff of polycrystalline gold. We also describe that the TRPES response of an oriented Bi(111)/HOPG sample is useful for fine-tuning the spatial overlap of the pump and probe beams even when p is as low as 30 nJ/mm 2 .

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Emergent photovoltage on SmB6 surface upon bulk-gap evolution revealed by pump-and-probe photoemission spectroscopy

Cited by 34 publications

References 52 publications

Hybridization gap formation in the Kondo insulatorYbB12observed using time-resolved photoemission spectroscopy

Hybridization gap formation in the Kondo insulatorYbB12observed using time-resolved photoemission spectroscopy

Revealing the ultrafast light-to-matter energy conversion before heat diffusion in a layered Dirac semimetal

High repetition pump-and-probe photoemission spectroscopy based on a compact fiber laser system

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