arXiv subjects

Junji Tominaga

Publications and source records attributed to Junji Tominaga.

15 recordsLinked to original sources

arXiv2019

All-optical detection of periodic structure of chalcogenide superlattice using coherent folded acoustic phonons

Chalcogenide superlattices (SL) consist of alternate stacking of GeTe and Sb$_{2}$Te$_{3}$ layers. The structure can become a 3D topological insulator depending on the constituent layer thicknesses, making the design of the SL period a central issue for advancing chalcogenide SL as potential candidates for spin devices as well as for optimization of the current generation of phase-change memory devices. Here we explore the periodic structure of chalcogenide SL by observing coherent folded longitudinal acoustic (FLA) phonons excited by femtosecond laser pulse irradiation. The peak frequency of the FLA modes was observed to change upon variation of the thickness of the GeTe layer, which was well reproduced by means of an elastic continuum model. In addition, a new SL structure is unveiled for a sample consisting of thin GeTe and Sb$_{2}$Te$_{3}$ layers, which suggests intermixing of Ge atoms. This all-optical technique based on observation of coherent FLA modes offers a non-destructive characterization of superlattice structures at atomic resolution.

cond-mat.mtrl-sci↗

arXiv2019

A topological phase buried in a chalcogenide superlattice monitored by a helicity dependent Kerr measurement

Chalcogenide superlattices (SL), formed by the alternate stacking of GeTe and Sb$_{2}$Te$_{3}$ layers, also referred to as interfacial phase change memory (iPCM), are a leading candidate for spin based memory device applications. Theoretically, the iPCM structure it has been predicted to form a 3D topological insulator or Dirac semimetal depending on the constituent layer thicknesses. Here, we experimentally investigate the topological insulating nature of chalcogenide SLs using a helicity dependent time-resolved Kerr measurement. The helicity dependent Kerr signal is observed to exhibit a four cycle oscillation with $π$/2 periodicity suggesting the existence of a Dirac-like cone in some chalcogenide SLs. Furthermore, we found that increasing the thickness of the GeTe layer dramatically changes the periodicity, indicating a phase transition from a Dirac semimetal into a trivial insulator. Our results demonstrate that thickness-tuned chalcogenide SLs can play an important role in the manipulation of topological states, which may open up new possibilities for spintronic devices based on chalcogenide SLs.

cond-mat.mes-hall↗

arXiv2019

Photon Energy Dependence of Kerr Rotation in Chalcogenide Superlattices

We report on pump-probe based helicity dependent time-resolved Kerr measurements of chalcogenide superlattices, consisting of alternately stacked GeTe and Sb$_{2}$Te$_{3}$ layers under infrared excitation. The Kerr rotation signal consists of the specular Inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE), both of which are found to monotonically increase with decreasing photon energy over a sub-eV energy range. Although the dependence of the SIFE can be attributed to a response function of direct third-order nonlinear susceptibility, the magnitude of the SOKE reflects cascading second-order nonlinear susceptibility resulting from electronic transitions from bulk valence band to interface-originating Dirac states of the superlattice.

cond-mat.mtrl-sci↗

arXiv2018

Zener Tunneling Breakdown in Phase-Change Materials Revealed by Intense Terahertz Pulses

We have systematically investigated the spatial and temporal dynamics of crystallization that occurs in the phase-change material Ge2Sb2Te5 upon irradiation with an intense THz pulse. THz pump and optical probe spectroscopy revealed that Zener tunneling induces a nonlinear increase in the conductivity of the crystalline phase. This result indicates that the large electric field associated with the THz pulses causes a nonlinear increase only at the edge of the crystallized area. The electric field concentrating in this area causes a temperature increase via Joule heating, which in turn leads to nanometer scale crystal growth parallel to the field and the formation of filamentary conductive domains across the sample.

cond-mat.mtrl-sci↗

arXiv2018

Coherent Dirac plasmons in topological insulators

We explore the ultrafast reflectivity response from photo-generated coupled phonon-surface Dirac plasmons in Sb$_2$Te$_3$ topological insulators several quintuple layers thick. The transient coherent phonon spectra obtained at different time frames exhibit a Fano-like asymmetric line shape of the $A^2_{1g}$ mode, which is attributed to quantum interference between continuum-like coherent Dirac-plasmons and phonons. By analyzing the time-dependent asymmetric line shape using the two-temperature model (TTM), it was determined that a Fano-like resonance persisted up to $\approx$1 ps after photo-excitation with a relaxation profile dominated by Gaussian decay at $\leq$200 fs. The asymmetry parameter could be well described by the TTM for $\geq$200 fs, therefore suggesting the coherence time of the Dirac plasmon is $\approx$200 fs.

cond-mat.str-el↗

arXiv2018

A cascading nonlinear magneto-optical effect in topological insulators

Topological insulators (TIs) are characterized by possessing metallic (gapless) surface states and a finite band-gap state in the bulk. As the thickness of a TI layer decreases down to a few nanometer, hybridization between the top and bottom surfaces takes place due to quantum tunneling, consequently at a critical thickness a crossover from a 3D-TI to a 2D insulator occurs. Although such a crossover is generally accessible by scanning tunneling microscopy, or by angle-resolved photoemission spectroscopy, such measurements require clean surfaces. Here, we demonstrate that a cascading nonlinear magneto-optical effect induced via strong spin-orbit coupling can examine such crossovers. The helicity dependence of the time-resolved Kerr rotation exhibits a robust change in periodicity at a critical thickness, from which it is possible to predict the formation of a Dirac cone in a film several quintuple layers thick. This method enables prediction of a Dirac cone using a fundamental nonlinear optical effect that can be applied to a wide range of TIs and related 2D materials.

cond-mat.mtrl-sci↗

arXiv2017

Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials

Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Femtosecond laser excitation and an ultrashort x-ray probe is used to show the temporal separation of electronic and thermal effects in a long-lived ($>$100 ps) transient metastable state of Ge$_{2}$Sb$_{2}$Te$_{5}$ with muted interatomic interaction induced by a weakening of resonant bonding. Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect allows the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit, and improved energy efficiency and reliability of phase-change memory technologies.

cond-mat.mtrl-sci↗

arXiv2015

Coherent gigahertz phonons in Ge$_{2}$Sb$_{2}$Te$_{5}$ phase-change materials

Using $\approx$40 fs ultrashort laser pulses, we investigate the picosecond acoustic response from a prototypical phase change material, thin Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST) films with various thicknesses. After excitation with a 1.53 eV-energy pulse with a fluence of $\approx$ 5 mJ/cm$^{2}$, the time-resolved reflectivity change exhibits transient electronic response, followed by a combination of exponential-like strain and coherent acoustic phonons in the gigahertz (GHz) frequency range. The time-domain shape of the coherent acoustic pulse is well reproduced by the use of the strain model by Thomsen et al. (Phys. Rev. B 34, 4129, 1986). We found that the decay rate (the inverse of the relaxation time) of the acoustic phonon both in the amorphous and in the crystalline phases decreases as the film thickness increases. The thickness dependence of the acoustic phonon decay is well modeled based on both phonon-defect scattering and acoustic phonon attenuation at the GST/Si interface, and it is revealed that those scattering and attenuation are larger in crystalline GST films than those in amorphous GST films.

cond-mat.mtrl-sci↗

arXiv2015

Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons

Multi-component chalcogenides, such as quasi-binary GeTe-Sb$_{2}$Te$_{3}$ alloys, are widely used in optical data storage media in the form of rewritable optical discs. Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST) in particular has proven to be one of the best-performing materials, whose reliability allows more than 10$^{6}$ write-erase cycles. Despite these industrial applications, the fundamental kinetics of rapid phase-change in GST remain controversial and active debate continues over the ultimate speed limit. Here we explore ultrafast structural transformation in a photo-excited GST superlattice, where GeTe and Sb$_{2}$Te$_{3}$ are spatially separated, using coherent phonon spectroscopy with pump-pump-probe sequences. By analysing the coherent phonon spectra in different time regions, complex structural dynamics upon excitation are observed in GST superlattice (but not in GST alloys), which can be described as the mixing of Ge sites from two different coordination environments. Our results suggest possible applicability of GST superlattice for ultrafast switching devices.

cond-mat.mtrl-sci↗

arXiv2015

Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb$_{2}$Te$_{3}$ superlattices

Femtosecond dynamics of coherent optical phonons in GeTe/Sb$_{2}$Te$_{3}$ superlattices (SLs), a new class of semiconductor SLs with three different states, have been investigated by using a reflection-type pump-probe technique at various lattice temperatures. The time-resolved transient reflectivity (TR) obtained in as-grown SLs exhibits the coherent A$_{1}$ optical modes at 5.10 THz and 3.78 THz, while only the single A$_{1}$ mode at 3.68 THz is observed in annealed SLs. The decay rate of the A$_{1}$ mode in annealed SLs is strongly temperature dependent, while that in as-grown SLs is not temperature dependent. This result indicates that the damping of the coherent A$_{1}$ phonons in amorphous SLs is governed by the phonon-defect (vacancy) scattering rather than the anharmonic phonon-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2014

Coherent phonon study of (GeTe)$_{l}$(Sb$_{2}$Te$_{3}$)$_{m}$ interfacial phase change memory materials

The time-resolved reflectivity measurements were carried out on the interfacial phase change memory (iPCM) materials ([(GeTe)$_{2}$(Sb$_{2}$Te$_{3}$)$_{4}$]$_{8}$ and [(GeTe)$_{2}$(Sb$_{2}$Te$_{3}$)$_{1}$]$_{20}$) as well as conventional Ge$_{2}$Sb$_{2}$Te$_{5}$ alloy at room temperature and above the RESET-SET phase transition temperature. In the high-temperature phase, coherent phonons were clearly observed in the iPCM samples while drastic attenuation of coherent phonons was induced in the alloy. This difference strongly suggests the atomic rearrangement during the phase transition in iPCMs is much smaller than that in the alloy. These results are consistent with the unique phase transition model in which a quasi-one-dimensional displacement of Ge atoms occurs for iPCMs and a conventional amorphous-crystalline phase transition takes place for the alloy.

cond-mat.mtrl-sci↗

arXiv2014

Picosecond strain dynamics in Ge$_{2}$Sb$_{2}$Te$_{5}$ monitored by time-resolved x-ray diffraction

Coherent phonons (CP) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, non-thermal switching in phase-change materials such as Ge$_{2}$Sb$_{2}$Te$_{5}$(GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100 ps x-ray pulses from the ESRF storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal, however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone implying the presence of transient non-equilibrium electron heating in addition to temperature driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase-transitions in GST.

cond-mat.mtrl-sci↗

arXiv2012

Polarization Dependent Optical Control of Atomic Arrangement in Multilayer Ge-Sb-Te Phase Change Materials

We report the optical perturbation of atomic arrangement in the layered GeTe/Sb_{2}Te_{3} phase change memory material. To observe the structural change, the coherent A_{1} mode of GeTe_{4} local structure is investigated at various polarization angles of femtosecond pump pulses with the fluence at < 78 μJ/cm^{2}. p-polarization found to be more effective in inducing the A_{1} frequency shift that can be either reversible or irreversible, depending on the pump fluence. The predominant origin of this shift is attributed to rearrangement of Ge atoms driven by anisotropic dissociation of the Ge-Te bonds along the [111] axis after the p-polarized pulse irradiation.

cond-mat.mtrl-sci↗

arXiv2011

Thermal conductivity of GeTe/Sb$_{2}$Te$_{3}$ superlattices measured by coherent phonon spectroscopy

We report on evaluation of lattice thermal conductivity of GeTe/Sb$_{2}$Te$_{3}$ superlattice (SL) by using femtosecond coherent phonon spectroscopy at various lattice temperatures. The time-resolved transient reflectivity obtained in amorphous and crystalline GeTe/Sb$_{2}$Te$_{3}$ SL films exhibits the coherent $A_{1}$ optical modes at terahertz (THz) frequencies with picoseconds dephasing time. Based on the Debye theory, we calculate the lattice thermal conductivity, including scattering by grain boundary and point defect, umklapp process, and phonon resonant scattering. The results indicate that the thermal conductivity in amorphous SL is less temperature dependent, being attributed to dominant phonon-defect scattering.

cond-mat.mtrl-sci↗

arXiv2011

Ultrafast optical manipulation of atomic arrangements in chalcogenide alloy memory materials

A class of chalcogenide alloy materials that shows significant changes in optical properties upon an amorphous-to-crystalline phase transition has lead to development of large data capacities in modern optical data storage. Among chalcogenide phase-change materials, Ge2Sb2Te5 (GST) is most widely used because of its reliability. We use a pair of femtosecond light pulses to demonstrate the ultrafast optical manipulation of atomic arrangements from tetrahedral (amorphous) to octahedral (crystalline) Ge-coordination in GST superlattices. Depending on the parameters of the second pump-pulse, ultrafast nonthermal phase-change occurred within only few-cycles (~ 1 ps) of the coherent motion corresponding to a GeTe4 local vibration. Using the ultrafast switch in chalcogenide alloy memory could lead to a major paradigm shift in memory devices beyond the current generation of silicon-based flash-memory.

cond-mat.mtrl-sci↗