High-pressure ultrafast dynamics has been recently developed, enabling the exploration of non-equilibrium properties of various quantum materials under high pressure. Particularly, by investigating the pressure dependence of time-resolved ultrafast dynamics, we have discovered a pressure-induced phonon bottleneck effect (PBE). To date, all reported PBEs are due to fully closed gaps, which was reflected in the simultaneous characteristic changes in both amplitude and lifetime of the phonon-phonon scattering slow relaxation component. However, as reflected through its connection to Euler disk, incompletely closed gaps can also induce PBEs. In this work, we report the first PBE due to a finite shrinking gap. As is known, it is challenging to directly observe high-pressure-induced variations in electronic band gaps due to the diamond anvil cell. Here, by investigating Sr2IrO4 in our previous work, we obtain an empirical formula for the pressure-induced energy gap variation at room temperature. Our quantitative analysis shows that the gap is finite shrinking rather than fully closed.

Controlling collective electronic states hold great promise for development of innovative devices. Here, we experimentally detect the modification of the charge density wave (CDW) phase transition within a 1T-TaS2 layer in a WS2/1T-TaS2 heterostructure using time-resolved ultrafast spectroscopy. Laser-induced charge transfer doping strongly suppresses the commensurate CDW phase, which results in a significant decrease in both the phase transition temperature (T-c) and phase transition stiffness. We interpret the phenomenon that photo-induced hole doping, when surpassing a critical threshold value of similar to 10(18) cm(-3), sharply decreases the phase transition energy barrier. Our results provide new insights into controlling the CDW phase transition, paving the way for optical-controlled novel devices based on CDW materials.

In many physical situations in which many-body assemblies exist at temperature T , a characteristic quantum-mechanical time scale of approximately ℏ / k B T can be identified in both theory and experiment, leading to speculation that it may be the shortest meaningful time in such circumstances. This behavior can be investigated by probing the scattering rate of electrons in a broad class of materials often referred to as “strongly correlated metals”. It is clear that in some cases only electron–electron scattering can be its cause, while in others it arises from high-temperature scattering of electrons from quantized lattice vibrations, i.e., phonons. In metallic oxides, which are among the most studied materials, analysis of electrical transport does not satisfactorily identify the relevant scattering mechanism at “high” temperatures near room temperature. We therefore employ a contactless optical method to measure thermal diffusivity in two Ru-based layered perovskites, Sr 3 Ru 2 O 7 and Sr 2 RuO 4 , and use the measurements to extract the dimensionless Lorenz ratio. By comparing our results to the literature data on both conventional and unconventional metals, we show how the analysis of high-temperature thermal transport can both give important insight into dominant scattering mechanisms and be offered as a stringent test of theories attempting to explain anomalous scattering.

Laser-induced electron coherence is a fascinating topic in manipulating quantum materials. Recently, it has been shown that laser-induced electron coherence in 2D materials can produce a third-order nonlinear optical response spatial self-phase modulation (SSPM), which has been used to develop a novel all-optical switching scheme. However, such investigations have mainly focused on electron coherence, whereas laser-induced hole coherence is rarely explored. Here, the observation of the optical Kerr effect in 3D Weyl semimetal TaAs flakes is reported. The nonlinear susceptibility (χ(3) ) is obtained, which exhibits a surprisingly high value (with χ o n e - l a y e r ( 3 ) \[{\bm{\chi }}_{{\bf one}{\bm{ - }}{\bf layer}}^{{\bf (3)}}\] = 9.9 × 10-9 e.s.u. or 1.4 × 10-16  m2  V-2 at 532 nm). This cannot be explained by the conventional electron mobility, but can be well understood by the unique high anisotropic hole mobility of TaAs. The wind-chime model and χ(3) carrier mobility correlation adequately explain the results, suggesting the crucial role of laser-induced nonlocal ac hole coherence. These observations extend the understanding of SSPM from 2D to 3D quantum materials with anisotropic carrier mobility and from electron coherence to hole coherence.

In many physical situations in which many-body assemblies exist at temperature $T$, a characteristic quantum-mechanical time scale of approximately $\hbar/k_{B}T$ can be identified in both theory and experiment, leading to speculation that it may be the shortest meaningful time in such circumstances. When this behaviour is investigated by probing the scattering rate of strongly interacting electrons in metals, it is clear that in some cases only electron-electron scattering can be its cause, while in others it arises from high-temperature scattering of electrons from quantised lattice vibrations, i.e. phonons. In metallic oxides, which are among the most studied materials, analysis of electrical transport does not satisfactorily identify the relevant scattering mechanism at 'high' temperatures near room temperature. We employ a contactless optical method to measure thermal diffusivity in two Ru-based layered perovskites, Sr$_3$Ru$_2$O$_7$ and Sr$_2$RuO$_4$, and use the measurements to extract the dimensionless Lorenz ratio. By comparing our results to the literature data on both conventional and unconventional metals we show how the analysis of high-temperature thermal transport can both give important insight into dominant scattering mechanisms, and be offered as a stringent test of theories attempting to explain anomalous scattering.

We describe an optical method to directly measure the position-dependent thermal diffusivity of reflective single crystal samples across a broad range of temperatures for condensed matter physics research. Two laser beams are used, one as a source to locally modulate the sample temperature, and the other as a probe of sample reflectivity, which is a function of the modulated temperature. Thermal diffusivity is obtained from the phase delay between source and probe signals. We combine this technique with a microscope setup in an optical cryostat, in which the sample is placed on a three-axis piezo-stage, allowing for spatially resolved measurements. Furthermore, we demonstrate experimentally and mathematically that isotropic in-plane diffusivity can be obtained when overlapping the two laser beams instead of separating them in the traditional way, which further enhances the spatial resolution to a micron scale, especially valuable when studying inhomogeneous or multidomain samples. We discuss in detail the experimental conditions under which this technique is valuable and demonstrate its performance on two stoichiometric bilayer ruthenates: Sr3Ru2O7 and Ca3Ru2O7. The spatial resolution allowed us to study the diffusivity in single domains of the latter, and we uncovered a temperature-dependent in-plane diffusivity anisotropy. Finally, we used the enhanced spatial resolution enabled by overlapping the two beams to measure the temperature-dependent diffusivity of Ti-doped Ca3Ru2O7, which exhibits a metal-insulator transition. We observed large variations of transition temperature over the same sample, originating from doping inhomogeneity and pointing to the power of spatially resolved techniques in accessing inherent properties.

We report a unique type of gap in a magnetic Weyl semimetal ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ where the electrons are spin polarized and preserve the spin-momentum locking feature of Weyl fermions. Such spin-polarized gaps are associated with the Weyl node annihilation, where a pair of Weyl nodes with opposite chirality touch each other at \ensuremath{\sim}210 meV above the Fermi energy. These are revealed by both time- and spin-resolved ultrafast spectroscopy experiments, combined with first-principles calculations. The spin-polarized gap opening is accompanied by a topological phase transition, and the gap magnitude exhibits an unconventional temperature dependence originated from the Weyl physics. Furthermore, we propose possible circularly polarized terahertz-midinfrared radiation from such a spin-polarized gap. Our results shed light on exploring the topological properties of excited states and endow application potentials for gapped materials based on chirality.

Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors. We investigate the ultrafast quasiparticle (QP) dynamics of an intercalated superconductor (Li1-xFex)OHFe1-ySe, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO3. We obtain the electron-phonon coupling (EPC) constant {\lambda}A1g (0.22 +/- 0.04), which well bridges that of bulk FeSe crystal and single-layer FeSe on SrTiO3. Moreover, we find that such a positive correlation between {\lambda}A1g and superconducting Tc holds among all known FeSe-based superconductors, even in line with reported FeAs-based superconductors. Our observation indicates possible universal role of EPC in the superconductivity of all known categories of iron-based superconductors, which is a critical step towards achieving a unified superconducting mechanism for all iron-based superconductors.

We employ the time resolved pump probe experiment to investigate the ultrafast dynamics in a topological semimetal molybdenum phosphide (MoP), which exhibits triple degenerate points in the momentum space. Two relaxation processes with the lifetime of 0.3 and 150 ps have been observed. We attribute the fast component to the electron-phonon scattering and the slow component to the phonon-phonon scattering, respectively. Temperature dependence investigation shows that both the lifetimes of the fast and slow components enhance slightly with increasing temperature. We also successfully generate and detect a thermal-stress-induced coherent acoustic phonon mode with a frequency of 0.033 THz, which does not vary with temperature. Our ultrafast spectroscopy investigation of the quasiparticle dynamics and the coherent phonon in MoP provides useful experimental facts and information about the overall excited state dynamics and the temperature dependence of electron-phonon coupling.

We report ultrafast quasiparticle (QP) dynamics of TaAs, from which we obtain its electron-phonon coupling (EPC) strength. Two relaxation components with lifetimes ${\ensuremath{\tau}}_{\mathrm{fast}}=0.48\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$ and ${\ensuremath{\tau}}_{\mathrm{slow}}=250\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$ are observed, each of which exhibits a smooth temperature dependence. Furthermore, we carry out a fluence-dependence investigation, based on which the quantitative analysis of the fast relaxation component yields a constant EPC strength \ensuremath{\lambda} with $\ensuremath{\lambda}\ensuremath{\langle}{\mathrm{\ensuremath{\Omega}}}^{3}\ensuremath{\rangle}/{\ensuremath{\langle}\mathrm{\ensuremath{\Omega}}\ensuremath{\rangle}}^{2}\ensuremath{\approx}5.2\phantom{\rule{0.16em}{0ex}}\mathrm{p}{\mathrm{s}}^{\ensuremath{-}1}$. In addition, our rotational polarization angle experiment demonstrates that the QP ultrafast dynamics is nearly isotropic under the given excitation geometry. Our investigation of the EPC strength accounts for the so far relatively rarely explored medium fluence regime, which can be generalized to various types of quantum materials.

Ultrafast spectroscopy is a powerful method to generate and control topological phase transitions and spin-polarized electrical currents in topological quantum materials. These light-induced novel physical properties originate from the topologically nontrivial states of Dirac and Weyl fermions. The topological semimetal molybdenum phosphide (MoP) exhibits double and triple degenerate points in the momentum space. We present the preliminary results of spin-polarized electrical currents and optical response investigations of MoP. We design and construct an experimental setup to perform the photocurrent generation and control by circularly polarized light in topological insulator Bi2Se3. The results compare well with those reported, which confirms the validity and reliability of our experimental setup. Further, we conduct the photocurrent experiment on MoP by using 400 nm laser pulses for excitation and successfully detect the current signals at different sample positions. We attribute the observed currents to photo-induced thermal currents (not the photo current associated with the triple degenerate topological properties), which facilitates generating and controlling photocurrents in MoP in the future investigation. Our thermal current investigations are of essence for further exploring the photocurrents in various types of topological quantum materials.

Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd3As2 is carried out. An acoustic phonon (AP) mode with central frequency f = 0.037 THz (i.e., 1.23 cm(-1) or 0.153 meV) is unambiguously generated and detected, which we attribute to laser-induced thermal strain. An AP chirping (i.e., variation of the phonon frequency) is clearly detected, which is ascribed to heat capacity variation with time. By comparing our experimental results and the theoretical model, we obtain a chirping time constant, which is 31.2 ps at 6K and 19.8 ps at 300K, respectively. Significantly, we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping, instead of a Fano resonance. Moreover, we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence, as well as temperature, which endows Cd3As2 application potentials in thermoelectric devices.

Received 14 June 2017DOI:https://doi.org/10.1103/PhysRevB.95.239905©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPhononsTopological materialsPhysical SystemsDirac semimetalTechniquesRaman spectroscopyTime-resolved infrared spectroscopyCondensed Matter, Materials & Applied Physics

This article explores the adsorbate distributions in the adlayer for a model A-B2 system of heterogeneous catalysis, i.e., A + 1/2B2 → AB, via kinetic Monte Carlo (KMC) simulations. In comparison with our previous work on the A-B model (Sun, F.; Huang, W.; Li, J. Structural characteristics of the adlayer in heterogeneous catalysis. Chem. Eng. Sci. 2016, 153, 87-92), species B2 here brings about significant new features due to its special site requirement during adsorption and desorption and a different stoichiometric ratio in reactions. The effects of various kinetic processes on the adsorbate distribution are found to be similar to those in the A-B system; that is, both desorption and diffusion (besides adsorption) processes contribute to the adlayer uniformity while reactions account for clustering. However, desorption exhibits a stronger role than diffusion in homogenizing the adlayer, which is opposite to the finding in the previous A-B model. Under a fixed partial pressure, different reaction and desorption rate constants can lead to steady states with different dominant species, which has not been observed in the A-B system. The regime of species B poisoning shrinks as well, leading to the spreading of the coexisting regime, in comparison with the A-B model.

We report an ultrafast lattice dynamics investigation of the topological Dirac semimetal Cd3As2. A coherent phonon beating among three evenly spaced A(1g) optical phonon modes (of frequencies 1.80, 1.96, and 2.11 THz, respectively) is unambiguously observed. The two side modes originate from the counter helixes composing Cd vacancies. Significantly, such helix vacancy- induced phonon (HVP) modes experience prominent extra waning in their ultrafast dynamics as temperature increases, which is immune to the central mode. Above 200 K, the HVP becomes inactive, which may potentially affect the topological properties. Our results in the lattice degree of freedom suggest the indispensable role of temperature in considering topological properties of such quantum materials.

This paper explores the adsorbate distributions in the adlayer for a model system of heterogeneous catalysis, A+B→AB. Relevant kinetic mechanisms are revealed in terms of Shannon entropy via kinetic Monte Carlo (KMC) simulations. It is reckoned that reactions account for the clustering of the adsorbates, whereas diffusion and desorption tend to homogenize the adsorbate distribution besides adsorption, and diffusion exhibits a relatively stronger role. The clustering tendency seems to compete with the homogenizing one, showing alternate dominance in microscale space and time. The compromise between these two tendencies was reflected as the extremal tendencies at the mesoscale and the macroscale. However, such extremal tendencies can be maximization or minimization of Shannon entropy, depending on the choice of the initial state, implying the insufficiency of a single extremal function of Shannon entropy.

We used x-ray magnetic circular dichroism (XMCD) to probe the ferromagnetic properties of As p-symmetric (4p) states in the recently synthesized diluted magnetic semiconductor (Ba1-x K-x)(Zn1-yMny)(2)As-2 system under ambient-and high-pressure conditions. The As K-edge XMCD signal scales with the sample magnetization (dominated by Mn) and scales with the ferromagnetic ordering temperature Tc, and hence it is representative of the bulk magnetization. The XMCD intensity gradually decreases upon compression and vanishes at around 25 GPa, indicating quenching of ferromagnetism at this pressure. Transport measurements show a concomitant increase in conductivity with pressure, leading to a nearly metallic state at about the same pressure where magnetic order collapses. High-pressure x-ray diffraction shows an absence of structural transitions to 40 GPa. The results indicate that the mobility of doped holes, probed by both transport and x-ray absorption spectroscopy (4p band broadening), is intimately connected with the mechanism of magnetic ordering in this class of compounds and that its control using external pressure provides an alternative route for tuning the magnetic properties in diluted magnetic semiconductor materials.