Local atomic and electronic structures of the YbB12(001) surfaces are studied by scanning tunneling microscopy and spectroscopy at low temperatures for revealing the hybridized surface electronic states. The surface was prepared by annealing in an ultrahigh vacuum at 1200 degrees C for one hour. The annealing process preferentially removed the surface Yb atoms, and the surface exhibits a homogeneous c (2 & times; 2) structure. On the inhomogeneous surface prepared by 1400 degrees C annealing for several seconds, there are three types of local c (2 & times; 2) structures. The local tunneling spectra at 5 K near the Fermi energy on the surfaces exhibit characteristic features consisting of a co-tunneling lineshape due to the Kondo lattice formation and an additional central peak. On the homogeneous surface, the shape of the central peak depends on the tip-surface distance, suggesting the f -d hybridized nature of the surface electronic states.

To develop a feasible methodology to observe the dynamics of element-specific electronic structures, collective excitations, and lattice structures, we are going to implement a time-resolved resonant electron energy-loss spectroscopy (TR-rEELS), which is expected to pave the way for exploring unconventional phenomena in quantum materials. Before the implementation, to determine the optimal parameters of the probe electron beam and to verify the feasibility of this methodology, we evaluated the profile and the space-charge effect of the pulsed electron beam with a kinetic energy less than 2 keV based on both simulation and experiment. By controlling the initial electron beam parameters, we realized an energy width less than 2 eV with a calculated temporal width of ∼30 ps, enabling the time-resolved measurement, while maintaining a sufficient signal-to-noise ratio, and, thus, showed the feasibility of TR-rEELS. An experimental demonstration result of TR-EELS using this apparatus is also introduced.

At the quantum critical point of correlated materials, a non-Fermi liquid state appears where electron correlations continuously develop to very low temperatures. The relaxation time of the interacted electrons, namely quasiparticles, is scaled with the Planckian time, ℏ/kBT. However, there is a debate over whether heavy-fermion systems can obey the Planckian time. In the optical conductivity spectra, the Drude response will appear as the scaling of ℏω/kBT as the dynamical Planckian scaling (DPS). Here, we report the non-Fermi liquid behavior in the Drude response of a candidate for such materials, the quasi-kagome Kondo lattice CeRhSn. Even though the material shows a strong valence fluctuation, renormalized Drude responses observed at the photon energy below 100 meV are characterized by non-Fermi-liquid-like scattering rate 1/τ. The heavy carriers’ Drude response only for the Ce quasi-kagome plane obeyed DPS below 80 K, suggesting the anisotropic quantum criticality with the strong c-f hybridization.

The photo-induced band structure variation of a rare-earth-based semiconductor, samarium monosulfide (SmS), was investigated using high-harmonic-generation laser-based time-resolved photoelectron spectroscopy. A nonlinear photo-induced band shift of the Sm 4f multiplets was observed. The first one is a shift to the high-binding-energy side due to a large surface photovoltage (SPV) effect of approximately 93 meV, comparable to the size of the bulk band gap, with a much longer relaxation time than 0.1 ms. The second one is an ultrafast band shift to the low binding energy side, which is in the opposite direction to the SPV shift, suggesting an ultrafast valence transition from divalent to trivalent Sm ions due to photo-excitation. The latter energy shift was approximately 58 meV, which is consistent with the energy gap shift from ambient pressure to the boundary between the black insulator and golden metallic phase with the application of pressure. This suggests that the photo-induced valence transition can reach the phase boundary, but other effects are necessary to realize the golden metallic phase.

In recent years, coherent electrons driven by light fields have attracted significant interest in exploring novel material phases and functionalities. However, observing coherent light-field-driven electron dynamics in solids is challenging because the electrons are scattered within several tens of femtoseconds in ordinary materials, and the coherence between light and electrons is disturbed. This study presents the light-field-driven dynamics by applying a THz pulse (similar to 1 ps) to the Weyl semimetal Co3Sn2S2, which has a relatively long coherent time (several hundred femtoseconds to several picoseconds). As the electric-field intensity of the irradiating THz pulse was increased, the reflected/emitted THz wave changed from being similar to the incident THz wave to an asymmetric electric field. This asymmetric electric field emission suggests the generation of non-Ohmic direct current via coherent acceleration, and the fact that its intensity dependence is proportional to the square of the electric field suggests electronic excitation by the Landau-Zener transition, a characteristic of the light-field picture.

Phase-changing materials have been a cornerstone of condensed matter physics for decades. A quintessential example is iron-rhodium (FeRh), which undergoes a first-order phase transition from antiferromagnetic to ferromagnetic states near room temperature. The pivotal aspect of this transition is a marked alteration in electrical conductivity. However, its underlying origin still remains elusive, largely due to the difficulties of directly probing fundamental transport during this phase transition. In this study, we investigate the fundamentals of FeRh's electrical transport employing terahertz time-domain spectroscopy (THz-TDS). Leveraging the Drude model, we discerned the distinct contributions of extrinsic (momentum scattering time, tau) and intrinsic (charge density, n, and effective mass, m*) factors to electrical conductivity independently. Notably, our investigation unveiled a sharp alteration in n and m* during the phase transition, contrasting with the gradual monotonic decrease of tau with rising temperature. Consequently, our findings provide compelling evidence that the conductivity change in FeRh during the phase transition originates from a restructuring of its band structure. This work provides a crucial step towards a comprehensive understanding of the electrical transport changes occurring during the phase transition, offering valuable insights into the behaviour of phase changing materials. Phase-changing materials such as FeRh, undergoing a first-order phase transition from antiferromagnetic to ferromagnetic near room temperature, are attractive for various applications. Here, terahertz time-domain spectroscopy provides evidence that the conductivity change in FeRh during the phase transition originates from a restructuring of its band structure.

The temperature dependence of the dielectric properties and phonon behavior of gadolinium gallium garnet (GGG) and yttrium aluminum garnet (YAG) single crystals were analyzed by terahertz time-domain spectroscopy (THz-TDS), Fourier transform infrared spectroscopy (FT-IR), and density-functional-theory (DFT) calculations. A custom air-plasma-based THz-TDS system was calibrated using the well-studied MgO single crystal, and the results in the low-frequency range were validated by a commercial high-resolution FT-IR system. Consequently, the THz-TDS and FT-IR measurement results for GGG and YAG were in good agreement. Due to the high absorption in the phonon response range, only the TO1 and TO4 phonons of GGG and the TO1 phonon of YAG were observed. A manual phase adjustment was introduced in analyzing GGG in order to calculate reliable values of dielectric constants. The Lorentz oscillator model was employed to identify the phonon modes. With increasing temperature, the phonon frequencies slightly shift to lower frequencies, the damping parameters generally increase, and the oscillator strengths stay constant. The theoretical calculations, based on DFT and a shell model, in which the thermal expansion of the crystals was considered, explain the temperature-dependent phonon frequency shift and are in good agreement with the experimental results. Finally, the phonon behavior was evaluated using the Gr & uuml;neisen parameter and compared with the experimental data. The results show that thermal expansion contributes significantly to phonon frequency shift, and the minor contribution from phonon anharmonicity increases with increasing temperature.

A strongly-correlated insulator, samarium mono-sulfide (SmS), presents not only the pressure-induced insulator-to-metal transition (IMT) with the color change from black to golden-yellow but also current-induced IMT (CIMT) with negative resistance. To clarify the origin of the CIMT of SmS, the electronic structure change has been investigated by optical reflectivity and angle-integrated photoelectron spectra by applying an electric current. At lower temperatures than about 100 K, where the nonlinear $V$-$I$ curve has been observed, the carrier density rapidly increases, accompanied by decreasing relaxation time of carriers with increasing current. Then, the direct gap size increases, and the mean valence changes from Sm$^{2+}$-dominant SmS to the mixed-valent one with increasing current. These results suggest that the CIMT originates from increasing the Sm $4f$-$5d$ hybridization intensity induced by the applied current.

To investigate the role of the excitons for the origin of the pressure-induced phase transition (BGT) from the black-colored insulator (BI) to the golden-yellow-colored metal (GM) of samarium monosulfide, optical reflectivity, Sm 3d x-ray absorption spectroscopy (XAS), and x-ray diffraction (XRD) with the creation of excitons by photoexcitation (PE) are reported. In the pump-probe reflectivity measurement, following a huge reflectivity change of about 22%, three different relaxation times with a vibration component were observed. The fast component with the relaxation time (tau) of less than 1 ps is due to the excitation and relaxation of electrons into the conduction band, and the slowest one with tau>several 100 ps originates from the appearance of the photo-induced (PI) state. The components with tau similar to 10 ps and vibration originate from the appearance of the PI state and the interference between the reflection lights at the sample surface and the boundary between the BI and PI states, suggesting that the electronic structure of the PI phase is different from that of the BI state. XAS spectra indicate that the Sm mean valence is shifted from the Sm2+ dominant to the intermediate between Sm2+ and Sm3+ by PE but did not change to that of the GM phase across BGT, consistent with the reflectivity data. The XRD result after PE shows that the PI state has much less lattice contraction than the GM phase. These results suggest that the BGT cannot be achieved solely by creating excitons after PE but requires other effects, such as a lattice contraction.

We have systematically studied the strong correlation effects in A-site ordered perovskites CaCu3Ti4-xRuxO12 (x = 0, 1, 3.5, and 4) by using photoemission and inverse photoemission spectroscopies. In x = 0, 1, 3.5, the peak positions of the strongly correlated Cu 3d states around -3.8 eV and Ti 3d states around 3.6 eV little change. On the other hand, in x = 4, the Cu 3d states split into two peaks around -2.5 and -4 eV. These indicate that Ti plays an important role to retain the strong correlation effects. In addition, the multiplet structures of Cu 3d final states from -8 to -14 eV become weak as Ru increases, indicating the reduction of the localized characters of Cu 3d states. At the Fermi level, we observe the absence of spectral weight in x = 0, 1 and the development of Ru 4d in-gap states in x = 3.5, 4, which give rise to the metal-insulator transition between x = 1 and x = 3.5.

CeCoSi with no local inversion symmetric crystal structure (P4/nmm) exhibits a phase transition of unknown origin (Hidden Order: HO) at about 12 K (T0) above the antiferromagnetic transition temperature (TN = 9.4 K). The electronic structure change across T0 was investigated with high-precision optical reflection spectroscopy. The optical spectrum changed from a typical metallic behavior above T0 to a gap-like structure at around 15 meV below T0. The gap-like structure was unchanged across TN except for the narrowing of the Drude component of carriers due to the suppression of magnetic fluctuations. This result suggests a slight change from the typical metallic electronic structure above T0 to that with an energy gap near the Fermi level in the HO phase. The change in electronic structure in the HO phase was concluded to be due to electron/valence instability.

The electronic structure changes of SmS surfaces under potassium (K) doping are elucidated using synchrotron-based core-level photoelectron spectroscopy and angle-resolved photoelectron spectroscopy (ARPES). The Sm core-level and ARPES spectra indicate that the Sm mean valence of the surface increased from the nearly divalent to trivalent states, with increasing K deposition. Carrier-induced valence transition (CIVT) from Sm$^{2+}$ to Sm$^{3+}$ exhibits a behavior opposite to that under conventional electron doping. Excess electrons are trapped by isolated excitons, which is inconsistent with the phase transition from the black insulator with Sm$^{2+}$ to the gold metal with Sm$^{3+}$ under pressure. This CIVT helps to clarify the pressure-induced black-to-golden phase transition in this material, which originates from the Mott transition of excitons.

Abstract Backgrounds Diagnostic criterion of left ventricular hypertrophy (LVH) on 12-leads electrocardiogram (ECG) were historically established. However, there was no study to evaluate the criterion comparing with machine learning methods. Purpose To verify the historical criterion of LVH on ECG, and to compare them with diagnosis by machine learning. Methods First, consecutive 60 patients with LVH were recruited, and one to one matching with age and sex to patients with normal cardiac function was performed. Finally, 120 patients (69.6 ± 12.6years, 38men per group) were enrolled. LVH was defined as at least one LV wall (septum, posterior wall, apex) showed thickness over 15mm on ultrasound echocardiography. No sinus rhythm, and wide QRS cases were excluded. The accuracy of historical criterion and ECG predictors were calculated by an assessment of whether a predictor was higher/lower than the cut-off value of receivor operating characteristics curve analysis (ROC). Ten machine learning methods were built using the significant predictors of logistic regression with 10-times cross validation. Results By logistic regression analysis, 77 significant predictors were extracted, and 22 predictors showed their area under ROC (AUROC) over 0.700. Among historical criterion, Cornell voltage showed high accuracy (0.783) and AUROC (0.808). Conversely, among AI methods, light gradient boosting machine demonstrated higher accuracy (0.843) and random forest method higher AUROC (0.882). Shapley additive explanation method (SHAP) demonstrated that V2/V2 S-wave amplitude and I/V5 T-wave amplitude played essential roles to build the AI models. Conclusions AI diagnosis on ECG for LVH showed powerful diagnostic performance comparing historical criterion.

The Kondo effect between localized f-electrons and conductive carriers leads to exotic physical phenomena. Among them, heavy-fermion (HF) systems, in which massive effective carriers appear due to the Kondo effect, have fascinated many researchers. Dimensionality is also an important characteristic of the HF system, especially because it is strongly related to quantum criticality. However, the realization of the perfect two-dimensional (2D) HF materials is still a challenging topic. Here, we report the surface electronic structure of the monoatomic-layer Kondo lattice YbCu2 on a Cu(111) surface observed by synchrotron-based angle-resolved photoemission spectroscopy. The 2D conducting band and the Yb 4f state, located very close to the Fermi level, are observed. These bands are hybridized at low-temperature, forming the 2D HF state, with an evaluated coherence temperature of about 30 K. The effective mass of the 2D state is enhanced by a factor of 100 by the development of the HF state. Furthermore, clear evidence of the hybridization gap formation in the temperature dependence of the Kondo-resonance peak has been observed below the coherence temperature. Our study provides a new candidate as an ideal 2D HF material for understanding the Kondo effect at low dimensions.

Abstract Backgrounds Diagnosis of multi-vessel coronary artery disease (MVD) on single photon emission computed tomography (SPECT) is recognized to be difficult. We tried to classify patients who were suspected coronary artery stenosis by artificial intelligence (AI) clustering on parameters of electrocardiogram (ECG) gated SPECT (gated-SPECT), and evaluated their predictive value for MVD. Methods We enrolled consecutive 335 patients (median:74years [68, 79], 255man) who underwent adenosine stress ECG gated-SPECT and coronary angiography in 3-months. No sinus rhythm patients were excluded. MVD was defined as at least two main branch coronary artery stenosis whose fractional flow reserve over 0.80 which was evaluated by coronary angiography. Univariate logistic regression analysis extracted significant predictors among many paremeters of the SPECT (summed score, left ventricular (LV) volume, systolic and diastolic function, and degree of LV dyssynchrony), and using them, AI clustering (K-means) was performed. Results Breakdown of MVD was as follows: MVD was 48 (double vessel disease 30, LMT stenosis 5, and triple vessel 13). Single vessel disease was 91, and no lesion was 196 cases. Clustering was performed using 21 significant parameters of the SPECT and clinical characteristics (Diabetes, LVEF, heart rate(HR), SD-TES, MDSV, 1/3 mean filling rate, peak filling ratio on adenosine stress, LVEF, HR, SD-TES, MDSV at rest, and other 10 predictors). Number of clusters was set as 4 by elbow plotting method. Prevalence of MVD was significantly different in the 4 clusters (Odds ratio: Cluster B/C/D to A, 3.05/3.61/6.60, respectively). Compared with cluster A, clusteter B/C/D showed increased summed stress score, chronic kidney disease, SD-TES (=left ventricular end-systolic dyssynchrony parameter) on stress, decreased left ventricular ejection fraction and 1/3 mean filling rate. Conclusions AI clustering on gated-SPECT had powerful predictive value for MVD.

Abstract Funding Acknowledgements Type of funding sources: None. Background At onset of acute heart failure (AHF), various clinical fundamental parameters including vital sign, laboratory data, or initial treatment were investigated, and we can roughly estimate the prognosis. However, machine learning method for prediction of the prognosis was not studied. Purpose To elucidate prognostic value of machine learning for AHF comparing conventional statistical model. Methods We enrolled consecutive 300 patients with AHF (79.5 ± 12.1 years, 158 Males). Patients with acute coronary syndrome, mechanical circulatory support cases, and cardio-pulmonary arrest cases were excluded. The patients were randomly divided into 80% (240 cases) and 20% (60 cases), and the former was used as train data, and the latter as validation data. Objective variable was set as cardiac death in one year. First, logistic regression analysis with Akaike’s information criterion (AIC) was performed, and extracted predictive parameters. The predictive model for the cardiac prognosis was constructed by cut-off value of ROC curve analysis of propensity score was calculated. Next, machine learning (random forest method and deep learning) to build predictive model was performed with the predictors. Finally, accuracy of each predictive model was compared. Results Thirty cases showed cardiac death in one year. Logistic regression with AIC extracted 8 predictors, and the cut off-value of propensity score with the 6 parameters was 0.110. The accuracy was 0.714 and area under ROC (AUROC) was 0.836. Conversely, random forest method demonstrated the accuracy as 0.927, AUROC 0.860. On deep learning, the accuracy was 0.937 and AUROC 0.901. The top 4 high feature importance of random forest were Cl/red blood cell count/pH/Anion Gap. However, accuracy of those predictors was lower than that of machine learning. Conclusion Machine learning was a powerful tool to predict cardiac prognosis of AHF, comparing with conventional statistical model. Abstract Figure. Statistical model

We investigated Heusler-type Ru2VAl, a candidate material for next-generation thermoelectric conversion, by first-principle calculations of its thermoelectric conversion properties and direct experimental observations of its electronic structures, employing photoemission and infrared spectroscopy. Our results show that Ru2VAl has a wider pseudogap near the Fermi level compared to Fe2VAl. Accordingly, a higher thermoelectric conversion performance can be expected in Ru2VAl at higher temperatures.

The topology and spin-orbital polarization of two-dimensional (2D) surface electronic states have been extensively studied in this decade. One major interest in them is their close relationship with the parities of the bulk (3D) electronic states. In this context, the surface is often regarded as a simple truncation of the bulk crystal. Here we show breakdown of the bulk-related in-plane rotation symmetry in the topological surface states (TSSs) of the Kondo insulator SmB6. Angle-resolved photoelectron spectroscopy (ARPES) performed on the vicinal SmB6(001)-p(2 × 2) surface showed that TSSs are anisotropic and that the Fermi contour lacks the fourfold rotation symmetry maintained in the bulk. This result emphasizes the important role of the surface atomic structure even in TSSs. Moreover, it suggests that the engineering of surface atomic structure could provide a new pathway to tailor various properties among TSSs, such as anisotropic surface conductivity, nesting of surface Fermi contours, or the number and position of van Hove singularities in 2D reciprocal space.

Spin-orbit interaction (SOI) in low-dimensional systems, namely Rashba systems and the edge states of topological materials, is extensively studied in this decade as a promising source to realize various fascinating spintronic phenomena, such as the source of the spin current and spin-mediated energy conversion. Here, we show the odd fluctuation in the spin-orbital texture in a surface Rashba system on Bi/InAs(110)-(2$\times$1) by spin- and angle-resolved photoelectron spectroscopy and a numerical simulation based on a density-functional theory (DFT) calculation. The surface state shows a paired parabolic dispersion with the spin degeneracy lifted by the Rashba effect. Although its spin polarization should be fixed in a particular direction based on the Rashba model, the observed spin polarization varies greatly and even reverses its sign depending on the wavenumber. DFT calculations also reveal that the spin directions of two inequivalent Bi chains on the surface change from nearly parallel (canted-parallel) to anti-parallel in real space in the corresponding wavevector region. These results point out an oversimplification of the nature of spin in Rashba and Dirac systems and provide more freedom than expected for spin manipulation of photoelectrons.

Bismuth (Bi) atomic layers are known as 2D topological materials with variety of the electronic structures and topological orders depending on the number of stacking layers. Recently, it is reported that few layers of Bi grown on semiconductor substrate InSb(111)B exhibit the Sierpiński-triangle (ST) fractal patterns on the surface. In this work, we have grown Bi layers on InSb(111)B and traced the evolution of the atomic and electronic structures of Bi. The surface atomic structures and growth modes were monitored by using reflective high-energy electron diffraction and core-level photoelectron spectroscopy. It is suggested that the single layer of the ST-phase Bi grows on InSb(111)B and the following Bi deposition causes layer-by-layer growth up to nominally 4 atomic layers. Diffuse band dispersion and quantum well states observed by angle-resolved photoelectron spectroscopy are consistent with the small surface domains and variation of the thickness even during the layer-by-layer growth region. The further Bi evaporation changes the growth mode to the 3D island formation. The unveiled growth behavior of Bi on InSb(111)B would provide a new interesting playground to study 2D topological electronic structure of quasi-periodic 2D atomic layers.