arXiv subjects

Daiki Tanaka

Publications and source records attributed to Daiki Tanaka.

12 recordsLinked to original sources

arXiv2021

A Novel Perspective for Positive-Unlabeled Learning via Noisy Labels

Positive-unlabeled learning refers to the process of training a binary classifier using only positive and unlabeled data. Although unlabeled data can contain positive data, all unlabeled data are regarded as negative data in existing positive-unlabeled learning methods, which resulting in diminishing performance. We provide a new perspective on this problem -- considering unlabeled data as noisy-labeled data, and introducing a new formulation of PU learning as a problem of joint optimization of noisy-labeled data. This research presents a methodology that assigns initial pseudo-labels to unlabeled data which is used as noisy-labeled data, and trains a deep neural network using the noisy-labeled data. Experimental results demonstrate that the proposed method significantly outperforms the state-of-the-art methods on several benchmark datasets.

cs.LG↗

arXiv2021

Quasi-Ballistic Thermal Conduction in 6H-SiC

The minimization of electronics makes heat dissipation of related devices an increasing challenge. When the size of materials is smaller than the phonon mean free paths, phonons transport without internal scatterings and laws of diffusive thermal conduction fail, resulting in significant reduction in the effective thermal conductivity. This work reports, for the first time, the temperature dependent thermal conductivity of doped epitaxial 6H-SiC and monocrystalline porous 6H-SiC below room temperature probed by time-domain thermoreflectance. Strong quasi-ballistic thermal transport was observed in these samples, especially at low temperatures. Doping and structural boundaries were applied to tune the quasi-ballistic thermal transport since dopants selectively scatter high-frequency phonons while boundaries scatter phonons with long mean free paths. Exceptionally strong phonon scattering by boron dopants are observed, compared to nitrogen dopants. Furthermore, orders of magnitude reduction in the measured thermal conductivity was observed at low temperatures for the porous 6H-SiC compared to the epitaxial 6H-SiC. Finally, first principles calculations and a simple Callaway model are built to understand the measured thermal conductivities. Our work sheds light on the fundamental understanding of thermal conduction in technologically-important wide bandgap semiconductors such as 6H-SiC and will impact applications such as thermal management of 6H-SiC-related electronics and devices.

cond-mat.mtrl-sci↗

arXiv2018

Joint Optimization Framework for Learning with Noisy Labels

Deep neural networks (DNNs) trained on large-scale datasets have exhibited significant performance in image classification. Many large-scale datasets are collected from websites, however they tend to contain inaccurate labels that are termed as noisy labels. Training on such noisy labeled datasets causes performance degradation because DNNs easily overfit to noisy labels. To overcome this problem, we propose a joint optimization framework of learning DNN parameters and estimating true labels. Our framework can correct labels during training by alternating update of network parameters and labels. We conduct experiments on the noisy CIFAR-10 datasets and the Clothing1M dataset. The results indicate that our approach significantly outperforms other state-of-the-art methods.

cs.CV↗

arXiv2012

Anisotropic spin-dynamics in the Kondo semiconductor CeRu2Al10

Spin dynamics in the new Kondo insulator compound CeRu2Al10 has been studied using unpolarized and polarized neutron scattering on single crystals. In the unconventional ordered phase forming below T0 = 27.3 K, two excitation branches are observed with significant intensities, the lower one of which has a gap of 4.8 +/- 0.3 meV and a pronounced dispersion up to about 8.5 meV. Comparison with RPA magnon calculations assuming crystal-field and anisotropic exchange couplings captures major aspects of the data, but leaves unexplained discrepancies, pointing to a key role of direction-specific hybridization between 4f and conduction band states in this compound.

cond-mat.str-el↗

arXiv2011

High-field magnetization and magnetic phase transition in CeOs2Al10

We have studied the magnetization of CeOs2Al10 in high magnetic fields up to 55 T for H // a and constructed the magnetic phase diagram for H // a. The magnetization curve shows a concave H dependence below T_max \sim40 K which is higher than the transition temperature T_0 \sim29 K. The magnetic susceptibility along the a-axis shows a smooth and continuous decrease down to \sim20 K below T_max \sim40 K without showing an anomaly at T_0. From these two results, a Kondo singlet is formed below T_max and coexists with the antiferro magnetic order below T_0. We also propose that the larger suppression of the spin degrees of freedom along the a-axis than along the c-axis below T_max is associated with the origin of the antiferro magnetic component.

cond-mat.str-el↗

arXiv2010

Magnetization and Magnetoresistance of CeRu2Al10 under High Magnetic Fields along c-Axis

We have studied the magnetization and magnetoresistance of CeRu2Al10 in the applied magnetic field H along the c-axis up to ~ 55 T. The magnetization M at low temperatures shows an H-linear increase with a small slope of M/H than that for H // a-axis up to ~ 55 T after showing a small anomaly at H ~ 4 T, which indicates that the critical field to the paramagnetic phase H_c^p is higher than 55 T for H // c-axis. The magnetization curves for H // a- and c-axes below the antiferro magnetic (AFM) transition temperature T0 behave as if the magnetic anisotropy in the AFM-ordered phase is small, although there exists a large magnetic anisotropy in the paramagnetic phase, which favors the easy magnetization axis along the a-axis. On the other hand, very recently, Khalyavin et al. have reported that the AFM order where the magnetic moment is parallel to the c-axis takes place below T0. These results indicate that the AFM order in this compound is not a simple one. The longitudinal magnetoresistance for H // c-axis at low temperatures shows no anomaly originating from the phase transition, but shows oscillations below 4.2 K. This oscillatory behavior below 4.2 K originates from the Shubnikov-de Haas oscillations, from which the cross section of the Fermi surface normal to the c-axis is estimated to be 1.0*10^14 cm-2, with no large effective mass. This is the first direct evidence of the existence of the Fermi surface below T0.

cond-mat.str-el↗

arXiv2010

Neutron scattering study of the long-range ordered state in CeRu2Al10

Elastic and inelastic neutron scattering measurements have been performed on powder and single-crystal samples of orthorhombic CeRu2Al10. The order forming below T0 = 27 K was identified as a long-range antiferromagnetic state with the wave vector k = (1,0,0). The magnetic spectral response in the ordered phase, measured on powder, is characterized by a spin gap and a pronounced peak at 8 meV, whose Q dependence suggests a magnetic origin. Both features are suppressed when temperature is raised to T0, and a conventional relaxational behavior is observed at 40 K. This peculiar spin dynamics is discussed in connection with recent magnetization results for the same compound.

cond-mat.str-el↗

arXiv2010

Long-range order and low-energy magnetic excitations in CeRu2Al10

The nature of the unconventional ordered phase occurring in CeRu2Al10 below T0 = 27 K was investigated by neutron scattering. Powder diffraction patterns show clear superstructure peaks corresponding to forbidden (h + k)-odd reflections of the Cmcm space group. Inelastic neutron scattering experiments further reveal a pronounced magnetic excitation developing in the ordered phase at an energy of 8 meV.

cond-mat.str-el↗

arXiv2010

High-Field Magnetization of CeRu2Al10

The magnetization measurements of CexLa1-xRu2Al10 (x = 1, 0.75) under the high magnetic field were performed in order to obtain the information for the long-range order (LRO) in CeRu2Al10. We successfully obtained the magnetic phase diagram of these two compounds for the applied magnetic field along the a-axis which is the magnetization easy axis, and found that the LRO for x = 1 disappears at ~50 T which is the critical field to the paramagnetic phase. For x = 0.75, the critical magnetic field decreases to ~37 T by La substitution. The magnetic phase diagram and magnetization curve are qualitatively consistent with the recent Hanzawa's mean field calculation results obtained by assuming the dimer of Ce ions whose crystalline electric field ground state has a large magnetic anisotropy. These results support the singlet pair formation scenario recently proposed by Tanida et al.. We also pointed out the possibility of the appearance of the field-induced magnetic phase between ~40 T and ~50 T for x = 1.

cond-mat.str-el↗

arXiv2010

Anisotropic Transport Properties of CeRu$_2$Al$_{10}$

We studied the transport properties of CeRu_2Al_10 single crystals. All the transport properties show largely anisotropic behaviors below T_0. Those along the a- and c-axes show similar behaviors, but are different from those along the b-axis. This suggests that the system could be viewed as a two-dimensional system. The results of the thermal conductivity and thermoelectric power could be explained by assuming the singlet ground state below T_0. However, the ground state is not simple but has some kind of structure within a spin gap.

cond-mat.str-el↗

arXiv2010

Existence of Fine Structure inside Spin Gap in CeRu2Al10

We investigate the magnetic field effect on the spin gap state in CeRu2Al10 by measuring the magnetization and electrical resistivity. We found that the magnetization curve for the magnetic field H//c shows a metamagnetic-like anomaly at H*~4 T below T_0=27 K, but no anomaly for H//a and H//b. A shoulder of the electrical resistivity at Ts~5 K for I//c is suppressed by applying a longitudinal magnetic field above 5 T. Many anomalies are also found in the magnetoresistance for Hkc below ~5 K. The obtained magnetic phase diagram consists of at least two or three phases below T_0. These results strongly indicate the existence of a fine structure at a low energy side in a spin gap state with the excitation energy of 8 meV recently observed in the inelastic neutron scattering experiments.

cond-mat.str-el↗

arXiv2010

Possible Long-Range Order with Singlet Ground State in CeRu2Al10

We investigate the thermal and transport properties of CexLa1-xRu2Al10 to clarify the origin of the recently discovered mysterious phase below T0=27 K in CeRu2Al10 where a large magnetic entropy is released, however, the existence of an internal magnetic field is ruled out by 27Al-NQR measurement. We find that T0 decreases with decreasing x and disappears at x~0.45. T0 of CeRu2Al10 is suppressed down to 26 K under H=14.5 T along the a-axis. These results clearly indicate that the transition has a magnetic origin and is ascribed to the interaction between Ce ions. Considering the results of specific heat, magnetic susceptibility, thermal expansion, and electrical resistivity and also 27Al NQR, we propose that the transition originates from the singlet pair formation between Ce ions. Although its properties in a Ce dilute region is basically understood by the impurity Kondo effect, CeRu2Al10 shows a Kondo-semiconductor-like behavior. The phase transition at T0 may be characterized as a new type of phase transition that appears during the crossover from the dilute Kondo to the Kondo semiconductor.

cond-mat.str-el↗