arXiv subjects

Kazumasa Miyake

Publications and source records attributed to Kazumasa Miyake.

At least 19 recordsLinked to original sources

arXiv2023

Threats, Vulnerabilities, and Controls of Machine Learning Based Systems: A Survey and Taxonomy

In this article, we propose the Artificial Intelligence Security Taxonomy to systematize the knowledge of threats, vulnerabilities, and security controls of machine-learning-based (ML-based) systems. We first classify the damage caused by attacks against ML-based systems, define ML-specific security, and discuss its characteristics. Next, we enumerate all relevant assets and stakeholders and provide a general taxonomy for ML-specific threats. Then, we collect a wide range of security controls against ML-specific threats through an extensive review of recent literature. Finally, we classify the vulnerabilities and controls of an ML-based system in terms of each vulnerable asset in the system's entire lifecycle.

cs.CR↗

arXiv2022

Magnetovolume Effect on the First-Order Metamagnetic Transition in UTe$_2$

The link between the metamagnetic transition and novel spin-triplet superconductivity of UTe$_2$ was discussed thermodynamically through magnetostriction measurements in a pulsed-magnetic field. We revealed a discontinuous magnetostriction across the metamagnetic transition at $μ_0H_{\rm m}\approx 35$~T for the applied magnetic fields along the crystallographic $b$ axis in the orthorhombic structure. The resultanting volume magnetostriction of $ΔV/V \approx-5.9\times 10^{-4}$ gives the initial pressure dependence of $H_{\rm m}$ by employing the Clausius-Clapeyron's equation, which agrees with previous pressure experiments. Further, significant anisotropic magnetostriction (AMS), derived by subtracting the averaged linear magnetostriction, was revealed. Contrary to the weakly field-dependent AMS along the $a$ axis, those along the $b$ and $c$ axes show strong field dependences with a similar magnitude but with opposite signs, indicating its lattice instability. The relationship between characteristic energy scales of magnetic fields and temperatures was discussed in terms of the Grüneisen parameters compared to the other $f$-electron systems. The volume shrinkage in UTe$_2$ at $H_{\rm m}$, contrary to the volume expansion in typical heavy fermion metamagnets, pushes to invoke the link with the valence instability related to the itinerant-localized dual nature of the U magnetism.

cond-mat.str-el↗

arXiv2022

Theory for Anomalous NMR Response in Pb_{1-x}Tl_{x}Te on Charge Kondo Effect

A theory for anomalous enhancement of NMR relaxation rate $1/T_{1}T$ of $^{125}$Te toward zero temperature observed in Pb$_{1-x}$Tl$_{x}$Te ($x$=0.01) is presented on the idea of the charge Kondo effect of valence skipping element Tl. It is found that such enhancement in $1/T_{1}T$ is caused through enhancement of the pair-hopping and inter-orbital interactions between 6s electrons localized on Tl site and conduction electrons doped in the hole band the semiconductor PbTe, which is the heart of the charge Kondo effect. It is also found that the Knight shift is not influenced in the temperature region where the relaxation rate is enhanced which is consistent with the experimental observation showing that the Korringa relation is apparently broken.

cond-mat.str-el↗

arXiv2022

Unconventional Non-Fermi Liquid Properties of Two-Channel Anderson Impurities System

A theory for treating the unconventional non-Fermi liquid temperature dependence of physical quantities, such as the resistivity, in the Pr-based two-channel Anderson impurities system is developed. It is shown that their temperature dependences are essentially the same as those in the pure lattice system except for the case of extremely low concentration of Pr ions that is difficult to realize by a controlled experiments. This result is consistent with recent observations in diluted Pr-1-2-20 system Y$_{1-x}$Pr$_x$Ir$_2$Zn$_{20}$ ($x=0.024,\,0.044,\,0.085,$ and $0.44$) reported in Yamane $et\ al$. Phys. Rev. Lett. ${\bf 121}$, 077206 (2018), and is quite different from that in the case of single-channel Anderson impurities system in which the crossover between behaviors of the local Fermi liquid and heavy Fermi liquid occurs at around moderate concentration of impurities as observed in Ce-based heavy fermion system La$_{1-x}$Ce$_x$Cu$_6$.

cond-mat.str-el↗

arXiv2021

On Pulsed Magnetic Field Measurements of Enhanced Sommerfeld Constant γand A-Coefficient around Metamagnetic Transition in UTe2

It is discussed from theoretical viewpoint why the aspects of enhancements in the A-coefficient in the resistivity\cite{Knafo}and the Sommerfeld constant γ\cite{AtsushiMiyake,Imajo} observed in UTe2 around the metamagnetic transition at H=H_m are so different. In particular, the discussions are focused on the reason why the H dependence of the A-coefficient reported in Ref.\citen{Knafo} and γreported in Ref.\citen{AtsushiMiyake}is almost symmetric around H=H_{\rm m}, while γ(H) at H\gsim H_m, reported in Ref.\citen{Imajo}, is considerably suppressed compared to that at H\lsim H_m. A key point to solve this seeming paradox is to take properly into account the difference in time scales of probes used in those measurements of pulsed magnetic field method with different time scales. Another crucial effect is the time dependent non-uniform magnetic fluctuations remaining in a certain period of time after the first-order metamagnetic transition has occurred. Indeed, the time scales of former measurements in Refs.\citen{Knafo,AtsushiMiyake} are far shorter than that of the latter one in Ref.\citen{Imajo}, which can solve the above paradox.

cond-mat.str-el↗

arXiv2020

On Sharp Enhancement of Effective Mass of Quasiparticles and Coefficient of T^{2} Term of Resistivity around First-Order Metamagnetic Transition Observed in UTe_{2}

The mechanism underlying the enhancement of the Sommerfeld coefficient of quasiparticles at the first-order metamagnetic transition in UTe_2, reported by Miyake et al. in J. Phys. Soc. Jpn. 88, 063706 (2019), is discussed theoretically by taking into account the ferromagnetic order-parameter fluctuations on the basis of the Landau theory of phase transition. We find that the enhanced ferromagnetic spin fluctuation gives rise to the enhancement of the effective mass of the quasiparticles or the Sommerfeld coefficient γ,which is consistent with the experimental observations. At the same time, the Kadowaki-Woods type scaling around the metamagnetic transition, reported by Imajo et al.in J. Phys. Soc. Jpn. 88, 083705 (2019) and Knafo et al. in J. Phys. Soc. Jpn. 88, 063705 (2019), is also understood semi-quantitatively by assuming reasonable values of parameters of Landau-type free energy reproducing key quantities characterizing the metamagnetic transition.

cond-mat.str-el↗

arXiv2020

Quantum Criticality of Valence Transition for the Unique Electronic State of Antiferromagnetic Compound EuCu2Ge2

The effect of pressure on the unique electronic state of the antiferromagnetic (AF) compound EuCu2Ge2 has been measured in a wide temperature range from 10 mK to 300 K by electrical resistivity measurements up to 10 GPa. The Neel temperature of TN = 15 K at ambient pressure increases monotonically with increasing pressure and becomes a maximum of TN = 27 K at 6.2 GPa but suddenly drops to zero at Pc = 6.5 GPa, suggesting the quantum critical point (QCP) of the valence transition of Eu from a nearly divalent state to that with trivalent weight. The rhomag0 and A values obtained from the low-temperature electrical resistivity based on the Fermi liquid relation of rhomag = rhomag0 + AT^2 exhibit huge and sharp peaks around Pc. The exponent n obtained from the power law dependence rhomag = rhomag0 + BT^n is clearly less than 1.5 at P = Pc = 6. 5 GPa, which is expected at the AF-QCP. These results indicate that Pc coincides with Pv, corresponding to the quantum criticality of the valence transition pressure Pv. The electronic specific heat coefficient, estimated from the generalized Kadowaki-Woods relation, is about 510 mJ/mol K^2 around Pc, suggesting the formation of a heavy-fermion state.

cond-mat.str-el↗

arXiv2020

Theory for Non-Fermi Liquid Temperature Dependence in Resistivity of Ce_xLa_{1-x} Cu_{5.62} Au_{0.38} (x=0.02-0.10) on the Local Quantum Valence Criticality of Ce Impurities

It was reported by Shiino et al in J. Phys. Soc. Jpn. 86, 123705 (2017) that Ce_xLa_{1-x} Cu_{5.62} Au_{0.38} (x=0.02-0.10) exhibits a new type of quantum criticality in both magnetic and thermal properties, which is the same as that observed in a series of materials exhibiting quantum critical valence fluctuations (QCVF), such as β-YbAlB_4, Yb_{15}Al_{34}Au_{51}, and so on. However, the temperature (T) dependence in the resistivity rho(T) for T<0.5K is quite anomalous, i.e., ρ(T)\propto (const.-T^{n}) with n \simeq 0.75 at x=0.05. We find that this anomalous exponent is given by n=2(1-ζ), with zeta being the weakly temperature dependent (0.5 \lsim ζ\lsim 0.7) critical exponent for the QCVF.The observed critical exponent n\simeq 0.75 at x=0.05 is reproduced by choosing ζ\simeq 0.63 which is consistent with the divergent behavior observed in the uniform magnetic susceptibility χ\propto T^{-ζ} with ζ\simeq 0.67 at x=0.02.

cond-mat.str-el↗

arXiv2020

Spin-orbit-phonon interaction as an origin of helical-symmetry breaking spin-triplet superconducting state

The excess increase of the local field distribution width $ΔH\equivσ/γ_μ$ (with $γ_μ=135.5\,$MHz/T being the gyromagnetic ratio of $μ^{+}$), and/or the spontaneous magnetic field $H_{\rm spon}$, have been reported in the superconducting state of a series of compounds more than 10 by $μ$SR (muon spin rotation or relaxation) experiment. Sizes of these quantities are of the order of 1G on the whole. We propose that the increase of the local field distribution $ΔH$ in those compounds, including ions with strong spin-orbit interaction of 5d electrons, such as La in LaNiGa$_2$ or Re in Re$_6$Zr, is possible through the cooperation of the spin-orbit coupling between the quasiparticles and ionic vibrations (phonons) and the conventional quasiparticles-phonon coupling, inducing the spin-triplet p-wave pairing with helical symmetry breaking superconducting state with chiral super spin current around stopped $μ^{+}$ site. The size of $ΔH$ is estimated as of the order of 1G, in consistent with experimental observations.

cond-mat.supr-con↗

arXiv2019

Non-Divergent Grüneisen Parameter in Quantum Critical Quasicrystal Yb$_{15}$Al$_{34}$Au$_{51}$: Reflection of Robustness of Quantum Criticality under Pressure

The mechanism of not diverging Grüneisen parameter in the quantum critical heavy-fermion quasicrystal (QC) Yb$_{15}$Al$_{34}$Au$_{51}$ is analyzed. We construct the formalism for calculating the specific heat $C_V(T)$, the thermal-expansion coefficient $α(T)$, and the Grüneisen parameter $Γ(T)$ near the quantum critical point of the Yb valence transition. By applying the framework to the QC, we calculate $C_V(T)$, $α(T)$, and $Γ(T)$, which explains the measurements. Not diverging $Γ(T)$ is attributed to the robustness of the quantum criticality in the QC under pressure. The difference in $Γ(T)$ at the lowest temperature between the QC and approximant crystal is shown to reflect the difference in the volume derivative of characteristic energy scales of the critical Yb-valence fluctuation and the Kondo temperature. Possible implications of our theory to future experiments are also discussed.

cond-mat.str-el↗

arXiv2019

Charge Transfer Effect under Odd-Parity Crystalline Electric Field

Charge-transfer effect under odd-parity crystalline electric field (CEF) is analyzed theoretically. In quantum-critical metal $β$-YbAlB$_4$, seven-fold configuration of B atoms surrounding Yb atom breaks local inversion symmetry at the Yb site, giving rise to the odd-parity CEF. Analysis of the CEF on the basis of hybridization picture shows that admixture of 4f and 5d wave functions at Yb with pure imaginary coefficient occurs, which makes magnetic-toroidal (MT) and electric-dipole (ED) degrees of freedom active. By constructing the minimal model for periodic crystal $β$-YbAlB$_4$, we show that the MT as well as ED fluctuation is divergently enhanced at the quantum critical point of valence transition simultaneously with critical valence fluctuations.

cond-mat.str-el↗

arXiv2019

Grüneisen Parameter and Thermal Expansion near Magnetic Quantum Critical Points in Itinerant Electron Systems

Complete expressions of the thermal-expansion coefficient $α$ and the Grüneisen parameter $Γ$ are derived on the basis of the self-consistent renormalization (SCR) theory. By considering zero-point as well as thermal spin fluctuation under the stationary condition, the specific heat for each class of the magnetic quantum critical point (QCP) specified by the dynamical exponent $z=3$ (FM) and $z=2$ (AFM) and the spatial dimension ($d=3$ and $2$) is shown to be expressed as $C_{V}=C_a-C_b$, where $C_a$ is dominant at low temperatures, reproducing the past SCR criticality endorsed by the renormalization group theory. Starting from the explicit form of the entropy and using the Maxwell relation, $α=α_a+α_b$ (with $α_a$ and $α_b$ being related to $C_a$ and $C_b$, respectively) is derived, which is proven to be equivalent to $α$ derived from the free energy. The temperature-dependent coefficient found to exist in $α_b$, which is dominant at low temperatures, contributes to the crossover from the quantum-critical regime to the Curie-Weiss regime and even affects the quantum criticality at 2d AFM QCP. Based on these correctly calculated $C_{V}$ and $α$, Grüneisen parameter $Γ=Γ_a+Γ_b$ is derived, where $Γ_a$ and $Γ_b$ contain $α_a$ and $α_b$, respectively. The inverse susceptibility coupled to the volume $V$ in $Γ_b$ gives rise to divergence of $Γ$ at the QCP for each class even though characteristic energy scale of spin fluctuation $T_0$ is finite at the QCP, which gives a finite contribution in $Γ_a=-\frac{V}{T_0}\left(\frac{\partial T_0}{\partial V}\right)_{T=0}$. General properties of $α$ and $Γ$ including their signs as well as the relation to $T_0$ and the Kondo temperature in temperature-pressure phase diagrams of Ce- and Yb-based heavy electron systems are discussed.

cond-mat.str-el↗

arXiv2019

Quantum Valence Criticality in Heavy Fermions on Periodic and Aperiodic Crystals

Progress of theories and experiments of the quantum valence criticality is overviewed focusing on recent discoveries of direct evidence of quantum critical point of valence transitions in periodic crystal $α$-YbAl$_{1-x}$Fe$_x$B$_4$ at $x=0.014$ and quasicrystal Yb$_{15}$(Al$_{1-x} $Ga$_{x}$)$_{34}$(Au$_{1-y}$Cu$_{y}$)$_{51}$ at $x=y=0$. The common quantum criticality as well as $T/B$ scaling behavior in the magnetization observed in the periodic crystal $β$-YbAlB$_4$ and quasicrystal Yb$_{15}$Al$_{34}$Au$_{51}$ is shown to be explained from the theory of critical Yb-valence fluctuations starting from the Hamiltonians for describing the low-energy electronic states in both systems.

cond-mat.str-el↗

arXiv2019

Grüneisen Parameter and Thermal Expansion by the Self-Consistent Renormalization Theory of Spin Fluctuations

The thermal expansion coefficient $α$ and the Grüneisen parameter $Γ$ near the magnetic quantum critical point (QCP) are derived on the basis of the self-consistent renormalization (SCR) theory of spin fluctuation. From the SCR entropy, the specific heat $C_{V}$, $α$, and $Γ$ are shown to be expressed in a simple form as $C_{V}=C_{\rm a}-C_{\rm b}$, $α=α_{\rm a}+α_{\rm b}$, and $Γ=Γ_{\rm a}+Γ_{\rm b}$, respectively, where $C_{\rm i}$, $α_{\rm i}$, and $Γ_{\rm i}$ $({\rm i}={\rm a}, {\rm b})$ are related with each other. As the temperature $T$ decreases, $C_{\rm a}$, $α_{\rm b}$, and $Γ_{\rm b}$ become dominant in $C_{V}$, $α$, and $Γ$, respectively. The inverse susceptibility of spin fluctuation coupled to the volume $V$ in $Γ_{\rm b}$ is found to give rise to the divergence of $Γ$ at the QCP for each class of ferromagnetism and antiferromagnetism (AFM) in spatial dimensions $d=3$ and $2$. This $V$-dependent inverse susceptibility in $α_{b}$ and $Γ_{\rm b}$ contributes to the $T$ dependences of $α$ and $Γ$, and even affects their criticality in the case of the AFM QCP in $d=2$. $Γ_{\rm a}$ is expressed as $Γ_{\rm a}(T=0)=-\frac{V}{T_0}\left(\frac{\partial T_0}{\partial V}\right)_{T=0}$ with $T_0$ being the characteristic temperature of spin fluctuation, which has an enhanced value in heavy electron systems.

cond-mat.str-el↗

arXiv2019

Charge Transfer Effect under Odd-Parity Crystalline Electric Field: Divergence of Magnetic Toroidal Fluctuation in $β$-YbAlB$_4$

A novel property of the quantum critical heavy fermion superconductor $β$-YbAlB$_4$ is revealed theoretically. By analyzing the crystalline electronic field (CEF) on the basis of the hybridization picture, odd parity CEF is shown to exist because of sevenfold configuration of B atoms around Yb, which breaks the local inversion symmetry. This allows onsite admixture of 4f and 5d wavefunctions with a pure imaginary coefficient, giving rise to the magnetic toroidal (MT) degree of freedom. By constructing a realistic minimal model for $β$-YbAlB$_4$, we show that onsite 4f-5d Coulomb repulsion drives charge transfer between the 4f and 5d states at Yb, which makes the MT fluctuation as well as the electric dipole fluctuation diverge simultaneously with the critical Yb-valence fluctuation at the quantum critical point of the valence transition.

cond-mat.str-el↗

arXiv2018

The Dominant Role of Critical Valence Fluctuations on High $T_{\rm c}$ Superconductivity in Heavy Fermions

Despite almost 40 years of research, the origin of heavy-fermion superconductivity is still strongly debated. Especially, the pressure-induced enhancement of superconductivity in CeCu$_2$Si$_2$ away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu$_2$Si$_2$ and several related compounds, optimal superconductivity occurs at the pressure of a valence crossover, which arises from a virtual critical end point at negative temperature $T_{\rm cr}$. In this context, we did a meticulous analysis of a vast set of top-quality high-pressure electrical resistivity data of several Ce-based heavy fermion compounds. The key novelty is the salient correlation between the superconducting transition temperature $T_{\rm c}$ and the valence instability parameter $T_{\rm cr}$, which is in line with theory of enhanced valence fluctuations. Moreover, it is found that, in the pressure region of superconductivity, electrical resistivity is governed by the valence crossover, which most often manifests in scaling behavior. We develop the new idea that the optimum superconducting $T_{\rm c}$ of a given sample is mainly controlled by the compound's $T_{\rm cr}$ and limited by non-magnetic disorder. In this regard, the present study provides compelling evidence for the crucial role of critical valence fluctuations in the formation of Cooper pairs in Ce-based heavy fermion superconductors besides the contribution of spin fluctuations near magnetic quantum critical points, and corroborates a plausible superconducting mechanism in strongly correlated electron systems in general.

cond-mat.str-el↗

arXiv2018

Renormalizations in unconventional superconducting states born of normal and singular Fermi-liquids

The density of low energy particle-hole excitations is non-analytic in a singular Fermi-liquid, but it is altered on entering a superconducting state in which, in the pure limit, it vanishes asymptotically at the chemical potential and in general is analytic. The single-particle excitations in the superconducting states are then quasi-particles so that a form of Landau theory may be constructed for thermodynamic and transport properties in the superconducting state. In this theory, the renormalization of measurable properties due to quasi-particle interactions, such as specific heat, compressibility, magnetic susceptibility, superfluid density, etc. changes in a temperature dependent fashion from the non-interacting theory. This is illustrated by showing the renormalization of these quantities and the relation between the parameters introduced to account for their temperature dependence. When the renormalizations in the normal state are large or singular, temperature dependence of properties in the superconducting states are then in general not useful for identifying the nodal character or symmetry of the superconducting state except for measurements at very low temperatures, upper limits of which are specified. The results obtained are expected to be useful in interpreting the experimental results for the temperature dependence of various properties in the superconducting state born of singular Fermi liquids.

cond-mat.str-el↗

arXiv2018

On anomalous temperature dependence of relaxation rate measured by \muSR in α-YbAl_{0.986}Fe_{0.014}B_4

Recently, it was reported by MacLaughlin et al. in Phys. Rev. B 93, 214421 (2016) that α-YbAl_{0.986}Fe_{0.014}B_4 exhibits an anomalous temperature dependence in the relaxation rate 1/T_{1} of $μ$SR, and stressed that such temperature dependence cannot be understood by the scenario based on the quantum critical valence transition (QCVT) while this compound exhibits a series of the non-Fermi liquid behaviors explained by the theory of the QCVT. In this paper, we point out that the anomalous temperature dependence in 1/T_{1} can be understood semi-quantitatively by assuming that the attraction of a screening cloud of conduction electrons about the μ^{+} induces a local magnetic moment arising from a 4f hole on the Yb ion, giving rise to the Kondo effect between heavy quasiparticles.

cond-mat.str-el↗