arXiv subjects

M. Yoshimura

Publications and source records attributed to M. Yoshimura.

At least 19 recordsLinked to original sources

arXiv2025

The nexus between negative charge-transfer and reduced on-site Coulomb energy in a correlated topological metal CoTe$_2$

The layered $3d$ transition metal dichalcogenide (TMD) CoTe$_2$ is a topological Dirac Type-II metal. However, the Co $3d$-bands in CoTe$_2$ do not exhibit the expected correlation-induced band narrowing seen in CoO. We address this conundrum by studying the electronic structure of CoTe$_2$ using hard x-ray photoemission spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS) and Resonant-PES. We quantify the on-site Coulomb energy $U_{dd}$ via single-particle partial density of states and the two-hole correlation satellite using valence band Resonant-PES), and obtain $U_{dd}$ = 3.0 eV for CoTe$_2$. Charge-transfer (CT) cluster model simulations of the measured core-level Co $2p$ PES and $L$-edge XAS spectra of CoTe\textsubscript{2} and CoO validate their contrasting electronic parameters:$U_{dd}$ and CT energy $\Delta$ are (3.0 eV, -2.0 eV) for CoTe\textsubscript{2}, and (5.0 eV, 4.0 eV) for CoO, respectively. The $d$-$p$ hybridization strength $T_{eg}$ for CoTe$_2$$<$CoO, and indicates that the reduced $U_{dd}$ in CoTe\textsubscript{2} is not due to $T_{eg}$. The increase in $d^n$-count$\sim$1 by CT from ligand to Co site in CoTe$_2$ is due to a negative-$\Delta$ and reduced $U_{dd}$. Yet, only because $U_{dd}$$>$$\big|\Delta\big|$, CoTe$_{2}$ becomes a topological metal with $p$$\rightarrow$$p$ type lowest energy excitations. The study reveals the nexus between negative-$\Delta$ and reduced $U_{dd}$ required for setting up the electronic structure framework for achieving topological behavior via band inversion in the correlated metal CoTe$_2$.

cond-mat.str-el↗

arXiv2025

Role of on-site Coulomb energy and negative-charge transfer in a Dirac semi-metal NiTe$_2$

Angle-resolved photoemission spectroscopy (ARPES) combined with band structure calculations have shown that the layered transition metal dichalcogenide(TMD) NiTe$_2$ is a type-II Dirac semimetal. However, conflicting conclusions were reported regarding the role of electron correlations in NiTe$_2$. We study core-levels and valence band electronic structure of single crystal NiTe$_2$ using soft and hard x-ray photoemission spectroscopy(SXPES, HAXPES), X-ray absorption spectroscopy(XAS) and Ni $2p-3d$ Resonant-PES to quantify electronic parameters in NiTe$_2$. The Ni $3d$ on-site Coulomb energy ($U_{dd}$) is quantified from measurements of the Ni $3d$ single particle density of states(DOS) and the two-hole correlation satellite. The Ni $2p$ core level and $L$-edge XAS spectra are analyzed by charge-transfer (CT) cluster model calculations using the experimental $U_{dd}$, and it shows that NiTe$_2$ exhibits a negative CT energy $\Delta$. A comparative analysis of NiO $L$-edge XAS confirms its well-known strongly correlated CT insulator character, with a larger $U_{dd}$ and positive $\Delta$. The $d$-$p$ hybridization strength $T_{eg}$ for NiTe$_2$$<$NiO, and shows that $T_{eg}$ is not responsible for reducing $U_{dd}$ in NiTe\textsubscript{2} compared to NiO. The negative-$\Delta$ and a reduced $U_{dd}$ leads to the increase in $d^n$ count on the Ni site in NiTe$_{2}$ by nearly one electron. However, importantly, since $U_{dd}$$>$$|\Delta|$, a finite repulsive $U_{dd}$ results in pushing $d$-states away from Fermi level and this is required to make NiTe$_{2}$ a moderately correlated Dirac semi-metal with band inversion in the $p$-$p$ type lowest energy excitations.

cond-mat.str-el↗

arXiv2024

Implications of neutrino species number and summed mass measurements in cosmological observations

We confront measurable neutrino degrees of freedom $N_{\rm eff}$ and summed neutrino mass in the early universe to particle physics at the energy scale beyond the standard model (BSM), in particular including the issue of neutrino mass type distinction. The Majorana-type of massive neutrino is perfectly acceptable by Planck observations, while the Dirac-type neutrino may survive in a restricted class of models that suppresses extra right-handed contribution to $\Delta N_{\rm eff} = N_{\rm eff} - 3$ at a nearly indistinguishable level from the Majorana case. There is a chance that supersymmetry energy scale may be identified in supersymmetric extension of left-right symmetric model if improved $N_{\rm eff}$ measurements discover a finite value. Combined analysis of this quantity with the summed neutrino mass helps to determine the neutrino mass ordering pattern, if measurement accuracy of order, $60 \sim 80\,$meV, is achieved, as in CMB-S4.

hep-ph↗

arXiv2024

Novel quantum spin liquid ground state in the trimer rhodate Ba$_4$NbRh$_3$O$_{12}$

Frustrated magnets offer a plethora of exotic magnetic ground states, including quantum spin liquids (QSLs), in which enhanced quantum fluctuations prevent a long-range magnetic ordering of the strongly correlated spins down to lowest temperature. Here we have investigated the trimer based mixed valence hexagonal rhodate Ba$_4$NbRh$_3$O$_{12}$ using a combination of dc and ac magnetization, electrical resistivity, specific heat, and muon spin rotation/relaxation ($\mu$SR) measurements. Despite the substantial antiferromagnetic exchange interactions, as evident from the Weiss temperature ($\theta_{\mathrm{W}}\sim -35$ to -45 K), among the Rh-local moments, neither long-range magnetic ordering nor spin-freezing is observed down to at least 50 mK, in ac-susceptibility, specific heat and ZF-$\mu$SR measurements (down to 0.26 K). We ascribe the absence of any magnetic transition to enhanced quantum fluctuations as a result of geometrical frustration arising out of the edge-sharing equilateral Rh-triangular network in the structure. Our longitudinal-field $\mu$SR result evidences persistent spin fluctuations down to 0.26~K, thus stabilizing a dynamic QSL ground state in Ba$_4$NbRh$_3$O$_{12}$. Furthermore, the magnetic specific heat ($C_{\mathrm{m}}$) data at low-$T$ reveal a significant $T$-linear contribution plus a quadratic $T$-dependence. A $T$-linear behavior is evocative of gapless spin excitations, while the $T^2$-term of $C_{\mathrm{m}}$ may indicate the Dirac QSL phenomenology of the spinon excitations with a linear dispersion.

cond-mat.str-el↗

arXiv2022

Primordial neutron star; a new candidate of dark matter

Z-boson exchange interaction induces attractive force between left-handed neutrino and neutron. The Ginzburg-Landau mean field calculation and the Bogoliubov transformation suggest that this attractive force leads to neutrino-neutron pair condensate and super-fluidity. When the result of super-fluid formation is applied to the early universe, horizon scale pair condensate may become a component of dark energy. A further accretion of other fermions from thermal cosmic medium gives a seed of primordial neutron stars made of proton, neutron, electron, and neutrino in beta-equilibrium. Primordial neutron stars may provide a mechanism of giving a part or the whole of the dark matter in the present universe, if a properly chosen small fraction of cosmic thermal particles condenses to neutrino-neutron super-fluid and primordial neutron star not to over-close the universe. The proposal can be verified in principle by measuring neutrino burst at primordial neutron star formation and by detecting super-fluid relic neutrinos in atomic experiments at laboratories.

hep-ph↗

arXiv2022

New types of instability and CP violation in electroweak theory

It is known that the Schwinger mechanism of vector-like QED theory is afflicted by a logarithmic singularity under background electromagnetic field due to a hypothetical massless charged fermion. We extend singularity analysis to a more realistic case of the chiral electroweak theory, to show that the effective lagrangian under background gauge field at zero temperature exhibits a similar instability proportional to $\ln (1/m_{\nu}^2)$ with $m_{\nu}$ a small neutrino mass. Moreover, the effective lagrangian of chiral fermion loop contains CP violating pieces proportional to background gauge fields in odd powers of $\vec{E}_Z\cdot\vec{B}_Z$ or $(\vec{E}_{W^+}\cdot\vec{B}_{W^-}+ \vec{E}_{W^-}\cdot\vec{B}_{W^+})/2 $. This brings in a new source of CP violation and time-reversal symmetry violation in the standard particle theory independent of the Kobayashi-Maskawa phase of quark mass mixing matrix. The effective action in thermal equilibrium at finite temperature $T$ is then calculated under background SU(2)$\times $U(1) gauge fields in the spontaneously broken phase. An even more singular power-law behavior $\propto (m_{\nu} T)^{-5/2}$ is found and it contains CP violating term as well. The case of Majorana neutrino satisfies almost all necessary conditions to generate a large lepton number asymmetry, though not necessarily convertible to a baryon asymmetry due to lower cosmic temperatures at which this may occur.

hep-ph↗

arXiv2022

B $-$ L genesis by sliding inflaton

We propose a new mechanism of lepton (L) number asymmetry generation, hence offer an explanation of matter-antimatter imbalance when a significant amount of baryon number is later transformed from this L-number by known electroweak sphaleron mediated process. The basic theoretical framework is a recently proposed multiple scalar-tensor gravity that dynamically solves the cosmological constant problem. The L-asymmetry generation in one of two proposed scenarios is triggered by dynamical relaxation of scalar inflaton field towards the zero cosmological constant. CPT violation (C= charge conjugation, P = parity operation, T= time reversal) in the presence of a chemical potential gives the necessary time arrow, and lepton number violating scattering in cosmic thermal medium generates a net cosmological L-number via resonance formation. Another scenario is L-asymmetry generation from evaporating primordial black holes. These proposed mechanisms do not require CP violating phases in physics beyond the standard model: the new required physics is existence of heavy Majorana leptons of masses $ 10^{15} \sim 10^{17}$ GeV that realizes the seesaw mechanism. We identify the cosmological epoch of lepto-genesis in two scenarios, which may give the right amount of observed baryon to entropy ratio. It might even be possible to experimentally determine microscopic physics parameter, masses of three heavy Majorana leptons by observing astrophysical footprints of primordial black hole evaporation at specified hole masses.

hep-ph↗

arXiv2022

Stronger gravity in the early universe

Scalar-tensor theories of gravity that embrace conformal coupling to the scalar curvature are the focal point of cosmology on discussions of inflation and late-time accelerating universe. Although there exists a stringent nucleo-synthesis constraint on conformal gravity, one can formulate how to evade this difficulty by modifying the standard particle theory action consistently with the principles of gauge invariant quantum field theory. It is shown that stronger gravity at early epochs of cosmological evolution than previously thought of is inevitable in a class of conformal gravity models. This enhances discovery potentials of primordial gravitational wave emission and primordial black hole formation. The strong gravity effect may be enormous if massive clumps are energetically dominated by cold dark matter made of inflaton field, and created black holes may become a major candidate of cold dark matter.

gr-qc↗

arXiv2022

Dynamical relaxation of cosmological constant

A special class of conformal gravity theories is proposed to solve the long standing problem of the fine-tuned cosmological constant. In the proposed model time evolution of the inflaton field leaves behind a nearly vanishing, but finite value of dark energy density of order (a few meV)$^4$ to explain the late-time accelerating universe. A multiple scalar inflaton field is assumed to have a conformal coupling to the Ricci scalar curvature in the lagrantian, which results in, after a Weyl rescaling to the Einstein metric frame, modification of inflaton kinetic and potential terms along with its coupling to Higgs fields in the standard model. One may define an effective cosmological $\Lambda$ function in the Einstein metric frame, which controls, when the potential is added, a slow-roll inflation and subsequent oscillation at the potential minimum of a spontaneous symmetry breaking phase. The inflaton oscillation accompanies particle production towards thermalized hot big-bang universe. At the same time zero-point quantum fluctuation of second inflaton field is generated,and its accumulated fluctuation gives rise to a symmetry restoration, pushing back the inflaton field towards the infinity. This gives a dynamical relaxation of vanishing effective cosmological constant. Both inhomogeneous inflaton modes and their collapsed black holes of primordial origin are good candidates of cold dark matter.

hep-ph↗

arXiv2021

Bifurcated symmetry breaking in scalar-tensor gravity

We present models that simultaneously predict presence of dark energy and cold dark matter along with slow-roll inflation. The dark energy density is found to be of order $({\rm a \;few \;meV})^4$, and the mass of dark matter constituent is $\approx 1\,$ meV. These numbers are given in terms of the present value of Hubble constant $H_0$ and the Plank energy $1/\sqrt{16 \pi G_N}$: they are $(H_0 M_{\rm P})^2$ for the energy density and $(H_0 M_{\rm P})^{1/2}$ for the dark matter constituent mass. The basic framework is a multi-scalar tensor gravity with non-trivial conformal coupling to the Ricci scalar curvature in the lagrangian density. The key for a right amount of dark energy is to incorporate in a novel way the spatially homogeneous kinetic contribution of Nambu-Goldstone modes in a spontaneously broken multi-scalar field sector. Proposed theories are made consistent with general relativity tests at small cosmological distances, yet are different from general relativity at cosmological scales. Dark matter is generated as spatially inhomogeneous component of the scalar system, with roughly comparable amount to the dark energy. In some presented models a cosmological bifurcation of symmetry breaking of scalar sector is triggered by the spontaneous breaking of electroweak SU(2) $\times $ U(1) gauge symmetry, hence the separation occurring simultaneously at the electroweak phase transition. The best experimental method to test presented models is to search for the fifth-force type of scalar exchange interaction with a force range, $O(10^{-2})$ cm, whose coupling to matter is basically of gravitational strength.

gr-qc↗

arXiv2021

Quantum dots as a probe of fundamental physics: Deviation from exponential decay law

We explore a possibility of measuring deviation from the exponential decay law in pure quantum systems. The power law behavior at late times of decay time profile is predicted in quantum mechanics, and has been experimentally attempted to detect, but with failures except a claim in an open system. It is found that electron tunneling from resonance state confined in man-made atoms, quantum dots, has a good chance of detecting the deviation and testing theoretical predictions. How initial unstable state is prepared influences greatly the time profile of decay law, and this can be used to set the onset time of the power law at earlier times. Comparison with similar process of nuclear alpha decay to discover the deviation is discussed, to explain why there exists a difficulty in this case.

quant-ph↗

arXiv2021

Parity violating magnetization at neutrino pair emission using trivalent lanthanoid ions

A new detection method using magnetization generated at triggered radiative emission of neutrino pairs (RENP), $ |e \rangle \rightarrow | g \rangle + \gamma + \sum_{ij}\nu_i \bar{\nu}_j $ (atomic de-transition from state $|e \rangle $ to state $|g \rangle$ emitting sum of neutrino pairs $\sum_{ij}\nu_i\bar{\nu}_j$ accompanied by a photon $\gamma$), is investigated in order to determine unknown neutrino properties; absolute neutrino masses of $\nu_i$ and Majorana/Dirac distinction. Magnetization associated with RENP events has parity violating component intrinsic to weak interaction enforced by crystal field effect in solids, and greatly helps background rejection of quantum electrodynamic (QED) origin even when these backgrounds are amplified. In proposed experiments we prepare a coherently excited body of trivalent lanthanoid ions, Er$^{3+}$ (a best candidate ion so far found), doped in a transparent dielectric crystal. The magnetic moment $\mu \langle \vec{S}\cdot\vec{k} \rangle/k $ arising from generated electron spin $\vec{S}$ parallel to trigger photon direction $\vec{k}/k$ is parity odd, and is absent in QED processes. The generated magnetic field of order nano gauss is stored in crystals long after pair emission event till spin relaxation time. An improved calculation method of coherent rate and angular distribution of magnetization is developed in order to incorporate finite size effect of crystal target beyond the infinite size limit in previous calculations.

hep-ph↗

arXiv2021

Selective Enhancement in Phonon Scattering leads to High Thermoelectric Figure of Merit in ZnO -- Graphene Oxide Core-shell Nanohybrids

ZnO is a promising candidate as an environment friendly thermoelectric (TE) material. However, the poor TE figure of merit (zT) needs to be addressed to achieve significant TE efficiency for commercial applications. Here we demonstrate that selective enhancement in phonon scattering leads to increase in zT of RGO encapsulated Al-doped ZnO core shell nanohybrids, synthesized via a facile and scalable method. The incorporation of 1 at% Al with 1.5 wt% RGO into ZnO (AGZO) has been found to show significant enhancement in zT (=0.52 at 1100 K) which is an order of magnitude larger compared to that of bare undoped ZnO. Photoluminescence and X-ray photoelectron spectroscopy measurements confirm that RGO encapsulation significantly quenches surface oxygen vacancies in ZnO along with nucleation of new interstitial Zn donor states. Tunneling spectroscopy reveals that the band gap of ~ 3.4 eV for bare ZnO reduces effectively to ~ 0.5 eV upon RGO encapsulation, facilitating charge transport. The electrical conductivity enhancement also benefits from the more than 95% densification achieved, using the spark plasma sintering method, which aids reduction of GO into RGO. The same Al doping and RGO capping synergistically brings about drastic reduction of thermal conductivity, through enhanced phonon-phonon and point defect-phonon scatterings. These opposing effects on electrical and thermal conductivities enhances the power factors as well as the zT value. Overall, a practically viable route for synthesis of oxide - RGO TE material which could find its practical applications for the high-temperature TE power generation.

cond-mat.mtrl-sci↗

arXiv2020

Persistent magnetization at neutrino pair emission

Measurement of parity violating magnetization is proposed as a means to determine neutrino properties such as Majorana/Dirac distinction and absolute neutrino masses. The process we use is radiative neutrino pair emission from a collective and coherent body of lanthanoid ions doped in host crystals. A vector-component of electron spin flip parallel to photon direction emitted from ion excited state generates this type of magnetization which is stored for a long time in crystals till spin relaxation time.

hep-ph↗

arXiv2019

Macro-coherent radiative emission of neutrino pair between parity-even atomic states

A new scheme to determine the neutrino mass matrix is proposed using atomic de-excitation between two states of a few eV energy spacing. The determination of the smallest neutrino mass of the order of 1 meV and neutrino mass type, Majorana or Dirac, becomes possible, if one can coherently excite more than 1 gram of atoms using two lasers.

hep-ph↗

arXiv2019

Neutrino mass spectroscopy using Er$^{3+}$ ions placed at inversion center of host crystals

We propose neutrino mass spectroscopy using Er$^{3+}$:Cs$_2$NaYF$_6$ or :Y$_2$O$_3$ crystal placed in hollow of a Bragg fiber as a target system. Unknown neutrino parameters and properties such as the lightest neutrino mass, Majorana/Dirac distinction, and CP violating phases can be explored by measuring scattered photons ($\gamma$) along the excitation (and fiber) axis by varying Raman trigger ($\gamma_0$) directions, in Er$^{3+}$ de-excitation process from $|e\rangle $ state to $|g\rangle $ state; $|e\rangle \,, | e\rangle + \gamma_0 \rightarrow | g\rangle + \gamma + \nu_i\bar{\nu}_j$, $\nu_i\,, i = 1, 2,3$ being a mass-resolved neutrino state. Rates and required level of QED background rejection are calculated using measured data of the target system.

hep-ph↗

arXiv2019

Divalent lanthanoid ions in crystals for neutrino mass spectroscopy

Electron spin flip in atoms or ions can cause neutrino pair emission, which provides a method to explore still unknown important neutrino properties by measuring spectrum of emitted photon in association, when electroweak rates are amplified by a phase coherence among participating atoms. Two important remaining neutrino issues to be determined are the absolute neutrino mass (or the smallest neutrino mass in the three-flavor scheme) and the nature of neutrino masses, either of Dirac type or of Majorana type. Use of Raman scattered photon was recently proposed as a promising tool for this purpose. In the present work we continue along this line to further identify promising ion targets in crystals, calculate neutrino pair emission rates, and study how to extract neutrino properties from Raman scattered photon angular distribution. Divalent lanthanoid ions in crystals, in particular Sm$^{2+}$, are the most promising, due to (1) its large number density, (2) sharp optical lines, (3) a variety of available ionic levels. Rejection of amplified quantum electrodynamic backgrounds is made possible to controllable levels by choosing a range of Raman trigger direction, when Sm$^{2+}$ sites are at O$_h$ inversion center of host crystals such as SrF$_2$.

hep-ph↗

arXiv2019

Raman stimulated neutrino pair emission

A new scheme using macroscopic coherence is proposed from a theoretical point to experimentally determine the neutrino mass matrix, in particular the absolute value of neutrino masses, and the mass type, Majorana or Dirac. The proposed process is a collective, coherent Raman scattering followed by neutrino-pair emission from an excited state $|e\rangle$ of a long lifetime to a lower energy state $|g\rangle$; $\gamma_0 + | e\rangle \rightarrow \gamma + \sum_{ij} \nu_i \bar{\nu_j} + | g\rangle $ with $ \nu_i \bar{\nu_j}$ consisting of six massive neutrino-pairs. Calculated angular distribution has six $(ij)$ thresholds of massive neutrino-pair emission which show up as steps at different angles in the distribution. Angular locations of thresholds and event rates of the angular distribution make it possible to experimentally determine the smallest neutrino mass to the level of less than 1 meV (accordingly all three masses using neutrino oscillation data) , the mass ordering pattern , normal or inverted, and to distinguish whether neutrinos are of Majorana or Dirac type. Event rates of neutrino-pair emission, when the mechanism of macroscopic coherence amplification works, may become large enough for realistic experiments by carefully selecting certain types of target atoms or ions doped in crystals. The problem to be overcome is macro-coherently amplified quantum electrodynamic background of the process, $\gamma_0 + | e\rangle \rightarrow \gamma +\gamma_2 + \gamma_3+ | g\rangle $, when two extra photons, $\gamma_2\,, \gamma_3$, escape detection. We illustrate our idea using neutral Xe and trivalent Ho ion doped in dielectric crystals.

hep-ph↗