arXiv subjects

T. Mito

Publications and source records attributed to T. Mito.

At least 19 recordsLinked to original sources

arXiv2023

Efficient NMR measurement and data analysis supported by the Bayesian inference : The case of the heavy fermion compound YbCo2Zn20

We propose a data-driven technique to infer microscopic physical quantities from nuclear magnetic resonance(NMR) spectra, in which the data size and quality required for the Bayesian inference are investigated. The $^{59}$Co-NMR measurement of YbCo$_2$Zn$_{20}$ single crystal generates complex spectra with 28 peaks. By exploiting the site symmetry in the crystal structure, the isotropic Knight shift $K_{iso}$ and nuclear quadrupole resonance(NQR) frequency $\nu_Q$ were estimated to be $K_{iso} = 0.7822 \pm 0.0090 \%$, $\nu_Q = 2.008 \pm 0.016$ MHz ( T = 20 K, H $\simeq$ 10.2 T) by analyzing only 30 data points from one spectrum. The estimation of $\nu_Q$ is consistent with the precise value obtained in the NQR experiment. Our method can significantly reduce the measurement time and the computational cost of data analysis in NMR experiments.

cond-mat.mtrl-sci↗

arXiv2020

Nonmagnetic-magnetic transition and magnetically ordered structure in SmS

SmS, a prototypical intermediate valence compound, has been studied by performing high-pressure nuclear magnetic resonance measurements on a $^{33}$S-enriched sample. The observation of an additional signal below 15-20 K above a nonmagnetic-magnetic transition pressure $P_{\rm c2} \approx 2$ GPa gives evidence of a magnetic transition. The absence of a Curie-term in the Knight shift near $P_{\rm c2}$ indicates that the localized character of $4f$ electrons is entirely screened and the mechanism of the magnetic ordering is not described within a simple localized model. Simultaneously, the line shape in the magnetically ordered state is incompatible with a spin density wave order. These suggest that the magnetic order in SmS may require an understanding beyond the conventional framework for heavy fermions. The fact that hyperfine fields from the ordered moments cancel out at the S site leads us to a conclusion that the ordered phase has a type II antiferromagnetic structure.

cond-mat.str-el↗

arXiv2019

Pressure induced evolution of band structure in black phosphorus studied by $^{31}$P-NMR

Two-dimensional layered semiconductor black phosphorus (BP), a promising pressure induced Dirac system as predicted by band structure calculations, has been studied by $^{31}$P-nuclear magnetic resonance. Band calculations have been also carried out to estimate the density of states $D(E)$. The temperature and pressure dependences of nuclear spin lattice relaxation rate $1/T_1$ in the semiconducting phase are well reproduced using the derived $D(E)$, and the resultant pressure dependence of semiconducting gap is in good accordance with previous reports, giving a good confirmation that the band calculation on BP is fairly reliable. The present analysis of $1/T_1$ data with the complemental theoretical calculations allows us to extract essential information, such as the pressure dependences of $D(E)$ and chemical potential, as well as to decompose observed $1/T_1$ into intrinsic and extrinsic contributions. An abrupt increase in $1/T_1$ at 1.63GPa indicates that the semiconducting gap closes, resulting in an enhancement of conductivity.

cond-mat.mtrl-sci↗

arXiv2019

Evidence for gradual evolution of low-energy fluctuations underlying the first-order structural and valence order in YbPd

The valence orders at $T_a=125$ K and $T_b=105$ K in the cubic compound YbPd have been investigated by $^{105}$Pd-nuclear magnetic resonance (NMR) measurements. Significant decrease in the density of states at the Fermi energy below $T_a$ is evident from the measurement of nuclear spin lattice relaxation rate $1/T_1$, suggesting that the instabilities of Fermi surface are associated with the transitions. Moreover we observed the unusual evolution of low-energy fluctuations toward the valence transition at $T_a$ behind its drastic first-order nature. The structural transition accompanying the valence order may occur as a result of cooperative effect of Fermi surface and valence instabilities.

cond-mat.str-el↗

arXiv2016

Novel Kondo-like behavior near magnetic instability in SmB$_6$ : temperature and pressure dependences of Sm valence

We report a systematic study of Sm valence in the prototypical intermediate valence compound SmB$_6$. Sm mean valence, $v_{\rm Sm}$, was measured by X-ray absorption spectroscopy as functions of pressure ($1<P<13$ GPa) and temperature ($3<T<300$ K). Pressure induced magnetic order (MO) was detected above $P_c = 10$ GPa by resistivity measurements. A shift toward localized $4f$ state with increasing $P$ and/or $T$ is evident from an increase in $v_{\rm Sm}$. However $v_{\rm Sm}$ at $P_c$ is anomalously far below 3, which differs from the general case of nonmagnetic-magnetic transition in Yb and Ce compounds. From the $T$ dependence of $v_{\rm Sm}(P,T)$, we found that $v_{\rm Sm}(P,T)$ consists of two different characteristic components: one is associated with low-energy electronic correlations involving Kondo like behavior, and the other with high-energy valence fluctuations.

cond-mat.str-el↗

arXiv2016

Mott transition in the A15 phase of Cs$_{3} $C$_{60}$: absence of pseudogap and charge order

We present a detailed NMR study of the insulator to metal transition induced by an applied pressure $p$ in the A15 phase of Cs$_{3}$C$_{60}$. We evidence that the insulating antiferromagnetic (AF) and superconducting (SC) phases only coexist in a narrow $p$ range. At fixed $p$, in the metallic state above the SC transition $T_c$, the $^{133}$Cs and $^{13}$C NMR spin lattice relaxation data are seemingly governed by a pseudogap like feature. We prove that this feature, also seen in the $^{133}$Cs NMR shift data is rather a signature of the Mott transition, which broadens and smears out progressively for increasing $(p,T)$. The analysis of the variation of the quadrupole splitting $\nu _{Q}$ of the $^{133}$Cs NMR spectrum precludes any cell symmetry change at the Mott transition and only monitors a weak variation of lattice parameter. These results open an opportunity to consider theoretically the Mott transition in a multiorbital three dimensional system well beyond its critical point.

cond-mat.str-el↗

arXiv2014

NMR study of the Superconducting gap variation near the Mott transition in Cs$_{3}$C$_{60}$

Former extensive studies of superconductivity in the \textit{A}$_{3}$C$_{60}$ compounds, where \textit{A} is an alkali, have led to consider that Bardeen Cooper Schrieffer (BCS) electron-phonon pairing prevails in those compounds, though the incidence of electronic Coulomb repulsion has been highly debated. The discovery of two isomeric fulleride compounds Cs$_{3}$C$_{60}$ which exhibit a transition with pressure from a Mott insulator (MI) to a superconducting (SC) state clearly re-opens that question. Using pressure ($p$) as a single control parameter of the C$_{60}$ balls lattice spacing, one can now study the progressive evolution of the SC properties when the electronic correlations are increased towards the critical pressure $p_{c}$ of the Mott transition. We have used $^{13}$C and $^{133}$Cs NMR measurements on the cubic phase A15-Cs$_{3}$C$_{60}$ just above $p_{c}=5.0(3)$ kbar, where the SC transition temperature $T_{c}$ displays a dome shape with decreasing cell volume. From the $T$ dependence below $T_{c}$ of the nuclear spin lattice relaxation rate $(T_{1})^{-1}$ we determine the electronic excitations in the SC state, that is $2Δ$, the SC gap value. We find that $2Δ$ increases with decreasing $p$ towards $p_{c}$, where $T_{c}$ decreases on the SC dome, so that $2Δ/k_{B}T_{c}$ increases regularly upon approaching the Mott transition. These results bring clear evidence that the increasing correlations near the Mott transition are not significantly detrimental to SC. They rather suggest that repulsive electron interactions might even reinforce elecron-phonon SC, being then partly responsible for the large $T_{c}$ values, as proposed by theoretical models taking the electronic correlations as a key ingredient.

cond-mat.supr-con↗

arXiv2004

Unconventional Superconductivity in Heavy Fermion Systems

We review the studies on the emergent phases of superconductvity and magnetism in the $f$-electron derived heavy-fermion (HF) systems by means of the nuclear-quadrupole-resonance (NQR) under pressure. These studies have unraveled a rich variety of the phenomena in the ground state of HF systems. In this article, we highlight the novel phase diagrams of magnetism and unconventional superconductivity (SC) in CeCu$_2$Si$_2$, HF antiferromagnets CeRhIn$_5$, and CeIn$_3$. A new light is shed on the difference and common features on the interplay between magnetism and SC on the magnetic criticality.

cond-mat.str-el↗

arXiv2004

Novel superconductivity on the magnetic criticality in heavy-fermion systems : a systematic study of NQR under pressure

We report the discovery of exotic superconductivity (SC) and novel magnetism in heavy-fermion (HF) compounds, CeCu$_2$Si$_2$, CeRhIn$_5$ and CeIn$_3$ through nuclear-quadrupole-resonance (NQR) measurements under pressure ($P$). The exotic SC in a homogeneous CeCu$_2$Si$_2$ revealed antiferromagnetic critical fluctuations at the border to antiferromagnetism (AFM) or marginal AFM. The uniform mixed phase of SC and AFM in CeCu$_2$(Si$_{1-x}$Ge$_x$)$_2$ emerges on a microscopic level, once a tiny amount of 1%Ge($x=0.01$) is substituted for Si to expand its lattice. The application of minute pressure ($P\sim 0.19$ GPa) suppresses the sudden emergence of the AFM caused by doping Ge. The persistence of the low-lying magnetic excitations at temperatures lower than $T_c$ and $T_N$ is ascribed due to the uniform mixed phase of SC and AFM. Likewise, the $P$-induced HF superconductor CeRhIn$_5$ coexists with AFM on a microscopic level in $P = 1.5$ - 1.9 GPa. The unconventional gapless nature of SC in the low-lying excitation spectrum emerges due to the uniform mixed phase of AFM and SC. By contrast, in CeIn$_3$, we propose that the magnetic excitations such as spin-density fluctuations induced by the first-order phase transition from the AFM to the paramagnetism (PM) might mediate attractive interaction to form the Cooper pairs in the novel phase of AFM.

cond-mat.str-el↗

arXiv2003

Exotic superconductivity in the coexistent phase of antiferromagnetism and superconductivity in CeCu2(Si0.98Ge0.02)2: A Cu-NQR study under hydrostatic pressure

We report a pressure ($P$) effect on CeCu$_2$(Si$_{0.98}$Ge$_{0.02}$)$_2$ where an antiferromagnetic (AFM) order at $T_N \sim$ 0.75 K coexists with superconductivity below $T_c \sim$ 0.4 K\@. At pressures exceeding $P = 0.19$ GPa, the AFM order is suppressed, which demonstrates that the sudden emergence of AFM order due to the Ge doping is ascribed to the intrinsic lattice expansion. The exotic superconductivity at $P = 0$ GPa is found to evolve into a typical heavy-fermion one with a line-node gap above $P = 0.91$ GPa\@. We highlight that the anomalous enhancement in nuclear spin-lattice relaxation rate $1/T_1$ that follows a $T_1T$ = const. behavior well below $T_c$ at $P$ = 0 GPa is characterized by the persistence of low-lying magnetic excitations, which may be inherent to the coexistent state of antiferromagnetism and superconductivity.

cond-mat.str-el↗

arXiv2003

Gapless Magnetic and Quasiparticle Excitations due to the Coexistence of Antiferromagnetism and Superconductivity in CeRhIn$_5$ : A study of $^{115}$In-NQR under Pressure

We report systematic measurements of ac-susceptibility, nuclear-quadrupole-resonance spectrum, and nuclear-spin-lattice-relaxation time ($T_1$) on the pressure ($P$)- induced heavy-fermion (HF) superconductor CeRhIn$_5$. The temperature ($T$) dependence of $1/T_1$ at $P$ = 1.6 GPa has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at $T_N = 2.8$ K and $T^{MF}_c$ = 0.9 K. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below $T_c^{onset} = 2$ K, but $T_c^{MF} = 0.9$ K, followed by a $T_1T$ = const law. These results point to the unconventional characteristics of SC coexisting with AFM. We highlight that both of the results deserve theoretical work on the gapless nature in low-lying excitation spectrum due to the coexistence of AFM and SC and the lack of the mean-field regime below $T_c^{onset} = 2$ K.

cond-mat.str-el↗

arXiv2003

Anisotropic spin fluctuations in heavy-fermion superconductor CeCoIn5: In-NQR and Co-NMR studies

We report In-NQR and Co-NMR experiments of CeCoIn$_5$ that undergoes a superconducting transition with a record high $T_{\rm c}$ = 2.3 K to date among heavy-fermion superconductors. At zero magnetic field, an anomalous temperature ($T$) dependence of nuclear spin-lattice relaxation rate $1/T_1$ of $^{115}$In is explained by the relation $1/T_1\propto T\cdotχ_Q(T)^{3/4}$ based on the anisotropic spin-fluctuations model in case of the proximity to an antiferromagnetic (AFM) quantum critical point (QCP). The novel behavior of $1/T_1\sim T^{1/4}$ over a wide $T$ range of $T_{\rm c} < T < 40$ K arises because the staggered susceptibility almost follows the Curie law $χ_Q(T)\propto 1/(T+θ)$ with $θ= 0.6$ K and hence $1/T_1\propto T/(T+0.6)^{3/4}\sim T^{1/4}$ for $θ< T$. We highlight that the behavior $1/T_1\sim T^{1/4}$ is due to the proximity to the anisotropic AFM QCP relevant with its layered structure, and is not associated with the AFM QCP for isotropic 3D systems. We have also found that the AFM spin fluctuations in CeCoIn$_5$ are suppressed by small magnetic field so that $θ=0.6$ K at $H$=0 increases to $θ= 2.5$ K at $H$ = 1.1 T, reinforcing that CeCoIn$_5$ is closely located at the QCP.

cond-mat.str-el↗

arXiv2003

Coexistence or Separation of the Superconducting, Antiferromagnetic, and Paramagnetic Phases in Quasi One-Dimensional (TMTSF)2PF6 ?

We report on experimental studies of the character of phase transitions in the quasi-1D organic compound (TMTSF)2PF6 in the close vicinity of the borders between the paramagnetic metal PM, antiferromagnetic insulator AF, and superconducting SC states. In order to drive the system through the phase border P_0(T_0), the sample was maintained at fixed temperature T and pressure P, whereas the critical pressure P_0 was tuned by applying the magnetic field B. In this approach, the magnetic field was used (i) for tuning (P-P_0), and (ii) for identifying the phase composition (due to qualitatively different magnetoresistance behavior in different phases). Experimentally, we measured R(B) and its temperature dependence R(B,T) in the pressure range (0 - 1)GPa. Our studies focus on the features of the magnetoresistance at the phase transition between the PM and AF phases, in the close vicinity to the superconducting transition at T~1K. We found pronounced history effects arising when the AF/PM phase border is crossed by sweeping the magnetic field: the resistance depends on a trajectory which the system arrives at a given point of the P-B-T phase space. In the transition from the PM to AF phase, the features of the PM phase extends well into the AF phase. At the opposite transition from the AF to PM phase, the features of the AF phase are observed in the PM phase. These results evidence for a macroscopically inhomogeneous state, which contains macroscopic inclusions of the minority phase. When the system is driven away from the transition, the homogeneous state is restored; upon a return motion to the phase boundary, no signatures of the minority phase are observed up to the very phase boundary.

cond-mat.str-el↗

arXiv2002

Coexistence of Antiferromagnetism and Superconductivity near the Quantum Criticality in Heavy Fermion Compound CeRhIn$_5$

We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in a heavy-fermion compound CeRhIn$_5$ under pressure $P=1.75$ GPa. The onset of the magnetic order is evidenced from a clear split of $^{115}$In-NQR spectrum due to the spontaneous internal field below the Néel temperature $T_N=2.5$ K. Simultaneously, bulk SC below $T_c=2.0$ K is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic level.

cond-mat.str-el↗

arXiv2002

^{115}In-NQR evidence for unconventional superconductivity in CeIn_3 under pressure

We report evidence for unconventional superconductivity in CeIn_3 at a pressure P = 2.65 GPa above critical pressure (P_c ~ 2.5 GPa) revealed by the measurements of nuclear-spin-lattice-relaxation time (T_1) and ac-susceptibility (ac-chi). Both the measurements of T_1 and ac-chi have pointed to a superconducting transition at T_c = 95 mK, which is much lower than an onset temperature T_{onset} = 0.15 K at zero resistance. The temperature dependence of 1/T_1 shows no coherence peak just below T_c, indicative of an unconventional nature for the superconductivity induced in CeIn_3.

cond-mat.str-el↗

arXiv2002

Coexistence of antiferromagnetism and superconductivity in heavy-fermions systems

We report the novel pressure(P)-temperature(T) phase diagrams of antiferromagnetism (AF) and superconductivity (SC) in CeRhIn$_5$, CeIn$_3$ and CeCu$_2$Si$_2$ revealed by the NQR measurement. In the itinerant helical magnet CeRhIn$_5$, we found that the Néel temperature $T_N$ is reduced at $P \geq$ 1.23 GPa with an emergent pseudogap behavior. The coexistence of AF and SC is found in a narrow P range of 1.63 - 1.75 GPa, followed by the onset of SC with line-node gap over a wide P window 2.1 - 5 GPa. In CeIn$_3$, the localized magnetic character is robust against the application of pressure up to $P \sim$ 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations. In CeCu$_2$Si$_2$, the SC and AF coexist on a microscopic level once its lattice parameter is expanded. We remark that the underlying marginal antiferromagnetic state is due to collective magnetic excitations in the superconducting state in CeCu$_2$Si$_2$. An interplay between AF and SC is discussed on the SO(5) scenario that unifies AF and SC. We suggest that the SC and AF in CeCu$_2$Si$_2$ have a common mechanism.

cond-mat.str-el↗

arXiv2001

Pressure-Temperature Phase Diagram of Antiferromagnetism and Superconductivity in CeRhIn5 and CeIn3 : In-NQR Study under Pressure

We report the novel pressure($P$) - temperature($T$) phase diagram of antiferromagnetism and superconductivity in CeRhIn$_5$ and CeIn$_3$ revealed by the $^{115}$In nuclear-spin-lattice-relaxation ($T_1$) measurement. In the itinerant magnet CeRhIn$_5$, we found that the Néel temperature $T_N$ is reduced at $P \geq$ 1.23 GPa with an emergent pseudogap behavior. In CeIn$_3$, the localized magnetic character is robust against the application of pressure up to $P \sim$ 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations.

cond-mat.str-el↗

arXiv2001

Unexpected Metallic-like Behavior of the Resistance in the Dielectric Spin Density Wave State in (TMTSF)2PF6

We report unexpected features of the transport in the dielectric spin density wave (SDW) phase of the quasi one-dimensional compound (TMTSF)_2PF_6: the resistance exhibits a maximum and a subsequent strong drop as temperature decreases below approximately 2K. The maximum in R(T) is not caused by depinning or Joule heating of the SDW. The characteristic temperature of the R(T) maximum and the scaling behavior of the resistance at different magnetic fields B evidence that the non-monotonic R(T) dependence has an origin different from the one known for the quantum Hall effect region of the phase diagram. We also found that the borderline T_0(B,P) which divides the field induced SDW region of the P-B-T phase diagram into the hysteresis and non-hysteresis domains, terminates in the N=1 sub-phase; the borderline has thus no extension to the SDW N=0 phase.

cond-mat.mes-hall↗