arXiv subjects

D. Aoki

Publications and source records attributed to D. Aoki.

88 records · Page 5Linked to original sources

arXiv2008

A signature of hidden order in URu2Si2 : the excitation at the wavevector Q0 = (100)

Simultaneous neutron-scattering and thermal expansion measurements on the heavy-fermion superconductor URu2Si2 under hydrostatic pressure of 0.67 GPa have been performed in order to detect the successive paramagnetic, hidden order, and large moment antiferromagnetic phases on cooling. The temperature dependence of the sharp low energy excitation at the wavevector Q_0=(100) shows that this excitation is clearly a signature of the hidden order state. In the antiferromagnetic phase, this collective mode disappears. The higher energy excitation at the incommensurate wavevector Q_1=(1.4,0,0) persists in the antiferromagnetic phase but increases in energy. The collapse of the inelastic neutron scattering at Q_0 coincides with the previous observation of the disappearance of superconductivity.

cond-mat.str-el↗

arXiv2007

Skutterudite Results Shed Light on Heavy Fermion Physics

Only few selected examples among the great diversity of anomalous rare earth skutterudite are reviewed. Focus is first given on PrFe4P12 in comparison with URu2Si2. For PrFe4P12, great progress has been made on determining the nature of the order parameter (OP). A non magnetic order parameter with a multipolar component emerges here while for URu2Si2 the nature of the so-called hidden order remains mysterious. The two systems have several similarities in their temperature--pressure (T, P) and magnetic field--temperature (H, T) phase diagrams, in their spin dynamics, in their nesting character and in their high sensitivity to impurities. Advances on one side must stimulate new views on the other. Besides general considerations on the choice of the OP, a simple basic problem is the treatment of the Kondo coupling in a system with low charge carrier number for the cases of uncompensated and compensated semi-metal. An interesting problem is also the possible decoupling between exciton modes and itinerant carriers.

cond-mat.str-el↗

arXiv2007

Collapse of antiferromagnetism in CeRh2Si2 : volume versus entropy

The thermal expansion of the heavy fermion compound CeRh2Si2 has been measured under pressure as a function of temperature using strain gages. A large anomaly associated to the Neel temperature has been detected even above the suspected critical pressure Pc = 1.05 GPa where no indication of antiferromagnetism has been observed in calorimetry experiments sensitive to the entropy change. An unexpected feature is the pressure slowdown of the antiferromagnetic-paramagnetic transition by comparison to the fast pressure collapse predicted for homogeneous first order quantum phase transition with one unique pressure singularity at Pc. A large pressure dependance is observed in the anisotropy of the thermal expansion measured parallel or perpendicular to the c axis of this tetragonal crystal. The Fermi surface reconstruction associated to the first order transition produces quite different pressure response in the transport scattering measured along different crystallographic directions. A brief discussion is made on other examples of first order quantum transitions in strongly correlated electronic systems : MnSi and CeCoIn5.

cond-mat.str-el↗

arXiv2007

Magnetic structure of CeRhIn$_{5}$ under magnetic field

The magnetically ordered ground state of CeRhIn$_{5}$ at ambient pressure and zero magnetic field is an incomensurate helicoidal phase with the propagation vector $\bf{k}$=(1/2, 1/2, 0.298) and the magnetic moment in the basal plane of the tetragonal structure. We determined by neutron diffraction the two different magnetically ordered phases of CeRhIn$_{5}$ evidenced by bulk measurements under applied magnetic field in its basal plane. The low temperature high magnetic phase corresponds to a sine-wave structure of the magnetization being commensurate with $\bf{k}$=(1/2, 1/2, 1/4). At high temperature, the phase is incommensurate with $\bf{k}$=(1/2, 1/2, 0.298) and a possible small ellipticity. The propagation vector of this phase is the same as the one of the zero-field structure.

cond-mat.str-el↗

arXiv2006

Interplay between different states in heavy fermion physics

Calorimetry experiments under high pressure were used to clarify the interplay between different states such as superconductivity and antiferromagnetism in CeRhIn5, spin density wave and large moment antiferromagnetism in URu2Si2. Evidences are given on the re-entrance of antiferromagnetism under magnetic field in the superconducting phase of CeRhIn5 up to pc = 2.5 GPa where the Neel temperature will collapse in the absence of superconductivity. For URu2Si2 measurements up to 10 GPa support strongly the coexistence of spin density wave and large moment antiferromagnetism at high pressures.

cond-mat.str-el↗

arXiv2005

Coexistence of antiferromagnetism and superconductivity in CeRhIn5 under high pressure and magnetic field

We report on detailed ac calorimetry measurements under high pressure and magnetic field of CeRhIn5. Under hydrostatic pressure the antiferromagnetic order vanishes near p_c*=2 GPa due to a first order transition. Superconductivity is found for pressures above 1.5 GPa inside the magnetic ordered phase. However, the superconductivity differ from the pure homogeneous superconducting ground state above 2 GPa. The application of an external magnetic field H || ab induces a transition inside the superconducting state above pc* which is strongly related to the re-entrance of the antiferromagnetism with field. This field-induced supplementary state vanishes above the quantum critical point in this system. The analogy to CeCoIn5 is discussed.

cond-mat.str-el↗

arXiv2005

Magnetism and superconductivity of heavy fermion matter

The interplay of magnetism and unconventional superconductivity (d singlet wave or p triplet wave) in strongly correlated electronic system (SCES) is discussed with recent examples found in heavy fermion compounds. A short presentation is given on the formation of the heavy quasiparticle with the two sources of a local and intersite enhancement for the effective mass. Two cases of the coexistence or repulsion of antiferromagnetism and superconductivity are given with CeIn3 and CeCoIn5. A spectacular example is the emergence of superconductivity in relatively strong itinerant ferromagnets UGe2 and URhGe. The impact of heavy fermion matter among other SCES as organic conductor or high Tc oxide is briefly pointed out.

cond-mat.str-el↗

arXiv2005

Evidence for a new magnetic field scale in CeCoIn5

The Nernst coefficient of CeCoIn5 displays a distinct anomaly at H_K ~ 23 T. This feature is reminiscent of what is observed at 7.8 T in CeRu2Si2, a well-established case of metamagnetic transition. New frequencies are observed in de Haas-van Alphen oscillations when the field exceeds 23 T, which may indicate a modification of the Fermi surface at this field.

cond-mat.str-el↗

arXiv2005

Anisotropic superconducting gap in transuranium superconductor PuRhGa$_{5}$: Ga NQR study on a single crystal

^{69,71}Ga NMR/NQR studies have been performed on a single crystal of the transuranium superconductor PuRhGa_{5} with Tc ~ 9 K. We have observed a ^{69}Ga NQR line at ~29.15 MHz, and assigned it to the $4i$ Ga site using the NMR results. The ^{69}Ga NQR spin-lattice relaxation rate 1/T_{1} shows no coherence peak just below Tc, but obeys a T^{3} behavior below Tc. This result strongly suggests that PuRhGa_{5} is an unconventional superconductor having an anisotropic superconducting gap.

cond-mat.supr-con↗

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↗

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

Itinerant metamagnetism of CeRu2Si2 : bringing out the dead. Comparison with the new Sr3Ru2O7 case

Focus is given on the macroscopic and microscopic experimental works realized during a decade on the clear case of itinerant metamagnetism in the heavy fermion paramagnetic compound CeRu2Si2 . Emphasis is made on the feedback between the band structure, the exchange coupling and the lattice instability. Sweeps in magnetic field, pressure and temperature feel the pseudogap of this strongly correlated electronic system as well as its equivalent CeRu2Ge2 at a fictitious negative pressure. Some mysteries persist as the complete observation of the FS above the metamagnetic field HM and the detection of the dynamical ferromagnetic fluctuation near HM. The novelty of the bilayer ruthenate Sr3Ru2O7 is discussed by comparison. Despite differences in spin and electronic dimensionality many common trends emerge.

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

Unique Spin Dynamics and Unconventional Superconductivity in the Layered Heavy Fermion Compound CeIrIn_5:NQR Evidence

We report measurements of the ^{115}In nuclear spin-lattice relaxation rate (1/T_1) between T=0.09 K and 100 K in the new heavy fermion (HF) compound CeIrIn_5. At 0.4 K < T < 100 K, 1/T_1 is strongly T-dependent, which indicates that CeIrIn_5 is much more itinerant than known Ce-based HFs. We find that 1/T_1T, subtracting that for LaIrIn_5, follows a 1/(T+θ)^{3/4} variation with θ=8 K. We argue that this novel feature points to anisotropic, due to a layered crystal structure, spin fluctuations near a magnetic ordering. The bulk superconductivity sets in at 0.40 K below which the coherence peak is absent and 1/T_1 follows a T^3 variation, which suggests unconventional superconductivity with line-node gap.

cond-mat.supr-con↗

arXiv2001

Pressure-induced anomalous magnetism and unconventional superconductivity in CeRhIn5 : 115In-NQR Study under Pressure

We report $^{115}$In nuclear-quadrupole-resonance (NQR) measurements of the pressure($P$)-induced superconductor CeRhIn$_5$ in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field $H_{int}$ at the In site is substantially reduced from $H_{int}=1.75$ kOe at P=0 to 0.39 kOe at $P=1.23$ GPa, while the Néel temperature slightly changes with increasing $P$. This suggests that either the size in the ordered moment $M_{Q}(P)$ or the angle $θ(P)$ between the direction of $M_{Q}(P)$ and the tetragonal $c$ axis is extrapolated to zero at $P^*=1.6 \pm 0.1$ GPa at which a bulk SC transition is no longer emergent. In the SC state at $P=2.1$ GPa, the nuclear spin-lattice relaxation rate $^{115}(1/T_1)$ has revealed a $T^3$ dependence without the coherence peak just below $T_c$, giving evidence for the unconventional superconductivity. The dimensionality of the magnetic flutuations in the normal state are also discussed.

cond-mat.str-el↗