Hidekazu Aoki

Exotic Heavy-Fermion State in Filled Skutterudite SmOs4Sb12

In this paper recent substantial progress in applying the density-matrix renormalization-group (DMRG) to the simulation of the time-evolution of strongly correlated quantum systems in one dimension is reviewed. Various approaches to generating a time-evolution numerically are considered. The key focus of this review is on current strategies to circumvent the limitations of the quite small subspace well approximated by DMRG, by either enlarging or changing it as time evolves. All these approaches can be extended to the simulation of mixed, i.e. finite-temperature states. While this paper is phrased almost entirely in standard DMRG language, I finish by considering the alternative formulation of time-evolutions in the language of matrix product states which is less well-known but conceptually more powerful.Specific heat and transport measurements have revealed an unconventional heavy-fermion (HF) state in SmOs 4 Sb 12 single crystals. The electronic specific-heat coefficient (γ=0.82 J/K 2 mol) and the coefficient ( A ) of the quadratic temperature dependence of electrical resistivity are largely enhanced, although the ratio A γ -2 is reduced from the Kadowaki–Woods ratio of HF materials. Both γ and A do not show any significant decrease in applied field in contrast with Ce-based HF compounds, suggesting an unconventional origin of the heavy quasiparticles. A weak ferromagnetic ordering sets in below ∼3 K, probably originating in the itinerant quasiparticles.Specific heat and transport measurements have revealed an unconventional heavy-fermion (HF) state in SmOs 4 Sb 12 single crystals. The electronic specific-heat coefficient (γ=0.82 J/K 2 mol) and the coefficient ( A ) of the quadratic temperature dependence of electrical resistivity are largely enhanced, although the ratio A γ -2 is reduced from the Kadowaki–Woods ratio of HF materials. Both γ and A do not show any significant decrease in applied field in contrast with Ce-based HF compounds, suggesting an unconventional origin of the heavy quasiparticles. A weak ferromagnetic ordering sets in below ∼3 K, probably originating in the itinerant quasiparticles.

Field-Induced Gap Formation in Yb4As3.

The field-induced gap observed in the specific heat of Yb 4 As 3 is interpreted as being due to the effective staggered field introduced by the alternating g -tensor and alternating Dzyaloshinsky-Moriya interactions. This mechanism also provides a natural explanation for the anomalous increase of magnetic susceptibility at low temperatures. A specific prediction is made for the dependence of the gap on the field direction.

Nodal Structures of Heavy Fermion Superconductors Probed by the Specific-Heat Measurements in Magnetic Fields

Heavy electron superconductors mostly have anisotropic gap function which vanishes (has nodes) for certain directions in the momentum space. Since the nodal structure is closely related to the pairing mechanism, its experimental determination is very important. In anisotropic superconductors, low energy spectrum of the quasiparticles in the vortex state much depends on the nodal structure. In particular, the electronic specific heat has been demonstrated to exhibit the characteristic dependence on the angle between the field and the nodal direction, thereby the nodal structure can be probed experimentally. In this contribution, we present our recent experimental results on the field-orientation dependence of the specific heat on f electron superconductors CeRu 2 , CeCoIn 5 , PrOs 4 Sb 12 , and URu 2 Si 2 , and discuss their gap structures.

Nonmagnetic Ground State in Fully Filled PrxFe4Sb12 (x=1.0) Synthesized under High Pressure

We have succeeded in synthesizing the filled skutterudite Pr x Fe 4 Sb 12 with the Pr-site filling fraction x close to 1 under high pressure. The residual resistivity of ∼24 µΩ·cm is several times smaller than that of samples with x ∼0.8 synthesized under ambient pressure by independent research groups, indicating that highly improved sample quality can be achieved by reducing the Pr-site vacancy. From the transport, magnetic, and thermal measurements of the present sample with x ∼1, we have found a singlet ground state of the crystalline electric field (CEF) for 4 f -electrons and no phase transition down to 0.15 K, in contrast to the reported magnetic ordering near 5 K in the samples with x ∼0.8. The effect of the Pr-site vacancy on the CEF level scheme and the 3 d -electron density of states near the Fermi level is discussed as a possible origin of such a drastic change in the magnetic ground state.

Strongly correlated electron system without free carrier Yb4(As1-xPx)3

Abstract Mixed valence compound Yb4As3 which shows the charge ordering of Yb3+ and Yb2+ around 300 K has been considered to be a heavy fermion system with extremely low carrier concentration. So the main interest in this system is to know whether the heavy fermion behavior surely comes from such low carrier concentration. To make this situation clear, single crystals of Yb4(As1-xPx)3 were grown and their transport properties, magnetic susceptibility and specific heat were measured. With increasing concentration of phosphor the system becomes semiconductive, however, the specific heat and magnetic susceptibility are almost the same as those of pure Yb4As3. This result shows that the large γ-value of Yb4As3 is not due to the mass enhancement as seen in usual heavy fermion systems but arises from another magnetic correlation.

Low-energy excitations of the semimetallic one-dimensional S=1/2 antiferromagnet Yb4As3 ☆

We investigate a new route to quasi-one-dimensional spin-chain systems which originates from the charge-ordering transition out of a homogeneous mixed-valence state. We present evidence that Yb4As3 and P, Sb doped mixed crystals are well described by this mechanism. Many thermodynamic and low-temperature transport heavy-fermion-like properties can be explained by the existence of low-lying quasi-one-dimensional spin excitations in the Yb3+-chains. The observation of soliton excitations in a transverse external field gives further support to the existence of spin chains in Yb4As3. We also present recent results on spin-glass behavior at very low temperature caused by the interchain coupling and disorder. In addition, the yet unexplained magnetotransport effects are discussed.

Itinerant-Electron Weak Ferromagnetism in La-Based Filled Skutterudite LaFe4As12

High-quality samples of LaFe 4 As 12 with a residual resistivity ratio of 64 have been successfully synthesized under high pressure ∼4 GPa at 1100 K. The magnetization, specific heat, electrical resistivity, and Hall coefficient of the samples were measured. All the investigated properties exhibit evident anomalies at 5.2 K, thereby indicating a ferromagnetic transition. The large Rhodes–Wohlfarth ratio (≡ the ratio of the effective moment deduced from the Curie constant to the saturation moment per magnetic ion) of 32 suggests that LaFe 4 As 12 is the first La-based filled skutterudite that exhibits itinerant-electron weak ferromagnetism, which is realized by the high density of states at the Fermi level D ( E F ). The ferromagnetism is unexpected within the simple Stoner-type argument, because D ( E F ) lies between those of an ordinary Pauli paramagnet LaFe 4 P 12 and an enhanced Pauli paramagnet LaFe 4 Sb 12 in the LaFe 4 X 12 (X = P, As, Sb) series. We discuss this unexpected ferromagnetic transition...

Systematic study in mixed valence system Yb4(As1−xSbx)3

Abstract In Yb pnictides with anti-Th 3 p 4 structure, the valence of Yb decreases from about 2.25 in Yb 4 As 3 to about 2 in Yb 4 Bi 3 with increasing weight of pnictogen. In Yb 4 As 3 , charge ordering around 300 K is observed and in the charge ordered phase heavy fermion behavior is seen in spite of extremely low carrier concentration. On the other hand, Yb 4 Sb 3 is a valence fluctuating system. So, to make systematic studies from Yb 4 As 3 to Yb 4 Sb 3 we grew single crystals of Yb 4 (As 1− x Sb x ) 3 . With increasing Sb content, the absolute value of the resistivity decreases due to the increase of the number of carriers and the charge ordering temperature decreases reflecting the increase of the screening effect, and at 29% charge ordering seems to disappear. The γ-value, however, is still large even in Sb 29% sample. The large γ-value of Yb 4 (As 1− x Sb x ) 3 is considered to be independent ofthe carrier concentration.

Thermodynamic and transport properties of the one-dimensional S=½ antiferromagnet Yb4As3

Abstract The semimetallic quasi-one-dimensional S= 1 2 antiferromagnet Yb4As3 has been studied by performing low-temperature (T) and high magnetic-field (B) measurements of the specific heat, C(T,B), magnetization, M(T,B), AC-susceptibility, χAC(T,B), and electrical resistivity, ρ(T,B). At finite transverse magnetic fields, a gap Δ(B) is induced in the low-energy magnetic excitation spectrum. Our C(T,B) measurements reveal a Δ(B)∼B2/3 dependence for B⩽9xa0T, in accordance with predictions of the quantum sine-Gordon model. At higher fields the Δ(B) curve levels-off gradually. In the isothermal magnetization taken at 0.6xa0K no saturation occurs up to 60xa0T. We also present new results on spin-glass behavior below 0.15xa0K caused by a weak ferromagnetic interchain coupling and disorder. Finally, we concentrate on the electrical transport properties. Shubnikov-de Haas oscillations, arising from a low-density system of mobile As-4p holes, are recorded in magnetic fields up to 60xa0T. We estimate the effective mass and the mean-free path of these carriers and discuss spin-splitting effects.

Magnetocaloric Effect Study on the Pyrochlore Spin Ice Compound Dy2Ti2O7 in a [111] Magnetic Field

Field variation of the entropy S ( H ) of the spin ice compound Dy 2 Ti 2 O 7 has been examined by means of magnetocaloric effect measurements in a magnetic field H ∥[111], with attention focused o...