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We report on the magnetic, transport, and thermal properties of a cage-like compound CeFe 2 Al 10 that crystallizes in the orthorhombic YbFe 2 Al 10-type structure. A broad peak in the magnetic susceptibility at 70 K indicates that CeFe 2 Al 10 is a valence fluctuation compound. The electrical resistivity and the Hall coefficient exhibit sharp upturns below 20 K, where the thermopower shows a rapid decrease. These low-temperature anomalies in the transport properties resemble those of a typical Kondo semiconductor CeRhSb. These features indicate the formation of a hybridization gap in CeFe 2 Al 10 on cooling below 20 K. The energy gap is estimated as 15 K from the thermal activation energy of the resistivity. The magnetic contribution of the specific heat shows a Schottky-type maximum at 30 K that provides another evidence for the gap formation in CeFe 2 Al 10 .

We have measured the electrical resistivity of a single crystal of YbCo 2 Zn 20 at pressures up to 2.37 GPa and at temperatures from 50 mK to 300 K. Above a critical pressure P c ($ 1 GPa), we have found a resistivity anomaly at T M ($ 0:15 K at 1 GPa) that increases with the pressure. At the ambient pressure, the system shows a nonmagnetic ground state described by the Fermi-liquid model. The T 2 coefficient of the electrical resistivity A strongly increases with the pressure upon approaching P c . However, in the vicinity of P c , the temperature dependence of the resistivity deviates from the Fermiliquid description. These observations suggest that the application of hydrostatic pressure induces a magnetically ordered state for P ! P c and T T M .In intermetallic compounds, including Ce and Yb, the hybridization between the 4f and itinerant conduction-band electrons induces the instability of magnetic moments and charge configurations. In recent years, one of the most interesting topics is the ground state properties of heavy fermion metals located at or close to a magnetic quantum critical point (QCP). 1) The application of external pressure is one of the important tools for controlling the electronic configurations as well as chemical pressure. In the case of heavy fermion Ce compounds exhibiting antiferromagnetic order, such as CeIn 3 and CePd 2 Si 2 , 2) the magnetic order is suppressed by applying pressure. Interestingly, unconventional superconductivity appears in the vicinity of the QCP at which the magnetic ordering temperature is decreased to zero.So far, pressure-induced magnetic transitions have been observed in some Yb-based compounds. [3][4][5][6][7] The key point here is that Yb ions fluctuate between the nonmagnetic Yb 2þ (J ¼ 0) and the magnetic Yb 3þ (J ¼ 7=2) states. Since the ionic volume of the magnetic Yb 3þ state is smaller than that of the nonmagnetic Yb 2þ one, applying pressure stabilizes the magnetic Yb 3þ configuration and induces the appearance of a magnetically ordered state in contrast to the Ce case. However, the critical pressure for these compounds is as high as or higher than 6 GPa, which prevents us from understanding the physics in the vicinity of the magnetic QCP because of difficulties in high-pressure experiments.The series of compounds YbT 2 Zn 20 (T = Fe, Co, Ru, Rh, Or, Ir) belongs to a new heavy fermion system crystallizing in the cubic CeCr 2 Al 20 structure. 8,9) All these compounds show an enhanced Sommerfeld coefficient of the specific heat exceeding 400 mJ/(mol K 2 ). The high-temperature magnetic susceptibility of these compounds follows the Curie-Weiss law with the effective moments close to the value for the free Yb 3þ ion ( eff ¼ 4:54 B ), although there is no indication of magnetic order down to 20 mK. In the YbT 2 Zn 20 family, YbCo 2 Zn 20 exhibits some notable features as follows. The low temperature electrical resistivity and specific heat can be described by the formulas ¼ 0 þ AT 2 and C=T ¼ constant, as expected from the Fermiliquid behavior; the value...

We report results of 11 B NMR and susceptibility measurements on the quasi 2D frustrated dimer spin system SrCu2(BO3)2 under high pressure. At 2.4 GPa and in a magnetic field of 7 T, NMR lines split with decreasing temperature in two steps. A gradual splitting below T =30 K breaking the four-fold symmetry of magnetic response is followed by a further sudden splitting below 3.6 K. The latter indicates a magnetic phase transition, which is also marked by a kink in the susceptibility at 1.44 GPa. From the magnetic hyperfine shift data, we conclude that the low-T phase has a doubled unit cell containing two types of dimers, one in a nearly singlet state and the other with a finite magnetization down to T =0.KEYWORDS: SrCu2(BO3)2, Shastry-Sutherland model, high pressure, NMR, phase transition A variety of exotic phenomena has been discovered in the quasi two dimensional dimer spin system SrCu 2 (BO 3 ) 2 .1, 2 It has an alternating stack of the magnetic CuBO 3 layers (Figs. 1(a) and 1(b)) and the nonmagnetic Sr layers.3, 4 The magnetic layer containing orthogonal arrays of spin-1/2 Cu 2+ dimers is a realization of the 2D Shastry-Sutherland model,where J (J ′ ) is the intradimer (interdimer) Heisenberg exchange interaction. The ground state of this model is obvious in two limiting cases: the dimer singlet phase for J ′ /J << 1 and the Néel ordered phase for J ′ /J >> 1. The dimer singlet phase is known to be stable up to (J ′ /J) c =0.68. [6][7][8] Various experiments have established that SrCu 2 (BO 3 ) 2 has a dimer singlet ground state at ambient pressure and zero magnetic field 1, 9, 10 with the energy gap of 33 K 11-13 and J ′ /J=0.60-0.64. 14, 15Frustration in the Shastry-Sutherland model strongly suppresses the kinetic energy of triplets.6 Indeed SrCu 2 (BO 3 ) 2 has an extremely small width of the triplet dispersion (∼0.2meV 12,13 scattering. It also leads to the magnetization plateaus at 1/8, 1/4, and 1/3 of the saturated magnetization in high magnetic fields, 19 where triplets crystalize in commensurate superlattices due to mutual repulsion. 20-23Since J ′ /J in SrCu 2 (BO 3 ) 2 is close to the critical value, tuning the exchange parameters, e.g. by applying pressure, might enable us to explore the phase diagram of the Shastry-Sutherland model, which is still an open issue. A plaquette singlet phase was proposed to exist between the dimer singlet and the Néel ordered phase.8, 24 Alternatively, instability of two-triplet bound * E-mail address: twac@issp.u-tokyo.ac.jp † Present address: Hitachi Medical Corporation, Kashiwa, Chiba ‡ E-mail address: masashi@issp.u-tokyo.ac.jp states 25 may lead to a spin nematic phase. Further variation may arise from the Dzyaloshinski-Moriya interaction beyond the Shastry-Sutherland model. 26In spite of such interest, only a few experiments under pressure have been reported to date. Magnetic susceptibility data up to P =0.7 GPa indicates reduction of the energy gap extrapolating to zero near P =2.5-3.0 GPa. 27The X-ray study shows a tetragonal to monoclinic structural transit...

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Spin dynamics as well as static properties of the one-dimensional J-J ′ model (S = 1/2, J > 0 and 0 ≤ α = J ′ /J ≤ 0.5) are studied by the exact diagonalization and the recursion method of finite systems up to 26 sites. Especially, the dynamical structure factor S(q, ω) is investigated carefully for various values of α. As α increases beyond the gapless-gapful critical value αc = 0.2411, there appear features definitely different from the Heisenberg model but the same with the Majumdar-Ghosh model. Some of these features depend only on the value of α and not on δ: a parameter introduced for the coupling alternation. By comparing these results with a recent inelastic neutron scattering spectrum of an inorganic spin-Peierls compound CuGeO3 [M. Arai et al. : Phys. Rev. Lett. 77 (1996) 3649], it is found that the frustration by J ′ in CuGeO3 is unexpectedly strong (α = 0.4-0.45), and at least α must be larger than αc to some extent. The value of J is evaluated at ∼ 180K consistent with other estimations. The coupling alternation is extremely small. This large frustration is a primary origin of the various anomalous properties CuGeO3 possesses. For comparison we refer also to α ′ -NaV2O5.

We present a quantum theory for one-dimensional spin-1/2 multiferroics, where the vector spin chirality couples with the electric polarization. Based on exact diagonalization and bosonization, it is shown that quantum fluctuations appreciably reduce the chiral ordering amplitude and the associated ferroelectric polarization. This yields nearly collinear spin correlations in short-range scales, in qualitative agreement with recent neutron scattering experiments. There appear gapless chirality excitations described by phasons and new solitons, which can be experimentally verified from the low-energy dielectric response.When the chiral symmetry is spontaneously broken by magnetic interactions in Mott insulators, a ferroelectric polarization appears through the spin-orbit coupling. This new prototype of multiferroic behavior has been discovered in a spin-2 helimagnet TbMnO 3 , 1) and attracted a current great interest for both its fundamental importance and its potential application to an electrical control of spins. 2, 3) This magnetoelectric coupling between the vector spin chirality and the polarization 4-6) is ubiquitous in Mott insulators. 7) Of our particular interest is recently discovered multiferroic behavior in one-dimensional (1D) frustrated spin-1/2 magnets, LiCuVO 4 8-11) and LiCu 2 O 2 . 12-14) These have opened an intriguing issue of strong quantum fluctuations in multiferroics due to the low dimensionality and the spin-1/2 nature, as suggested by recent neutron scattering experiments. 14,15) Another important aspect in multiferroics is that low-frequency magnetic excitations can be probed from the dielectric functions ε ii (ω) through the magnetoelectric coupling, [16][17][18] as actually measured for RMnO 3 . 19, 20) Namely, a local flip of the chirality induces the charge dynamics. Particularly, in the 1D spin-1/2 multiferroics, a pair of such local defects may propagate as new elementary excitations. This is reminiscent of charged solitons in 1D band insulators, e.g., polyacetylene. 21) However, the chirality-induced charge dynamics in the 1D multiferroics has a much smaller energy scale and may produce the low-frequency dielectric response.In this Letter, we for the first time reveal the novel quantum nature of 1D spin-1/2 multiferroics, by examining a simple but realistic frustrated spin model. It is shown that the chiral phase can appear near the SU (2)-symmetric case, which explains the experimentally observed ferroelectricity. Significantly, this multiferroic state is characterized by (i) a tiny amplitude of the the chiral long-range order (LRO), (ii) almost collinear spin correlations, and (iii) gapless chiral solitons in the low-frequency dielectric response.The minimal model for the 1D spin-1/2 multiferroics, especially for LiCuVO 4 , 22) is given by a simple spin Hamiltonian with nearest-and second-nearest-neighbour exchange couplings, J 1 and J 2 :Here, S j represents the spin operator on the site 0 0.2 0.4 0.6 0.8 1 1.2 J 1 /J 2 ∆ -4 -2 0 2 4 0 0.2 0.4 0.6 0.8 1 J 1 /J 2 ∆ gapless chiral...