Search

Low dimensional strongly correlated electron systems exhibit a variety of exotic phenomena such as fractional quantum Hall effects, high-temperature superconductivity, and topological phase transitions. However, a problem in modern condensed-matter physics is the difficulty to compare theories with experiments, because of the absence of an ideal experimental system whose properties can be controlled in a systematic way. Here we show a state-of-the-art silicon technology can provide a platform to investigate a one-dimensional quantum system, where various theoretical predictions are available on the basis of mathematically rigid models. We have found unusual transport properties in a field-effect-transistor with a wide and short hole-channel under strong electric fields at low temperatures. By gate-induced doping in the transistor, we discovered new current plateaus and negative differential conductances against drain voltages. We have also found anomalous gate leakage currents which increases upon reducing temperatures and reducing the external electric fields. This provides evidence of the broken symmetry of quantum dipoles formed at the gate interface, which increases local electric fields coming from molecular mean-fields. We obtained phase diagrams of the field-induced phase transitions, which will be correlated with a one-dimensional quantum dipole model.

We discovered anomalous transport properties in a conventional Si based Metal-Oxide-Semiconductor Field-Effect-Transistor with a wide and narrow hole-channel at low temperatures. We found the quantum dipole formed at the thin gate interface is responsible for the phase transition. We discuss its potential to use for a quantum simulator as a test bed to examine various theoretical concepts in condensed-matter physics.

The scaling of Silicon (Si) technology is approaching the physical limit, where various quantum effects such as direct tunnelling and quantum confinement are observed, even at room temperatures. We have measured standard Complementary Metal-Oxide-Semiconductor Field-Effect-Transistors (CMOSFETs) with wide and short channels at low temperatures to observe single electron/hole characteristics due to local structural disturbances such as roughness and defects. In fact, we observed Coulomb blockades in sub-threshold regimes of both {\it p}-type and {\it n}-type Si CMOSFETs, showing the presence of quantum dots in the channels. The stability diagrams for the Coulomb blockade were explained by the potential minima due to poly-Si grains. We have also observed sharp current peaks at narrow bias windows at the edges of the Coulomb diamonds, showing resonant tunnelling of single carriers through charge traps.

We have found a systematic way to identify the bias conditions to observe the Random-Telegraph-Noise (RTN) in advanced Metal-Oxide-Semiconductor Field-Effect-Transistors (MOSFETs). We measured a p-type MOSFET at 2K, and found narrow bias conditions to observe the RTN presumably caused by charge trapping and de-trapping, which were only observed at low temperatures. It will pave the way to address the nature of a trap, which will be useful to understand the mechanism of RTN to secure the reliability.

BaCuSi 2 O 6 can be regarded as an almost ideal realization of an S = 1 2 system of weakly interacting spin dimers with spin-singlet ground state and gapped excitation spectrum. We argue that the fine structure observed in low-temperature EPR spectra of BaCuSi 2 O 6 is a fingerprint of triplet excitations (excitons). Analyzing the angular dependence of the exciton modes allows us to precisely calculate the zero-field splitting within the triplet states and, correspondingly, the anisotropy parameter, D = 0.07 cm - 1 . The proposed procedure can be applied for studying anisotropy effects in a large number of S = 1 2 gapped quantum antiferromagnets with dimerized or alternating spin structure.

We have measured the magnetic susceptibilities of single crystals of ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{GeO}}_{3}$ with extremely low Zn concentration $(x)$ lower than $x=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at very low temperatures to investigate the spin-Peierls and antiferromagnetic transitions. The results show that the undoped ${\mathrm{CuGeO}}_{3}$ has no antiferromagnetic phase down to 12 mK and there exists an antiferromagnetic long-range order with the easy axis along the $c$ axis for $x$ down to $1.12(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}.$ The minimum observed N\'eel temperature was 0.0285 K for the $x=1.12(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ sample. From the concentration dependence of the N\'eel temperature it is concluded that there is no critical concentration for the occurrence of the antiferromagnetic long-range order. This indicates that the dimerization sustains the coherence of the antiferromagnetic phase of the spin polarization in impurity-doped systems and is consistent with the theory of the impurity-doped spin-Peierls system. The temperature dependence of the susceptibilities at $Tg{T}_{N}$ of all samples indicates that the magnetic correlations between localized spins are enhanced by a relatively large interchain interaction of ${\mathrm{CuGeO}}_{3}.$

We study a spin-Peierls gap and a bound and a resonant states of two magnetic excitations in Cu 1- x Zn x GeO 3 and CuGe 1- y Si y O 3 . Breakdown of the selection rule in the doped samples enables us to observe the spin-Peierls gap excitation in the first-order Raman process. It is found that this excitation shifts to lower frequency and merges into the tail of the elastic scattering together with its line broadening with increasing dopant concentration. The bound state of a particle-hole pair is observed just below the bottom of the second-order Raman band in pure CuGeO 3 . It changes into a resonant state and disappears rapidly as the concentration of the dopant is increased, owing to the weakening of attractive interaction and the extreme increase of the dampings of the magnetic excitations. These features are demonstrated theoretically by using Green's function.

We report an observation of a commensurate-incommensurate phase transition in the Dzyaloshinskii-Moriya spiral antiferromagnet Ba${}_{2}{\mathrm{CuGe}}_{2}{\mathrm{O}}_{7}$. The transition is induced by an external magnetic field applied along the $c$ axis of the tetragonal structure, i.e., in the plane of spin rotation. Bulk magnetic measurements and neutron-diffraction experiments show that the transition occurs in a critical field ${H}_{c}\ensuremath{\approx}2.1$ T. Experimental results for the period of the magnetic structure and magnetization as functions of magnetic field are in quantitative agreement with our exact analytical solution for Dzyaloshinskii's model of commensurate-incommensurate transitions in spiral magnets.

The magnetic structure of Ba{sub 2}CuGe{sub 2}O{sub 7} is investigated by neutron diffraction in magnetic fields applied along several directions in the (a,b) plane of the crystal. In relatively weak fields, H{approx_lt}0.5 T, the propagation vector of the spin spiral rotates to form a finite angle with the field direction. This angle depends on the orientation of {bold H} itself. The rotation of the propagation vector is accompanied by a reorientation of the plane of spin rotation in the spiral. The observed behavior is well described by a continuous-limit form of a free-energy functional that includes exchange and Dzyaloshinskii-Moriya interactions, as well as the Zeeman energy and an empirical anisotropy term. {copyright} {ital 1997} {ital The American Physical Society}

Comprehensive neutron-scattering studies were carried out on a series of high-quality single crystals of Cu${}_{1\ensuremath{-}x}$Zn${}_{x}$GeO${}_{3}$. The Zn concentration $x$ was determined for each sample using electron probe microanalysis. The measured Zn concentrations were found to be 40--80 % lower than the target values. Nevertheless, the measured concentrations cover a wide range which enables a systematic study of the effects due to Zn doping. We have confirmed the coexistence of spin-Peierls (SP) and antiferromagnetic (AF) orderings at low temperatures and the measured phase diagram is presented. Most surprisingly, long-range AF ordering occurs even in the lowest available Zn concentration $x=0.42%$, which places important constraints on theoretical models of the AF-SP coexistence. Magnetic excitations are also examined in detail. The AF excitations are sharp at low energies and show no considerable broadening as $x$ increases, indicating that the AF ordering remains long ranged for $x$ up to 4.7%. On the other hand, the SP phase exhibits increasing disorder as $x$ increases, as shown from the broadening of the SP excitations as well as the dimer reflection peaks.

We report muon spin relaxation measurements of the doped spin-Peierls system $({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x})\ensuremath{-}({\mathrm{Ge}}_{1\ensuremath{-}y}{\mathrm{Si}}_{y}){\mathrm{O}}_{3}$. Spontaneous muon spin precession in zero applied field was observed, confirming the presence of antiferromagnetic order in this series of compounds. In contrast to usual antiferromagnets, muon spin precession is accompanied by a relaxation signal indicating a large spatial inhomogeneity of the ordered moment size. Assuming an exponential decay of the moment size away from the doping centers, we estimated a decay length of $\ensuremath{\xi}\ensuremath{\sim}10$ lattice units along the chain. We suggest that both Zn and Si doping induces the same maximum moment size around the doping center.

Bulk magnetic measurements and inelastic neutron scattering were used to investigate the magnetic properties of ${\mathrm{BaCuSi}}_{2}$${\mathrm{O}}_{6}$, a quasi-two-dimensional antiferromagnet with a square-lattice bilayer structure. The compound was shown to have a spin-singlet dimerized ground state with a gap \ensuremath{\Delta}\ensuremath{\approx}4.5 meV in the magnetic excitation spectrum. The dispersion of the gap modes was measured and found to be temperature dependent and totally suppressed at temperatures higher than the characteristic gap energy. The observed excitations are analogous to magnetoexcitons in light rare-earth compounds, but are an intrinsic property of a simple Heisenberg Hamiltonian for the S=1/2 magnetic bilayer. The behavior is discussed in comparison to that of related systems, in particular, ${\mathrm{Cs}}_{3}$${\mathrm{Cr}}_{2}$${\mathrm{Br}}_{9}$.

We have measured the temperature dependence of low-frequency Raman spectra in ${\mathrm{CuGeO}}_{3}$, and have observed quasielastic scattering in the (c,c) polarization above the spin-Peierls transition temperature. We attribute this to the fluctuations of the energy density in the spin system. The magnetic specific heat and an inverse of the magnetic correlation length can be derived from the quasielastic scattering. The inverse of the magnetic correlation length is proportional to (T-${\mathrm{T}}_{\mathrm{SP}}$${)}^{1\mathrm{/}2}$ at high temperatures. We compare the specific heat with a competing-J model. This model cannot explain quantitatively both the magnetic specific heat and the magnetic susceptibility with the same parameters. The origin of this discrepancy is discussed.

We introduce our μSR investigations of spin‐gap systems, such as, (1) a 2‐leg spin‐ladder material SrCu2O3, (2) a Haldane material (S=1 spin‐chain) Y2BaNiO5, (3) a spin‐Peierls material CuGeO3, (4) a spin‐chain# material Sr6Ca8Cu24O41. All of these antiferromagnetic spin systems are characterized by a spin‐gap between the singlet ground‐state and the triplet first excited states. In the above‐mentioned materials, we confirmed the absence of magnetic order down to milli‐Kelvin regime, supporting the non‐magnetic feature of the ground‐state. If a spin‐gap system is doped with charges and/or vacancies at the spin site, unpaired spins are induced out of the singlet ground‐state. In some materials, doping completely destroys the singlet ground‐state and induces a bulk magnetic order. We report μSR investigations of doped materials as well, which clarifies the existence/absence of a magnetic order upon doping.

The {ital S}=1/2 Heisenberg antiferromagnet CaCuGe{sub 2}O{sub 6} was studied by inelastic neutron scattering on a powder sample in the temperature range 5{endash}300 K. A sharp symmetric magnetic inelastic peak was observed around {h_bar}{omega}=6 meV, and its {ital q} and temperature dependences were investigated. The absolute values of the energy-integrated magnetic intensity were determined. Despite the fact that from the structural point of view the material has a distinct one-dimensional arrangement of magnetic sites, the experimental results are surprisingly well described by an ensemble of weakly interacting antiferromagnetic dimers, which are composed of pairs of Cu{sup 2+} ions. {copyright} {ital 1996 The American Physical Society.}

Magnetizations of spin-Peierls cuprates Cu 1- x Zn x GeO 3 with x =0, 0.005, 0.010, and 0.020 were measured in order to investigate influence of impurities on a magnetic phase. A characteristic change of the magnetization related to a phase transition from dimerized to other phases was seen below a spin-Peierls-transition temperature. The magnetization above 16.0 T shows weak x and temperature dependences. A critical field associated with the phase transition decreases with increasing x . We derived a result that the effects of impurities on the magnetic phase are weaker than on the dimerized phase.

The magnetization was obtained as a function of temperature and magnetic field of a Cu 1− x Zn x GeO 3 single-crystal with x ∼ 0.04. The temperature dependence of the magnetic susceptibility in 0.1 T parallel to the c axis shows a clear cusp around 4.5 K and decreases rapidly below 4.5 K. There is no hysteresis between the data measured in zero-field-cooling and field-cooling processes. The field dependence of the magnetization parallel to the c axis at 2.0 K exhibits an apparent increase around 1.0 T, which corresponds to a spin-flop transition. The temperature and field dependences, the anisotropy, and the absence of the hysteresis undoubtedly prove the occurrence of an antiferromagnetic long-range order below 4.5 K instead of the spin-glass-like transition reported previously. Spins below the transition temperature are ordered almost parallel to the c axis. This is the first report of magnetic long-range order caused by nonmagnetic impurities.

Recently, a spin-Peierls transition was discovered in CuGeO3, which is the first inorganic compound with this type of transition [H. Hase, I. Terasaki and K. Uchinokura, Phys. Rev. Lett. 70 (1993) 3651]. As is the case for organic spin-Peierls materials, CuGeO3 has a magnetic phase in high magnetic fields. High-field magnetization was measured in pulse and steady magnetic fields. AC susceptibility measurements were also performed. The existence of the magnetic phase was confirmed. Effects of substitution of Zn for Cu were also investigated.

Phonons and magnetic excitations were studied as a function of temperature in the spin-Peierls phase of an inorganic material ${\mathrm{CuGeO}}_{3}$ by Raman scattering. All of the phonon Raman peaks observed at room temperature can be assigned on condition that the crystal symmetry is ${\mathit{D}}_{2\mathit{h}}^{5}$(Pbmm). Five peaks intrinsic to the spin-Peierls state were observed at 30, 105, 228, 370, and 818 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the spin-Peierls phase. The temperature dependence of the frequency of the 30-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ peak agrees approximately with 2\ensuremath{\Delta}(T), where \ensuremath{\Delta}(T) is the spin-Peierls gap. The Raman spectrum between 0 and 300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was compared with the density of states of the magnetic excitons. The 105- and 228-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ peaks were assigned to two-magnetic-exciton Raman scattering. The peaks at 370 and 818 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ were assigned to one-phonon Raman scattering of the folded ${\mathit{A}}_{\mathit{g}}$ modes, which occurs due to the formation of a dimerization in the spin-Peierls phase.

Electron spin resonance (ESR) was investigated on the spin-Peierls system CuGeO3 and antiferromagnetic Ba2Cu3O4Cl2 at X-band and far-infrared regions (FIR). The g-values of CuGeO3 were determined to be 2.17, 2.26 and 2.11 for H∥a, H∥b and H∥c-axes, respectively. The temperature dependence of FIR and X-band ESR spectra of CuGeO3 clarified the difference of character among uniform, dimerize and high-field magnetic phases. In the Ba2Cu3O4Cl2 system, the resonance field for H∥c drastically changes around 30K, and antiferromagnetic resonance (AFMR) was observed at 5 K.

We measured the magnetic-field dependence of the magnetizations of the spin-Peierls cuprates Cu 1− x Zn x GeO 3 with the x = 0, 0.005, 0.010 and 0.020. The characteristic change of the magnetization was observed and it is associated with the phase transitions from the dimerized to the other phases. When the magnetic phase diagram is expressed by the reduced variables gH /2 T SP (0) and T / T SP (0), the phase diagrams of Cu 1− x Zn x GeO 3 qualitatively agree with those of organic spin-Peierls systems and the theoretical one.

We have observed the nuclear quadrupole resonance (NQR) of Cu in the spin-Peierls system CuGeO 3 . The NQR signals have been observed at 34.23±0.02 MHz for 63 Cu and 31.66±0.02 MHz for 65 Cu at 4.2 K under zero external field. These signals persist down to 1.3 K with neither line splitting nor broadening due to the internal magnetic field. This fact indicates there is no long-range order of Cu spins. The 63 Cu nuclear spin-lattice relaxation rate 63 (1/ T 1 ) rapidly drops below 15 K which clearly demonstrates an opening of the energy gap in the excitation spectrum. However, no anomaly was detected in the Cu NQR frequency at the transition temperature. It was also found that 63 (1/ T 1 ) above 15 K smoothly extrapolates to a finite value at 0 K. This may suggest the spin dynamics in the paramagnetic state is described in terms of the one-dimensional antiferromagnetic Heisenberg model with S =1/2.

The magnetic susceptibilities of polycrystalline ${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{GeO}}_{3}$ were measured to study the effects of nonmagnetic impurities on the spin-Peierls transition. The transition temperature drastically reduces upon Zn doping and the spin-Peierls state collapses around x=0.03. The Zn doping causes another phase transition in the samples with 0.02\ensuremath{\le}x\ensuremath{\le}0.08, which proved to be spin-glass-like. The present Letter provides the first study of the spin-Peierls system with impurities.

$\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_{2}{\mathrm{O}}_{6}$, a $S=\frac{1}{2}$ quantum antiferromagnet with a double-layer structure of ${\mathrm{Cu}}^{2+}$ ions in a distorted planar-rectangular coordination and with a dimerized spin singlet ground state, has been studied by means of the electron paramagnetic resonance technique. It is argued that multiple absorptions observed at low temperatures are intimately related to a thermally activated spin-triplet exciton superstructure, which appears to be a characteristic feature of low-dimensional anisotropic spin-$\frac{1}{2}$ systems with the dimerized spin-singlet ground state. We showed that analysis of the angular dependence of exciton modes could be used for accurate estimation of anisotropy parameters in $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_{2}{\mathrm{O}}_{6}$; the procedure can be applied for a large number of $S=\frac{1}{2}$ quantum antiferromagnets. In addition, the temperature dependence of EPR intensity and linewidth in $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_{2}{\mathrm{O}}_{6}$ has been studied and discussed.

We measured and analyzed the transport properties of single-crystal ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$, which is a metallic transition-metal oxide consisting of a two-dimensional triangle lattice of Co. Reflecting the crystal structure, the resistivity is highly anisotropic between in- and out-of-plane directions, and the in-plane resistivity is as low as 200 $\ensuremath{\mu}\ensuremath{\Omega}$ cm at 300 K. Most strikingly, the in-plane thermoelectric power of ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$ is about 100 $\ensuremath{\mu}$V/K at 300 K, which is nearly ten times larger than that of typical metals. The large thermoelectric power and the low resistivity suggest that ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$ is a potential thermoelectric material.

The quasi-dimensional square-lattice antiferromagnet Ba2CuGe2O7 was studied by neutron scattering and bulk magnetic techniques. An incommensurate magnetic spiral structure with the propagation vector (1 + ξ,ξ,0) (ξ = 0.027) was observed below TN = 3.26 K. The spin dynamics can be adequately described by conventional spin-wave theory with two exchange constants: nearest-neighbor in-plane antiferromagnetic coupling J1 ≈ 0.48 meV and interplane ferromagnetic interaction J3 ≈ 0.013 meV. This set of exchange parameters apparently fails to explain the spiral order. The non-centrosymmetric crystal structure suggests that the incommensurate phase may be the result of a Dzyaloshinskii-Moriya instability of the Neel ground state.

A series of high-quality single crystals of Cu$_{1-x}$Zn$_x$GeO$_3$ have been examined by neutron scattering techniques. An antiferromagnetic (AF) ordering is confirmed for samples with $x\geq 0.02$, in complete agreement with previous reports. We show that the spin-Peierls (SP) phase transition persists to 6\% Zn, whereas previous magnetic susceptibility measurements reported a deterioration of the SP transition above 2\% Zn. We present some details of the successive transitions upon lowering temperature into the spin-Peierls phase which is followed by a transition into an antiferromagnetically ordered phase. Below the N\'eel temperature we observe the coexistence of the SP lattice dimerization and AF states., Comment: 9 pages, RevTeX manuscript, 4 ps figures included. Submitted to Phys. Rev. B. Also available from http://insti.physics.sunysb.edu/~mmartin/pubs.html

We measured Raman scattering below 400 cm−1 in Cu1−xZnxGeO3 (x = 0, 0.013, 0.017, and 0.0212) to study the Zn-doping effect in the spin-Peierls phase. When Zn atoms (S = 0) are doped, a two-magnetic-exciton bound state disappears in the spin-Peierls phase. In the sample of x = 0.0212, the folded phonon mode at 370 cm−1 and the two-magnetic-exciton Raman spectra below 250 cm−1 were observed even in the uniform phase. In pure CuGeO3, the quasi-elastic scattering was observed at high temperatures. The origin of this scattering is considered as a spin fluctuation interacting with lattice.

Besides being an ancient pigment, BaCuSi2O6 is a quasi-2D magnetic insulator with a gapped spin dimer ground state. The application of strong magnetic fields closes this gap, creating a gas of bosonic spin triplet excitations. The topology of the spin lattice makes BaCuSi2O6 an ideal candidate for studying the Bose-Einstein condensation of triplet excitations as a function of the external magnetic field, which acts as a chemical potential. In agreement with quantum Monte Carlo numerical simulations, we observe a distinct lambda anomaly in the specific heat together with a maximum in the magnetic susceptibility upon cooling down to liquid helium temperatures.

The AC susceptibility in the cuprate compound CuGeO3, in which the spin-Peierls (SP) transition occurs around 14K in the absence of magnetic field, was measured in the magnetic field region up to 20 T to establish the magnetic phase diagram of CuGeO3. It was found that the isofield AC susceptibility shows a “knee” or peak at the phase boundary between the SP and uniform (U) phases and that between another magnetic (M) and U phases. The SP-M phase transition was clearly observed in the isothermal AC susceptibility. The determined phase diagram is consistent with the theoretical one after Cross and those of three organic SP materials.

We measured the static magnetization of polycrystalline CuGeO3, which exhibits the spineierls transition around 14.0 K in the absence of the magnetic field. The field dependence of the magnetization below 14.0 K shows a characteristic nonlinear process associated with a phase transition, and phase boundaries between the dimerized phases and other phases were obtained. The magnetic phase diagram qualitatively agrees with that of an organic spin-Peierls material. The phase transition is of first order below 10.0 K, and of second order above 10.0 K. Above 10.0 K, the critical field decreases from 13 to 0 T with increasing temperature. On the other hand, it is nearly independent of temperature below 10.0 K.

The transition-metal oxide NaCo/sub 2/O/sub 4/ is a layered oxide in which CoO/sub 2/ and Na alternately stack along the c axis. Recently we have found that this compound shows large thermopower with low resistivity which are comparable to those of Bi/sub 2/Te/sub 3/. The negative transverse magnetoresistance and the strongly temperature-dependent Hall coefficient suggest that electron correlation dominates the conduction mechanism in NaCo/sub 2/O/sub 4/.

Magnetic susceptibility, high-field magnetization and inelastic neutron scattering experiments are used to study the magnetic properties of a new S=1 quasi-1-dimensional antiferromagnet PbNi2V2O8. Inter-chain interactions are shown to be almost, but not quite, strong enough to destroy the nonmagnetic singlet ground state and the energy gap in the magnetic excitation spectrum. Substituting nonmagnetic Mg$^{2+}$ (S=0) ions for Ni$^{2+}$ (S=1) induces a magnetically ordered state at low temperatures. To our knowledge this is the first observation of doping-induced long-range order in a Haldane-gap system., 5 pages including 4 figures

We report an observation of a commensurate-incommensurate phase transition in a Dzyaloshinskii-Moriya spiral magnet Ba_2CuGe_2O_7. The transition is induced by applying a magnetic field in the plane of spin rotation. In this experiment we have direct control over the strength of the commensurate potential, while the preferred incommensurate period of the spin system remains unchanged. Experimental results for the period of the soliton lattice and bulk magnetization as a function of external magnetic field are in quantitative agreement with theory., 4 pages, 4 figures, submitted to PRL

Recent discoveries of a new Haldane compound PbNi 2 V 2 O 8 and the spin-vacancy-induced antiferromagnetism (Uchiyama et al., Phys. Rev. Lett. 83 (1999) 632) were studied in more detail by heat capacity measurements. The existence of the antiferromagnetic transition was more firmly proved by these measurements. We also obtained temperature versus Mg-concentration phase diagram.

We have studied the Zn- and Si-doping effects on the 368 cm −1 folded phonon in CuGeO 3 by means of Raman scattering. With increasing dopant concentrations, the transition temperature decreases to a constant value, and the lattice distortion at T = 0 also decreases. In the doped samples, a broad Raman peak of the folded phonon still remains observable even above the transition temperature, indicating the increase of the short-range fluctuations of lattice dimerization. In α′ -NaV 2 O 5 , we have observed a folded phonon at 948 cm −1 in the spin-Peierls phase. The defect of the Na + ions also causes the decrease of the intensity and the increase of the half-width of the folded phonon.

We report the interatomic charge distribution (ICD) studied by laboratory powder x-ray diffraction in YBa 2 Cu 3 O 6.9 (BYCO) and CuGeO 3 . In contrast with the existence of two dimensional network of ICD from the Cu 3d and O 2pσ orbitals in BYCO, rather ionic feature of copper and oxygen atoms was found in CuGeO 3 together with the charge localization in the planar lattice of one-dimensional -Cu O 2 -linkage.

The magnetic susceptibility of single crystals Cu1−x M x GeO3 (M=Zn, Ni) were investigated to study the effects of impurities on the inorganic spin-Peierls material CuGeO3. We succeeded in the growth of Zn-doped single crystals with 0.007≤x≤0.09 and Ni-doped ones withx=0.033. We determined temperature vsx phase diagram of the Zn-doped single crystals, which is almost identical to that of the polycrystaline samples. In both Cu1−x Zn x GeO3 and Cu1−x Ni x GeO3, the antiferromagnetic transition occurs below 5 K. It is proved that the easy axis of Ni-doped CuGeO3 is nearly along thea axis, while that of Zn-doped CuGeO3 is along thec axis as previously reported. This difference of spin orderings between Zn-doped and Ni-doped CuGeO3 is caused by that Ni2+ ions carry spinS=1 while Zn2+ ions are nonmagnetic.

Linear Cu2+ (S = 12) inorganic chain compound CuGeO3 exhibits a spin-Peierls (SP) transition at 14 K. The present report describes the existence of the strong fluctuation and/or the formation of short-range order above 80 K associated with the appearance of the satellite reflection with h/2 k 32 (h:odd, k:integer). It may lead to the explanation of the difference between the χ(T) and the Bonner-Fisher law. The temperature dependence of the b lattice constant and the integral breadth at 040 reflection is consistent with the observation of the softening of the LA phonons along the b axis and the spontaneous strain Δb by Lorenzo et al.

We measured the magnetization of Cu1−xMgxGeO3. A spin-Peierls-transition temperature and a critical field associated with a transition to a magnetic phase reduce up to x = 0.06. A spin-glass-like transition was not seen, although it was observed in Cu1−xZnxGeO3. The effects of substitution of Mg are weaker than those of Zn.

We measured the magnetic susceptibility of single-crystal CuGeO 3 . The data revealed that the spin-Peierls transition occurs around 14 K in this cuprate, which is the first inorganic spin-Peierls material. The susceptibilities of polycrystalline Cu 1−x Zn x GeO 3 were also measured. We discovered the occurrence of a new spin-glass-like transition in the samples with x =0.02∼0.08 around 2∼5 K.