Successive orbital ordering transitions in NaVO2

Physical property measurements on samples of triangular-lattice NaVO2 reveal two successive orbital ordering transitions. At 300 K, the structure is rhombohedral. At 98 K, the system undergoes a second-order transition to a monoclinic phase in which the in-plane V-V distances separate into four short and two long bonds, corresponding to orbital ordering of one electron per V3+. Below 93 K, there is a first-order transition to a second monoclinic phase with four long and two short V-V bonds, consistent with orbital ordering of two electrons per V3+. Long range magnetic ordering of 0.98(2)mu_(B) per V3+ (3d(2)) sets in at the 93 K structural transition. The orbital ordering relieves the geometric frustration and leads to a magnetically ordered ground state.     

A new interpretation of the CO state in half-doped manganites: new results from neutron diffraction and synchrotron radiation experiments

In the R_1−xD_xMnO₃ (x0.5) manganites, the structural phase transition at T_CO is commonly interpreted as a concomitant charge and orbital ordering (CO/OO) process driven by a co-operative Jahn–Teller effect and Coulomb repulsion forces. The low-temperature phase is supposed to contain well-separated and ordered Mn³⁺ and Mn⁴⁺ ionic species in an NaCl-like pattern. Structure refinement, from a neutron diffraction experiment below T_CO on a Pr_0.6Ca_0.4MnO₃ single crystal, gives us a model for the displacement of atoms with respect to the high-temperature phase that invalidates the standard model based in the CO/OO picture. Our result is a non-centrosymmetric crystal structure with two non-equivalent MnO₆ octahedra, both being slightly elongated but displaying very similar average Mn–O distances (1.96 and 1.95 Å, respectively) and having off-centered Mn atoms. We argue that this is a proof of the absence of charge ordering in half-doped manganites in the sense of formation of separated Mn³⁺ and Mn⁴⁺ ionic species. A new qualitative interpretation of the CE-type spin ordering (SO) is proposed. The so-called CO transition is, in fact, a structural transition induced by the change in the mean free path of electrons that continue to be thermally activated below T_CO by forming ferromagnetic Mn–Mn pairs stabilized by a local double-exchange process. The CE SO pattern results from the ordering of these pairs formed at T_CO. High-resolution synchrotron powder diffraction shows a complex anisotropic/asymmetric strains appearing at the transition that can be phenomenologically fitted by additional phases. Complementary electron diffraction and microscopy have shown no trace of macroscopic phase separation.     

(1)H and (87)Rb nuclear magnetic resonance study of the order-disorder phase transition of RbHSeO(4) single crystals

The ferroelectric phase transition at T(C2) (=370K) in RbHSeO(4) has been studied by (1)H and (87)Rb solid-state NMR. Although not large, the spin-lattice relaxation time, T(1), and the spin-spin relaxation time, T(2), of rubidium and of the alpha- and beta-type protons show distinct change near the phase transition. The intensity of the signal due to the alpha-type protons decreases with increasing temperature, and the intensity of alpha-type protons is quite weak above 330K: at a temperature which is about 40K lower than the phase transition temperature, the ordering of the alpha-type protons occurs. The alpha-type protons in the ferroelectric phase lead to a noticeable change in the proton magnetic resonance spectra. Our study of the (1)H spectra shows that the ferroelectric phase transition in RbHSeO(4) is of order-disorder type and is due to the ordering of protons in hydrogen bonds.     

Fragile magnetic ground state in half-doped LaSr2Mn2O7

We investigated the orbital and antiferromagnetic ordering behaviors of the half-doped bilayer manganite La(2-2x)Sr(1+2x)Mn2O7 (x ? 0.5) by using Mn L(2,3)-edge resonant soft x-ray scattering. Resonant soft x-ray scattering reveals the CE-type orbital order below T(oo) ? 220 K, which shows partial melting behavior below T(m) ? 165 K. We also found coexistence CE- and A-type antiferromagnetic orders. Both orders involve the CE-type orbital order with nearly the same orbital character and are coupled with each other. These results manifest that the ground state with the CE-type antiferromagnetic order is easily susceptible to destabilization into the A-type one even with a small fluctuation of the doping level, as suggested by the extremely narrow magnetic phase boundaries at x ? 0.5±0.005.     

Orbital ordering transition in Ca2RuO4 observed with resonant X-ray diffraction

Resonant x-ray diffraction performed at the L(II) and L(III) absorption edges of Ru has been used to investigate the magnetic and orbital ordering in Ca2RuO4 single crystals. A large resonant enhancement due to electric dipole 2p-->4d transitions is observed at the wave-vector characteristic of antiferromagnetic ordering. Besides the previously known antiferromagnetic phase transition at T(N)=110 K, an additional phase transition, between two paramagnetic phases, is observed around 260 K. Based on the polarization and azimuthal angle dependence of the diffraction signal, this transition can be attributed to orbital ordering of the Ru t(2g) electrons. The propagation vector of the orbital order is inconsistent with some theoretical predictions for the orbital state of Ca2RuO4.     

One-dimensional instability in BaVS3

The 3d(1) system BaVS3 undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T(MI)=70 K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k(F)=c(*)/2 charge-density wave (CDW) instability of the d(z(2)) electron gas. We discuss the formation below T(MI) of an unconventional CDW state involving the condensation of the other V4+ 3d(1) electrons of the quasidegenerate e(t(2g)) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS3 and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.     

Zener polaron ordering variants induced by A-site ordering in half-doped manganites

We have studied the magnetism of the half-doped charge ordered manganite YBaMn2O6. A formation of ferromagnetic plaquettes of four Mn atoms in the charge ordered phase below T_{CO} approximately 480 K is inferred from high temperature magnetic susceptibility data and the magnetic structure, as determined by neutron powder diffraction at T=1.5 K. The results indicate that new fourfold Mn paramagnetic units form between T_{N}相似文献   

Charge/orbital ordering structure of Pr(1-x)Ca(x)MnO(3)(x = 3/8) examined by low-temperature transmission electron microscopy

The structural phase transition of Pr(1-x)Ca(x)MnO(3)(x = 3/8) was investigated by means of low-temperature transmission electron microscopy. Superlattice reflection spots with a modulation wave vector q(1) = (0,1/2,0) appeared below 230 K, indicating formation of the d(3x(2-r(2))/d(3y(2)-r(2)) type of charge/orbital ordering. Below 150 K, a new series of superlattice reflection spots with a modulation wave vector q(2) = (1/4,1/4,1/2) appeared, suggesting an additional ordering of excess 1/8 Mn(3+), necessary due to the deviation of x from 1/2, with the occupation of the d(3z(2-r(2)) type of e(g) orbital.     

Irreversible metamagnetic transition and magnetic memory in small-bandwidth manganite Pr(1-x)Ca(x)MnO3 (x = 0.0-0.5)

The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr(1-x)Ca(x)MnO(3) (with x = 0.0-0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM-FM ordering and spin-glass phase at the low doping range (x = 0.0-0.2) while increased hole doping (x = 0.3-0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.     

Charge-ordering signatures in the optical properties of beta-Na0.33V2O5

Temperature dependent optical spectra are reported for beta-Na0.33V2O5. The sodium ordering transition at T(Na)=240 K and, in particular, the charge ordering transition at T(MI)=136 K strongly influence the optical spectra. The metal-insulator transition at T(MI) leads to the opening of a pseudogap ( variant Planck's over 2pi omega=1700 cm(-1)) and to the appearance of a large number of optical phonons. These observations and the presence of a midinfrared band (typical for low dimensional metals) strongly suggest that the charge carriers in beta-Na0.33V2O5 are small polarons.     

Pr1/3Sr2/3FeO3中电荷有序现象的超声研究

系统研究了Pr1/3Sr2/3FeO3单相多晶样品在低温下的电荷输运性质和超声特性.电阻测量表明,Pr1/3Sr2/3FeO3在162K发生了电荷有序相变.而超声声速从室温开始就出现了明显的软化,在电荷有序温度附近达到最小值,随后又急剧硬化,同时伴随着一个巨大的衰减峰.分析指出这是由于电-声子相互作用导致的结果,该电-声子耦合可能起源于Fe4 的Jahn-Teller效应.     

Microstrain sensitivity of orbital and electronic phase separation in SrCrO3

An orbital ordering transition and electronic phase coexistence have been discovered in SrCrO3. This cubic, orbitally-degenerate perovskite transforms to a tetragonal phase with partial orbital order. The tetragonal phase is antiferromagnetic below 35-40 K, whereas the cubic phase remains paramagnetic at low temperatures. The orbital ordering temperature (35-70 K) and coexistence of the two electronic phases are very sensitive to lattice strain. X-ray measurements show a preferential conversion of the most strained regions in the cubic phase. This reveals that small fluctuations in microstrain are sufficient to drive long range separation of competing electronic phases even in undoped cubic oxides.     

Nonlocal Coulomb correlations in metals close to a charge order insulator transition

The charge ordering transition induced by the nearest-neighbor Coulomb repulsion V in the 1/4-filled extended Hubbard model is investigated using cellular dynamical mean-field theory. We find a transition to a strongly renormalized charge ordered Fermi liquid at V(CO) and a metal-to-insulator transition at V(MI)>V(CO). Short range antiferromagnetism occurs concomitantly with the CO transition. Approaching the charge ordered insulator, V approximately 相似文献   

Unique spin dynamics and unconventional superconductivity in the layered heavy fermion compound CeIrIn5: NQR evidence

We report measurements of the 115In nuclear spin-lattice relaxation rate ( 1/T1) between T = 0.09 and 100 K in the new heavy fermion (HF) compound CeIrIn5. At 0.4 < or = T< or = 100 K, 1/T1 is strongly T-dependent, which indicates that CeIrIn5 is much more itinerant than known Ce-based HFs. We find that 1/T1T, subtracting that for LaIrIn5, follows a (1 / T+straight theta)3/4 variation with straight theta = 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/T1 follows a T3 variation, which suggests unconventional superconductivity with line-node gap.     

Infrared spectroscopy of the charge ordering transition of Na0.5CoO2

We report infrared reflectivity study of charge ordering in a Na0.5CoO2 single crystal. In comparison with x=0.7 and 0.85 compounds, we found that the effective carrier density increases systematically with decreasing Na contents. The charge ordering transition only affects the optical spectra below 1000 cm(-1). A hump near 800 cm(-1) develops below 100 K, which is accompanied by the appearance of new lattice modes as well as the strong antiresonance feature of phonon spectra. These observations signify a polaronic characteristic of charge carriers. Below T(co), an optical gap develops at the magnitude of 2Delta approximately 3.5k(B)T(co) (T相似文献   

A new phase transition in 2H-TaSe2 — A dilatometric study

A new first-order phase transition of 2H-TaSe₂ has been observed at 113 K in thermal expansion measurements, using a high-resolution capacitance dilatometer. The first order nature of the lock-in transition at 93 K has been confirmed. The Charge-Density-Wave ordering transition at 123 K is observable only in the basal plane thermal expansion, emphasing the two-dimensional nature of the ordering.     

Metamagnetic phase transitions induced by static and pulsed fields in (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics

It has been found that the magnetic susceptibility of (Sm_0.5Gd_0.5)_0.55Sr_0.45MnO₃ ceramic samples in zero external magnetic field exhibits a sharp peak near the temperature of 48.5 K with a small temperature hysteresis that does not depend on the frequency of measurements and is characteristic of the phase transition to an antiferromagnetic state with a long-range charge orbital ordering, which is accompanied by an increase in the magnetic susceptibility with a decrease in the temperature. The magnetization isotherms in static and pulsed magnetic fields at temperatures below 60 K demonstrate the occurrence of an irreversible metamagnetic transition to a homogeneous ferromagnetic state with a critical transition field independent of the measurement temperature, which, apparently, is associated with the destruction of the insulating state with a long-range charge ordering. In the temperature range 60 K ?? T ?? 150 K, the ceramic samples undergo a magnetic-field-induced reversible phase transition to the ferromagnetic state, which is similar to the metamagnetic transition in the low-temperature phase and is caused by the destruction of local charge/orbital correlations. With an increase in the temperature, the critical transition fields increase almost linearly and the field hysteresis disappears. Near the critical fields of magnetic phase transitions, small ultra-narrow magnetization steps have been revealed in pulsed fields with a high rate of change in the magnetic field of ??400 kOe/??s.     

Low-Temperature Phase Transitions in the Trigonal Modification of Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 I 9

The trigonal (P-3 m1) modification of Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 I 9 have been studied using NQR, X-ray single crystal and powder pattern methods. Moreover, the heat capacity was measured in a wide temperature interval: 4-300 K. In Cs 3 Bi 2 Br 9 a second-order phase transition was found at T C = 96 K. The low-temperature phase is monoclinic (C12/c1), with the unit cell doubled along the [001] direction. Cs 3 Sb 2 I 9 has a sequence of phase transitions at T C = 85 K, T i = 78 K and T L = 72.1 K. The monoclinic structure below 85 K is isomorphic with the low-temperature structure of Cs 3 Bi 2 Br 9 . According to calorimetric data the lock-in transition at 72.1 K is discontinuous.     

Doping dependent evolution of magnetism and superconductivity in Eu(1-x)K(x)Fe2As2 (x = 0-1) and temperature dependence of the lower critical field H(c1)

We have synthesized polycrystalline samples of Eu(1-x)K(x)Fe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (T(c) = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent Tx phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field H(c1) of the superconducting polycrystalline samples. The values of H(c1)(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba(1-x)K(x)Fe2As2 and BaFe(2-x)Co(x)As2.     

Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR

We report bulk superconductivity (SC) in Eu(0.2)Sr(0.8)(Fe(0.86)Co(0.14))(2)As(2) single crystals by means of electrical resistivity, magnetic susceptibility and specific heat measurements with T(c) is approximately equal to 20 K and an antiferromagnetic (AFM) ordering of Eu(2+) moments at T(N) is approximately equal to 2.0 K in zero field. (75)As NMR experiments have been performed in the two external field directions (H is parallel to ab) and (H is parallel to c). (75)As-NMR spectra are analysed in terms of first-order quadrupolar interaction. Spin-lattice relaxation rates (1/T(1)) follow a T(3) law in the temperature range 4.2-15 K. There is no signature of a Hebel-Slichter coherence peak just below the SC transition, indicating a non-s-wave or s(±) type of superconductivity. In the temperature range 160-18 K 1/T(1)T follows the C/(T+θ) law reflecting 2D AFM spin fluctuations.