4,075 research outputs found

    Microscopic origin of local moments in a zinc-doped high-TcT_{c} superconductor

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    The formation of a local moment around a zinc impurity in the high-TcT_{c} cuprate superconductors is studied within the framework of the bosonic resonating-valence-bond (RVB) description of the tJt-J model. A topological origin of the local moment has been shown based on the phase string effect in the bosonic RVB theory. It is found that such an S=1/2S=1/2 moment distributes near the zinc in a form of staggered magnetic moments at the copper sites. The corresponding magnetic properties, including NMR spin relaxation rate, uniform spin susceptibility, and dynamic spin susceptibility, etc., calculated based on the theory, are consistent with the experimental measurements. Our work suggests that the zinc substitution in the cuprates provide an important experimental evidence for the RVB nature of local physics in the original (zinc free) state.Comment: The topological reason of local moment formation is given. One figure is adde

    Tunable temperature induced magnetization jump in a GdVO3 single crystal

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    We report a novel feature of the temperature induced magnetization jump observed along the a-axis of the GdVO3 single crystal at temperature TM = 0.8 K. Below TM, the compound shows no coercivity and remanent magnetization indicating a homogenous antiferromagnetic structure. However, we will demonstrate that the magnetic state below TM is indeed history dependent and it shows up in different jumps in the magnetization only when warming the sample through TM. Such a magnetic memory effect is highly unusual and suggesting different domain arrangements in the supposedly homogenous antiferromagnetic phase of the compound.Comment: 17 pages, 8 Figure

    Comment on "Localized behavior near the Zn impurity in YBa2Cu4O8 as measured by nuclear quadrupole resonance"

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    Williams and Kramer [Phys. Rev. B {\bf 64}, 104506 (2001)] have recently argued against the existence of staggered magnetic moments residing on several lattice sites around Zn impurities in YBCO superconductors. This claim, which is in line with an earlier publication by Williams, Tallon and Dupree [Phys. Rev. B {\bf 61}, 4319 (2000)], is however in contradiction with a large body of experimental data from different NMR groups. On the contrary, the authors argue in favor of a very localized spin and charge density on Cu sites first neighbors to Zn. We show that the conclusions of Williams and Kramer arise from erroneous interpretations of NMR and NQR data.Comment: 4 page

    Bose-Einstein condensation of triplons in the S=1 tetramer antiferromagnet K2Ni2(MoO4)3: A compound close to quantum critical point

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    The structure of K2Ni2(MoO4)3 consists of S=1 tetramers formed by Ni^{2+} ions. The magnetic susceptibility chi(T) and specific heat Cp(T) data on a single crystal show a broad maximum due to the low-dimensionality of the system with short-range spin correlations. A sharp peak is seen in chi(T) and Cp(T) at about 1.13 K, well below the broad maximum. This is an indication of magnetic long-range order i.e., the absence of spin-gap in the ground state. Interestingly, the application of a small magnetic field (H>0.1 T) induces magnetic behavior akin to Bose-Einstein condensation (BEC) of triplon excitations observed in some spin-gap materials. Our results demonstrate that the temperature-field (T-H) phase boundary follows a power-law (T-T_{N})propotional to H^(1/alpha) with the exponent 1/alpha close to 2/3, as predicted for BEC scenario. The observation of BEC of triplon excitations in small H infers that K2Ni2(MoO4)3 is located in the proximity of a quantum critical point, which separates the magnetically ordered and spin-gap regions of the phase diagram.Comment: 5 pages, 5 figures, Accepted in Phys. Rev. B Rapid Communication

    Antiferromagnetism of Zn2_2VO(PO4)2_4)_2 and the dilution with Ti4+^{4+}

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    We report static and dynamic properties of the antiferromagnetic compound Zn2_{2}(VO)(PO4_{4})2_{2}, and the consequences of non-magnetic Ti4+^{4+} doping at the V4+^{4+} site. 31^{31}P nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rate (1/T11/T_1) consistently show the formation of the long-range antiferromagnetic order below TN=3.83.9T_N= 3.8-3.9\,K. The critical exponent β=0.33±0.02\beta=0.33 \pm 0.02 estimated from the temperature dependence of the sublattice magnetization measured by 31^{31}P NMR at 9.4\,MHz is consistent with universality classes of three-dimensional spin models. The isotropic and axial hyperfine couplings between the 31^{31}P nuclei and V4+^{4+} spins are Ahfiso=(9221±100)A_{\rm hf}^{\rm iso} = (9221 \pm 100) Oe/μB\mu_{\rm B} and Ahfax=(1010±50)A_{\rm hf}^{\rm ax} = (1010 \pm 50) Oe/μB\mu_{\rm B}, respectively. Magnetic susceptibility data above 6.5\,K and heat capacity data above 4.5\,K are well described by quantum Monte-Carlo simulations for the Heisenberg model on the square lattice with J7.7J\simeq 7.7\,K. This value of JJ is consistent with the values obtained from the NMR shift, 1/T11/T_1 and electron spin resonance (ESR) intensity analysis. Doping Zn2_2VO(PO4)2_4)_2 with non-magnetic Ti4+^{4+} leads to a marginal increase in the JJ value and the overall dilution of the spin lattice. In contrast to the recent \textit{ab initio} results, we find neither evidence for the monoclinic structural distortion nor signatures of the magnetic one-dimensionality for doped samples with up to 15\% of Ti4+^{4+}. The N\'eel temperature TNT_{\rm N} decreases linearly with increasing the amount of the non-magnetic dopant.Comment: 13 pages, 12 figures, 2 table

    Chemical solution deposition of single phase BiFeO3 thin films on transparent substrates

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    The production of high quality BiFeO3 thin films on cost-effective transparent electrodes for visible light harvesting applications and devices remains a challenge. Here, we report the production of single-phase nanostructured BiFeO3 thin films via chemical solution deposition (CSD) on transparent conductive fluorine doped tin oxide FTO glass substrates. We show that the BiFeO3 is of high purity using a variety of analytical tools and that the as-obtained BiFeO3 thin films have a single grain single domain structure exhibiting ferroelectric switching under poling. The BiFeO3 samples show visible light absorption with a band gap of 2.7 eV under all processing conditions. By changing the annealing atmosphere it was possible to modify the photocurrent produced, which were (at 1.23 VNHE) 0.07 mA/cm2 (O2-annealed), 0.02 mA/cm2 (air-annealed) and 0.01 mA/cm2 (Ar-annealed). This indicates a change in the mobile carriers available. Our results show that it is possible to produce single phase BiFeO3 on a transparent conductive electrode system with controllable photoconductivity

    Quantum Monte Carlo study of a nonmagnetic impurity in the two-dimensional Hubbard model

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    In order to investigate the effects of nonmagnetic impurities in strongly correlated systems, Quantum Monte Carlo (QMC) simulations have been carried out for the doped two-dimensional Hubbard model with one nonmagnetic impurity. Using a bare impurity potential which is onsite and attractive, magnetic and single-particle properties have been calculated. The QMC results show that giant oscillations develop in the Knight shift response around the impurity site due to the short-range antiferromagnetic correlations. These results are useful for interpreting the NMR data on Li and Zn substituted layered cuprates.Comment: 10 pages, 7 figure
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