DCNQI-Cu Salts Research Articles

X-Ray photoemission study of an organic quasi-1D DCNQI-Cu salt

We have studied the electronic states of an organic quasi-one-dimensional conductor undeuterated (DMe-DCNQI) 2Cu by X-ray photoemission spectroscopy. The valence-band spectra taken using photon energies in the N 1s and Cu 2p core-absorption regions have shown resonance enhancement of overlapping Auger features. We have found a strong excitation energy dependence of the Auger emission intensity in the N 1s absorption region. By comparing valence-band photoemission spectra taken at various photon energies, spectral features have been assigned to atomic orbital characters of the DCNQI-Cu salt.

Effect of Strong Correlation on Metal-Insulator Transition of DCNQI-Cu Salts – Rigorous Treatment of the Local Constraint –

Electronic properties originated from the strong correlation have been examined for both metallic and insulating states in organic conductor, DCNQI-Cu salt by use of a periodic Anderson model with electron-phonon interaction. The local constraint imposed on the d -electron in the Cu atom is taken into account in terms of a method of the slave-boson. Compared with the previous mean-field calculation which corresponds to the global constraint, we have found novel aspects that the transition temperature is suppressed and that the magnetic susceptibility of the insulating state exhibits the temperature dependence of the Curie law. These results are in favor of the explanation of the experimental results.

Novel electronic states in the (DCNQI) 2Cu system

A series of molecular conductors (R 1,R 2-DCNQI) 2Cu exhibit unique physical properties associated with the interaction between the pπ organic one-dimensional band and the d-orbital of Cu atom. The DCNQI-Cu salts are classified into three types: type I salt shows a metallic behavior down to low temperature, type II salt shows a metal-insulator (M-I) transition, and type III salt shows a M-I-M (reentrant) transition. All the types of transitions are first-order. In addition, the DI-system (R 1=R 2=I) shows another metallic phase accompanied by the small resistivity anomaly in the higher-pressure region. There are two important parameters that govern the electronic states of this system. The one is the amount of charge transfer from Cu to DCNQI which is quite sensitive to pressure or substituent (R 1,R 2). The other parameter is an inter-chain LUMO···LUMO interaction which plays an important role in the iodine-containing system. This interaction significantly affects the electronic structure. A complicated P-T phase diagram of the DI-system could be explained by these two parameters.

Density of States of Strongly Correlated Electrons in DCNQI-Cu Salts

By making use of the periodic Anderson model with both the electron-phonon interaction and the transverse hopping, we examine the density of states at zero temperature for strongly correlated d electrons in DCNQI-Cu salt which undergoes a metal-insulator transition followed by three-fold periodicity of both a lattice distortion and a local spin. A slave-boson method is applied to the model expressed in terms of a hole picture in order to treat the local constraint strictly. There are both the coherent (band) component and the incoherent (localized) component in the metallic state. In the insulating state, all the electrons at the site with the local spin are incoherent and a small amount of both components remains at other sites.

High-resolution photoemission and x-ray absorption study of quasi-one-dimensional DCNQI-Cu salt

We have made a high-resolution photoemission (PES) and x-ray absorption (XAS) study of quasi-one-dimensional partially deuterated (DMe-DCNQI)2Cu. The PES spectra in the metallic phase exhibit a power-law dependence on the electron binding energy ω on a high energy scale (∼0.3 eV>|ω|>∼0.05 eV) or at high temperatures (T>∼300 K) while a weak but finite intensity is found at the Fermi level at low temperatures, indicating a weak 3D character. From the N 1s and Cu 2p XAS spectra, we clearly observe the mixed valence state of Cu and the strong anisotropy of the electronic structure although 3D character is also found.

High-resolution photoemission and x-ray absorption study of quasi-one-dimensional DCNQI-Cu salt

We have made a high-resolution photoemission (PES) and x-ray absorption (XAS) study of quasi-one-dimensional partially deuterated (DMe-DCNQI) 2Cu. The PES spectra in the metallic phase exhibit a power-law dependence on the electron binding energy ω on a high energy scale (∼0.3 eV>|ω|>∼0.05 eV) or at high temperatures (T>∼300 K) while a weak but finite intensity is found at the Fermi level at low temperatures, indicating a weak 3D character. From the N 1 s and Cu 2 p XAS spectra, we clearly observe the mixed valence state of Cu and the strong anisotropy of the electronic structure although 3D character is also found.

Re-entrant phenomena of the organic conductor, DCNQI-Cu salts

Partially deuterated molecular conductor, (DMe-DCNQI-da)2Cu (n=2, 3) exhibit successive metal-insulator-metal transitions at ambient pressure. We have investigated the re-entrant phenomena microscopically by the measurements of 1H-NMR relaxation rate, T1−1, and absorption lineshapes. The transitions are detected from the fact that T1−1 in the insulating state is large due to the appearance of Cu2+ local magnetic moments. In the low temperature metallic state, it has been observed that T1−1 does not follow the Korringa relation: T1T=const. The possibility of 1D-spin fluctuations is considered.

Magnetic structure in the antiferromagnetic state of DCNQI-Cu salts

Abstract The antiferromagnetic (AF) state of (DMe-DCNQI[3,3:1]d7)2Cu has been investigated microscopically by the measurements of 1H-NMR lineshapes and antiferromagnetic resonance (AFMR) on a single crystal sample. NMR line in the AF state was found to be composed of well-separated three pairs of peaks, while the 1H has a unique crystallographic site. From the angular dependence of the peak positions, the magnetic structure is determined; i) the Cu2+ ions are localized with three-fold periodicity, ii) the AF moments reside only at Cu2+ site, and iii) the hard axis of AF state is the c-axis. The angular dependence of AFMR field indicates that the easy axis is in the ab-plane and parallel to the symmetry axis of CN tetragon coordinating axis.

Systematic study of DCNQI-Cu salts by NMR at selective nuclear sites

In order to investigate electronic structure of the DCNQI-Cu systems at molecular site, we prepared 13 C labeled DMe-DCNQI and measured 13 C NMR line shapes and spin lattice relaxation rate. Comparison with the results on some partially and non-duterized DCNQI salts, lead us to get some important informations on the nature of the metallic phases.

Metal-insulator transition in organic conductor DCNQI-Cu salt

Abstract The first order metal-insulator transition of the mixed-valent conductor DCNQI-Cu salt has been studied theoretically by applying the slave boson method to the periodic Anderson model with the electron-phonon interaction. It is shown that both the lattice distortion and the local spin with three-fold periodicity appear simultaneously at the transition.

Synchrotron-radiation photoemission study of in-situ synthesized DCNQI( N, N′-dicyanoquinonediimine)-Cu salts

A photoemission study using synchrotron radiation of in-situ synthesized organic conductors (DMe-DCNQI) 2Cu and (MeBr-DCNQI) 2Cu is reported, where DMe-DCNQI and MeBr-DCNQI are dimethyl- N, N′-dicyanoquinonediimine and methyl, bromine- N, N′-dicyanoquinonediimine, respectively. The photon energy dependence of the valence band spectra is used to characterize the origin of the observed features. In both salts, the Cu 3 d derived states are situated at 3.5–4 eV binding energy, while the C and N 2 pσ states appear around 7.5 eV. The Br 4 p states are found around 5 eV in (MeBr-DCNQI) 2Cu. The states near the Fermi level have substantial C and N 2 pπ character consistent with extended Hückel molecular orbital calculations. The absence of a resonance at the Cu 3 p threshold suggests a small contribution of Cu 2+ configuration in DCNQI-Cu salts.

Peierls Transition of Strongly Correlated DCNQI-Cu Salts

The strongly correlated electronic states of the periodic Anderson model in the presence of the electron-phonon (e-p) interaction have been examined to understand the properties of the metal-insulator transition for DCNQI-Cu salts. By making use of the slave-boson mean-field theory based on the previous work [J. Phys. Soc. Jpn. 62 (1993) 3244], it is shown that the first-order Peierls transition from the metallic state with the heavy electron mass into the insulating state with the local spin takes place by varying the temperature and/or the e-p coupling constant.

Interplay of Charge Density and Spin Density in DCNQI-Cu Salts

Treating the periodic Anderson model with the electron-phonon (e-p) interaction in the mean field theory, we examine the insulating state in DCNQI-Cu salts as the function of both the temperature and the e-p coupling constant. By extending the work [J. Phys. Soc. Jpn. 61 (1992) 3322] to the case where the e-p interaction at the Cu site is added, it is shown that the phase transition into the coexistent state of charge density and spin density is accompanied by the large jump of their amplitudes in addition to the hysteresis.

Strongly Correlated Electrons in Mixed-Valence Conductors DCNQI-Cu Salts

Organic conductors, DCNQI-Cu salts, with a mixed valence Cu +4/3 on the average has been studied theoretically by applying the slave-boson method to the strongly correlated systems which consist of π-electrons of DCNQI molecule and d-electrons of Cu atoms. The ground state is calculated by use of the 1/N σ expansion method which treats the local constraint strictly. It is shown that the electron mass around the Fermi surface is enhanced in the presence of the charge density with the three-fold periodicity.

Charge and spin states of DCNQI-Cu salts

Magnetic susceptibility, lattice parameters, heat capacity and magnetoresistance under pressure were measured systematically as functions of temperature in (DMeDCNQI)2Cu and (DMeODCNQI)2Cu, which are metallic down to the lowest temperature measured. These compounds are found to be marginal metals where the copper valence is fluctuating giving marginal properties between a localized state having a magnetic spin and delocalized one in the d-π mixed band. A relatively low pressure or a small amount of lattice distortion brings the system into the well-defined localized state where a band gap opens at the Fermi level of the conduction band. The gap is ascribed to the periodic potential formed by the charge ordering of Cu2+. Origins of the so-called reentrance phenomena are examined in relation to the lattice distortion.

Magnetism of DCNQI-Cu salts

Magnetic properties of (DMe-DCNQI)2Cu, (DMeO-DCNQI)2Cu and (DBr-DCNQI)2Cu were investigated by 1H-NMR, ESR and magnetic susceptibility measurements. 1H-T1−1 was found to exhibit a step-like enhancement at the metal-insulator (M-I) transitions of DBr-salt at ambient pressure and of DMe- and DMeO-salts under pressure. This reflects the appearance of localized Cu2+ spins at the M-I transition. An antiferromagnetic (AF) ordering was observed in DBr- (at ambient pressure) and DMe-salts (under pressure). In both salts, 1H-NMR relaxation rates, T1−1, T2∗−1and T2−1, indicate the onset of inhomogeneous local field at the transition temperature, TN: The lineshape below TN is well explained by the assumption that the magnetic Cu2+-sites with μB form a regular three-fold lattice along the c-axis. In the DMe-salt, the pressure dependence of TN was determined. It was shown that the enhancement of static susceptibility in the metallic state of DMe-salt was due to the insulating domain induced by some stress within the sample.