Cu [ N Research Articles

Spin-liquid and magnetic phases in the anisotropic triangular lattice: The case ofκ−(ET)2X

The two-dimensional Hubbard model on the anisotropic triangular lattice, with two different hopping amplitudes $t$ and $t^\prime$, is relevant to describe the low-energy physics of $\kappa$-(ET)$_2$X, a family of organic salts. The ground-state properties of this model are studied by using Monte Carlo techniques, on the basis of a recent definition of backflow correlations for strongly-correlated lattice systems. The results show that there is no magnetic order for reasonably large values of the electron-electron interaction $U$ and frustrating ratio $t^\prime/t = 0.85$, suitable to describe the non-magnetic compound with X=Cu$_2$(CN)$_3$. On the contrary, N\'eel order takes place for weaker frustrations, i.e., $t^\prime/t \sim 0.4 \div 0.6$, suitable for materials with X=Cu$_2$(SCN)$_2$, Cu[N(CN)$_2$]Cl, or Cu[N(CN)$_2$]Br.

Optical Modulation of Effective On-Site Coulomb Energy for the Mott Transition in an Organic Dimer Insulator

We report an optical modulation of the effective on-site Coulomb energy U on a dimer (U_{dimer}) for achieving the Mott insulator-to-metal transition in kappa-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Br, as investigated by pump-probe spectroscopy. A reduction of U_{dimer} is optically induced by molecule displacement in the dimer under intradimer excitation. The mechanism of this metallization differs greatly from the photodoping-type mechanism reported previously. In contrast, a faster transition via the photodoping mechanism is detected for interdimer excitation. A metallic-domain-wall oscillation originating from the modulation of U_{dimer} was also observed near the critical end point of the Mott transition line.

The relation between magnetization and internal energy for ferromagnets Fe, Ni, and electron‐doped In2O3:Cr, antiferromagnetically ordered Zn Cu3(OH)6 Cl2, and the layered material κ‐(BEDT‐TTF)2 Cu[N(CN)2]Cl

AbstractGrout and March utilized the exact solution of the two‐dimensional Ising model to relate the internal energy E(T) to the magnetization M(T). These authors then used Bloch spin‐wave theory to relate E and M at low temperatures for both insulating and metallic ferromagnets in three dimensions.Ayuela and March, very recently, have used experimental data on the specific heat and the magnetization to show that for metallic Fe and Ni the E – M data collapse on to a single curve. A model with long‐range exchange interactions is presented to allow insight to be gained into this feature. Reference is also made briefly to the ferromagnetism of the electron‐doped In2 O3:Cr system.Two antiferromagnetically ordered materials are next considered, though now more qualitatively than for the ferromagnets. The first is the spin −1/2 kagome antiferromagnet ZnCu3(OH)6 Cl2, and the second is the κ‐phase layered material (BEDT‐TTF)2 Cu[N(CN)2]Cl. Anomalous low‐temperature specific heat Cv (T) is emphasized, with an enhancement that is reduced by application of a magnetic field. Further work is proposed, both experiment and theory, to understand the precise nature of the low‐lying magnetic excitations in the latter κ‐phase material. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Direct Observation of d-Wave Superconducting Gap in κ-(BEDT-TTF)2Cu[N(CN)2]Br with Scanning Tunneling Microscopy

Conducting and lateral surfaces of two-dimensional organic superconductor κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br were studied by electron tunneling using scanning tunneling microscopy. The V-shape tunneling conductance curve was observed at the conducting plane in the superconducting phase, attributed to the d -wave pairing symmetry. Tunneling spectra were successfully obtained at various lateral surfaces of crystals. The tunneling conductance curve varies systematically depending on tunneling direction. The conductance curve and its directional dependence are explained by the d -wave with nodes along the direction near π/4 from the k a - and k c -axes. The pairing symmetry is of the d x 2 - y 2 -wave. (If we take the same convention for the magnetic Brillouin zone as high- T c cuprates with the d x 2 - y 2 -symmetry, the pairing symmetry of κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br is the d x y -symmetry.) This is the same symmetry as that in the superconducting gap of κ-(BEDT-TTF) 2 Cu(NCS) 2 . The zero bias conductance pea...

The first polymorph, κ″-(ET) 2Cu[N(CN) 2]Cl, in the family of κ-(ET) 2Cu[N(CN) 2] X ( X=Cl, Br, I) radical cation salts

The first polymorph, κ″-(ET) 2Cu[N(CN) 2]Cl, of the well known Mott insulator κ-(ET) 2Cu[N(CN) 2]Cl has been prepared and its crystal and electronic structures have been examined. The polymorph has monoclinic symmetry in contrast with the orthorhombic one of the isostructural κ-salts of the family (ET) 2Cu[N(CN) 2] X ( X=Cl, Br, I). The monoclinic phase exhibits a layered structure in which the conducting layers are packed in a κ-type arrangement and the anion sheets consist of polymeric zigzag chains formed by Cu[N(CN) 2]Cl units. The main structural differences with the orthorhombic κ-salts are that the anion sheets are disordered and the ET molecules are less planar. The new polymorph shows metallic type resistivity down to 4.2 K.

Carrier Dynamics in κ-Type Organic Superconductors: Time-Resolved Observation

The κ-type organic superconductors have unique physical properties, many of which are yet to be understood. Most of their physical properties are closely related to carrier dynamics, which can be directly observed by ultrafast optical measurements in their nonequilibrium states. Time-resolved temperature-dependent reflectivity changes were measured on single crystals of an isostructural pair of organic charge transfer salts κ-(ET) 2 Cu[N(CN) 2 ]X, which are ambient- (X=Br) or high-pressure (X=Cl) superconductors. The latter compound in its ground state is a Mott insulator under ambient pressure. The obtained temperature dependences of relaxation times for both salts indicated the existence of an intermediate electronic state between those of metals and insulators at all measurement temperatures. Such an unusual electronic state coexisted with the superconducting and Mott insulating states in X=Br and X=Cl, respectively. In addition, the results for X=Br revealed the formation of a temperature-dependent we...

Low-dimensional compounds containing cyano groups. XVI. (Dicyanamido-κN1)bis(1,10-phenanthroline-κ2N,N′)copper(II) tetrafluoridoborate

The title compound, [Cu{N(CN)(2)}(C(12)H(8)N(2))(2)]BF(4), was prepared as part of our study of the shape of coordination polyhedra in five-coordinated copper(II) complexes. Single-crystal X-ray analysis reveals that the structure consists of [Cu{N(CN)(2)}(phen)(2)](+) cations (phen is 1,10-phenanthroline) and BF(4)(-) anions. The Cu centre is five-coordinated in a distorted trigonal bipyramidal manner by four N atoms of two phen ligands and one N atom of a dicyanamide anion, which is coordinated in the equatorial plane at a distance of 1.996 (2) A. The two axial Cu-N(phen) distances have similar values [average 1.994 (6) A] and are shorter than the two equatorial Cu-N(phen) bonds [average 2.09 (6) A]. This work demonstrates the effect of ligand rigidity on the shape of coordination polyhedra in five-coordinated copper(II) complexes.

Quasiparticles at the Verge of Localization near the Mott Metal-Insulator Transition in a Two-Dimensional Material

The dynamics of charge carriers close to the Mott transition is explored theoretically and experimentally in the quasi-two-dimensional organic charge-transfer salt, kappa-(BEDT-TTF)_(2)Cu[N(CN)_(2)]Br_(x)Cl_(1-x), with varying Br content. The frequency dependence of the conductivity deviates significantly from simple Drude model behavior: there is a strong redistribution of spectral weight as the Mott transition is approached and with temperature. The effective mass of the quasiparticles increases considerably when coming close to the insulating phase. A dynamical mean-field-theory treatment of the relevant Hubbard model gives good quantitative description of experimental data.

X-ray Irradiation-Induced Carrier Doping Effects in Organic Dimer–Mott Insulators

We report X-ray irradiation-induced carrier doping effects on the electrical conductivity in the organic dimer-Mott insulators $\kappa$-(ET)$_{2}$$X$ with $X =$ Cu[N(CN)$_{2}$]Cl and Cu$_{2}$(CN)$_{3}$. For $\kappa$-(ET)$_{2}$Cu[N(CN)$_{2}$]Cl, we have observed a large decrease of the resistivity by 40 % with the irradiation at 300 K and the metal-like temperature dependence down to about 50 K. The irradiation-induced defects expected at the donor molecule sites might cause a local imbalance of the charge transfer in the crystal. Such molecular defects result in the effective doping of carriers into the half-filled dimer-Mott insulators.

Lattice effects and entropy change at the Mott transition of a quasi-2D organic conductor

Layered organic charge-transfer salts of the κ - ( BEDT - TTF ) 2 X family have emerged as exemplary systems for exploring effects of strong electron–electron correlations. Here we report on high-resolution dilatometric investigations on a fully deuterated crystal of the X = Cu [ N ( CN ) 2 ] Br salt, where the Mott metal–insulator (MI) transition—a key phenomenon of strongly correlated electron systems—can be explored through temperature sweeps. Measurements of the uniaxial expansivities reveal discontinuous lattice changes at T MI ≃ 14 K , accompanied by a striking in-plane anisotropy. The entropy change at T MI , determined from the volume change, is small, indicating that T MI coincides with the Néel-temperature T N at this point in the phase diagram.

Specific-Heat Measurements of the Gap Structure of the Organic Superconductorsκ−(ET)2Cu[N(CN)2]Brandκ−(ET)2Cu(NCS)2

We present high resolution heat capacity measurements of the organic superconductors kappa-(ET)(2)Cu[N(CN)(2)]Br and kappa-(ET)(2)Cu(NCS)(2) in fields up to 14 T. We use the high field data to determine the normal state specific heat and hence extract the behavior of the electronic specific heat C(el) in the superconducting state in zero and finite fields. We find that in both materials for T/T(c) less or similar 0.3, C(el)(H=0) approximately T2 indicating d-wave superconductivity. The data are well described by a strong coupling d-wave model from our base temperature (T/T(c) approximately 0.1) right up to T(c). Our data help to resolve the controversy regarding the order parameter symmetry in these materials.

Anomalous Lattice Response at the Mott Transition in a Quasi-2D Organic Conductor

Discontinuous changes of the lattice parameters at the Mott metal-insulator transition are detected by high-resolution dilatometry on deuterated crystals of the layered organic conductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br. The uniaxial expansivities uncover a striking and unexpected anisotropy, notably a zero effect along the in-plane c axis along which the electronic interactions are relatively strong. A huge thermal expansion anomaly is observed near the end point of the first-order transition line enabling us to explore the critical behavior with very high sensitivity. The analysis yields critical fluctuations with an exponent alpha approximately 0.8+/-0.15 at odds with the novel criticality recently proposed for these materials [Kagawa et al., Nature (London) 436, 534 (2005)]. Our data suggest an intricate role of the lattice degrees of freedom in the Mott transition for the present materials.

Determining ethylene group disorder levels inκ−(BEDT−TTF)2Cu[N(CN)2]Br

We present a detailed structural investigation of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br at temperatures $T$ from 9 to 300 K. Anomalies in the $T$ dependence of the lattice parameters are associated with a glass-like transition previously reported at $T_g$ = 77 K. From structure refinements at 9, 100 and 300 K, the orthorhombic crystalline symmetry, space group {\it Pnma}, is established at all temperatures. Further, we extract the $T$ dependence of the occupation factor of the eclipsed conformation of the terminal ethylene groups of the BEDT-TTF molecule. At 300 K, we find 67(2) %, with an increase to 97(3) % at 9 K. We conclude that the glass-like transition is not primarily caused by configurational freezing-out of the ethylene groups.

High-Pressure Electronic State of κ-(BEDT-TTF)2Cu[N(CN)2]Br Probed by Hall Effect

We studied the normal-state properties of an organic superconductor, κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br, through the investigation of the Hall effect as functions of temperature and pressure. At low pressures, the Hall coefficient shows an apparent increase with lowering temperature and a following peak structure. In this pressure range, the Hall effect possibly correlates with the spin fluctuation. At high pressures, we observed a drastic decrease and sign change in the Hall coefficient. With the dissimilarity with the behavior of similar compounds such as κ-(BEDT-TTF) 2 Cu(NCS) 2 and κ-(BEDT-TTF) 2 Cu(N[CN] 2 )Cl, we concluded that this behavior originates from a superstructure below 200 K.

Pressure dependence of the phase separation in deuterated [formula omitted] at the Mott boundary

The pressure dependence of the optical reflectivity spectrum [ R ( ω ) ] of deuterated κ - ( BEDT - TTF ) 2 Cu [ N ( CN ) 2 ] Br ( d 44 ) at T = 4.5 K was determined. At the ambient pressure, d 44 under fast cooling conditions forms an antiferromagnetic insulator (AFI) phase, as confirmed by the wavenumber of the ν 3 ( a g ) vibration mode. At the pressures in the range of several MPa, the optical spectrum changes to that of a metal that matches a non-deuterated material consisting of AFI and PM (paramagnetic metallic) spectra. This indicates that the phase separation between the AFI and PM phases appears at intermediate pressures. The phase size of the separated phase is smaller than the instrumental spatial resolution of 50 μ m , based on the inability of our methods to record it.

Irreversibility line and critical current studies in κ‐(BEDT‐TTF)2Cu[N(CN)2]Br organic superconductor: effect of the structural transformation near 80 K

AbstractWe report systematic magnetic measurements on organic superconductor κ‐(BEDT‐TTF)2Cu[N(CN)2]‐Br. The irreversibility line Hirr(T) and the critical current density Jc were studied. Our results show the strong dependence of Hirr(T) and Jc on the cooling rate. The Hirr(T) data follow power dependence H ∝ (1 – T /T c)m where m depends on the sample cooling. The structural transformation which occurs at the vicinity of 80 K influences very strongly the physics of vortex lattice and the associated magnetic behavior. This has a pronounced effect on the critical current density and the superconducting transition temperature Tc. Rapid cooling through 80 K will freeze the high temperature magnetic phase to low temperatures and the presence of local magnetic moments suppresses the superconducting Tc. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlation between the critical current and the structural transformation near 80 K in the organic superconductor κ‐(BEDT‐TTF)2Cu[N(CN)2]Br

AbstractWe have systematically studied the critical current and the magnetic susceptibility as a function of the cooling rate through the order‐disorder transformation near 80 K for the κ‐(BEDT‐TTF)2Br organic superconductor. Our results show that the magnetic properties of these compounds depend strongly on the cooling rate Vc, and this disorder can influence very strongly the physics of the vortex lattice. The degree of ordering depends on the cooling rate through 80 K. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vortex dynamics in organic superconductor [formula omitted][formula omitted], [formula omitted

Organic superconductors κ -(ET) 2X (X=Cu(NCS) 2, Cu[N(CN) 2]Br) show anomalous elastic properties in both normal and superconducting states. We have measured the sound velocity of these salts near the superconducting transition temperature T c in the selected magnetic fields with different directions. Under the magnetic field, the sound velocity as a function of temperature shows remarkable elastic anomalies. It was mainly caused by interplay between the vortices and the elastic strain. The Δ C S - N , which is the difference of elastic constant C ( T ) in the superconducting state from the normal state, shows the softening toward lower temperature after following the jump at T c . This discontinuous jump at T c has been found on the longitudinal mode and on the transverse mode. It suggests that two-dimensional order parameter plays an important role in the superconductivity of κ -(ET) 2X system.

Phase separation in the vicinity of the surface ofκ−(BEDT−TTF)2Cu[N(CN)2]Brby fast cooling

Partial suppression of superconductivity by fast cooling has been observed in the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br by two means: a marked sample size effect on the magnetic susceptibility and direct imaging of insulating regions by scanning microregion infrared reflectance spectroscopy. Macroscopic insulating regions are found in the vicinity of the crystalline surface after fast cooling, with diameters of 50--100 $\mu$m and depths of a few $\mu$m. The very large in-plane penetration depth reported to date ($\sim$ 24--100 $\mu$m) can be explained by the existence of the insulating regions.

Synthesis, Crystal Structure, and Magnetic Properties of (2,2′-dipyridylamine)(X− ) Copper(II) Complexes (X− : Cyanate, Dicyanamide, Acetate, Thiocyanato)

The synthesis, structure, and magnetic properties of four 2,2′-dipyridylamine ligand (abbreviated as Hdpa) containing copper(II) complexes. There is one binuclear compound, which is [Cu2(μ1,1-NCO)2(NCO)2(Hdpa)2] (1), and three mononuclear compounds, which are [Cu{N(CN)2}2(Hdpa)2] (2), [Cu(CH3CO2)(Hdpa)2·N(CN)2] (3), and [Cu(NCS)(Acac)] (4). Compounds 1 and 4 crystallize in the monoclinic system, space group P2(1)/c and Z = 4, with a = 8.2465(6) Å, b = 9.3059(7) Å, c = 16.0817(12) Å, β = 91.090(1)°, and V = 1233.90(16) Å3 for 1 and a = 7.6766(6) Å, b = 21.888(3) Å, c = 10.4678(12) Å, β = 90.301(2)°, and V= 1758.8(4) Å3 for 4. Compounds 2 and 3 crystallize in the triclinic system, space group P-1 and Z = 1, with a = 8.1140(3) Å, b = 8.2470(3) Å, c = 9.3120(4) Å, β = 102.2370(10)°, and V = 592.63(4) Å3 for 2 and a = 7.4780(2) Å, b = 12.5700(3) Å, c = 13.0450(3) Å, β = 96.351(2)°, and V = 1211.17(5) Å3 for 3. Complex (1), the magnetic data was fitted by the Bleaney-Bowers equation (1). A very good fit was derived with J = 23.96, Θ = −1.5 (g = 1.97). Complex (1) shows the ferromagnetism. Complexes (2), (3) and (4) of have the it is the typical paramagnetic behavior of unpaired electrons. Under a low temperature around 25 K, complexes (2) and (3) show weak ferromagnetic behavior. They are the cause of hydrogen bonds.