Transport and structural properties of polymerized AC60 (A = K, Rb) under zero and high pressure conditions

60 display transport properties vastly different from their unpolymerized counterparts. We here describe structural (X-ray diffraction and transmission electron microscopy) and dc resistivity measurements of polymerized AC₆₀ (A = K, Rb) at zero and high pressures. The structural and electronic properties are rich, and evidence is found for unusual phase transitions in both materials. Received: 28 October 1996/Accepted: 13 December 1996     

Properties of a new ferroelectric crystal family: MCd(NO2)3 [M=NH4, K,Rb, Cs,Tl]

Single crystals of MCd(NO₂)₃, with M=NH₄, K, Rb, Cs, and Tl, respectively, were grown and investigated by means of optical and dielectric methods. The crystals show rhombohedral symmetry (possible space group R 3) and multi-domain patterns at ambient temperature. The compounds exhibit phase transitions above room-temperature of first (M=K) and second order (M=Rb, Cs, Tl), respectively, into cubic high-temperature phases. Observed dielectric anomalies and hysteresis loops point to ferroelectric properties and suggest a relaxational order-disorder mechanism of the nitrite ions to be responsible for the transitions. The MCd-(NO₂)₃-compounds are seen to represent a new family of ferroelectrics with a cubic prototypic phase closely related to the structural type of the perovskites. The transions are classified by symmetry considerations, and a possible structure of the cubic high-temperature phase with space group Pm3 is proposed.     

Electronic properties of ortho-polymer A1C60 (A = K,Rb) and related phases

1 C₆₀. Its singular structural, magnetic and electrical properties greatly enhanced the study of the AC₆₀ phases (A = Rb, K and \chem{Cs}). We report our continuous wave (cw) ESR results on the structural phase transformations, phase instabilities and the related kinetics of K₁C₆₀. We quantitatively explain the origin of the difference between Rb₁C₆₀ and K₁C₆₀ phase diagrams. The stability of OP-Rb₁C₆₀ together with the narrowness of its homogenous cw-ESR linewidth in the temperature range allow to perform pulsed ESR experiments. Pulsed ESR experiments measure precisely both the electronic spin-spin () and spin-lattice () relaxation times. Above , the equality of and , which are the only contributions to the cw-ESR linewidth, constitutes a convincing evidence of the metallic nature of OP-Rb₁C₆₀. The drastic increase (decrease) of the linewidth () below \valunit{40}{K} is interpreted in terms of an \qut{electronic dimerization}. Received: 12 September 1996/Accepted: 14 November 1996     

Investigation of Paramagnetic Centers in Fullerides A2MC60 and AM2C60 (A = K, Rb, M = Mg, Be)

Paramagnetic centers in heterofullerides with the composition A₂MC₆₀ and AM₂C₆₀ (A = K, Rb, M = Mg, Be) were investigated by the electron spin resonance (ESR) method. It was found that the ESR signal can be interpreted as a composition of two lines with different temperature dependence of ESR absorption magnitude. This gives an evidence of the presence of at least two different types of paramagnetic centers. Centers of the first type behave as localized spins, while the rest can be interpreted as conduction electrons. Authors' address: Vladimir A. Kulbachinskii, Physics Faculty, M. V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russian Federation     

Elastic properties of MBH4 (M=Na, K, Rb, Cs)

The elastic properties of MBH₄ (M=Na, K, Rb, Cs) are studied by first-principles calculations using a projected augmented plane-wave method and finite strain method. KBH₄ is found to be the most isotropic material among MBH₄, which can be attributed to the contribution of C₄₄. The Debye temperatures are also calculated using theoretical elastic constants and the change along the series is explained.     

High pressure calculation of the melting curve: Na,K, Rb and Cs

The melting of four alkali metals is investigated in the present paper, in the theoretical framework defined in a preliminary study on sodium. The calculations enable us to predict the existence of a very flat maximum on all four phase lines. The obtained results are in good agreement with experiment in the case of Na, K and Rb, and emphasize the importance of “d” effects and the dramatic influence of the electronic transition on the high pressure fusion curve of Cs.     

Electronic structures and magnetic order of ordered-Fe-vacancy ternary iron selenides TlFe1.5Se2 and AFe1.5Se2 (A=K, Rb, or Cs)

By the first-principles electronic structure calculations, we find that the ground state of the Fe-vacancies ordered TlFe(1.5)Se(2) is a quasi-two-dimensional collinear antiferromagnetic semiconductor with an energy gap of 94 meV, in agreement with experimental measurements. This antiferromagnetic order is driven by the Se-bridged antiferromagnetic superexchange interactions between Fe moments. Similarly, we find that crystals AFe(1.5)Se(2) (A=K, Rb, or Cs) are also antiferromagnetic semiconductors but with a zero-gap semiconducting state or semimetallic state nearly degenerated with the ground states. Thus, rich physical properties and phase diagrams are expected.     

Near threshold photoionization of excited alkali atoms Ak(np) (Ak = Na, K, Rb, Cs; n = 3-6)

The effect of the polarization of the atomic core by the outer electron on near threshold photoionization of excited alkali atoms Ak(np) (Ak = Na-Cs; n=3-6) is investigated. Partial and total cross-sections for photo-ionization of the np-electron were computed utilizing the configuration interaction technique with Pauli-Fock atomic orbitals (CIPF) and including the long range core polarization potential (CP). To calculate the core polarization potential the variational principle is applied. Comparison with previous theoretical results and with available experimental data is made for the total cross-section , for the electron angular distribution parameter , for the ratio of the reduced electric dipole matrix elements and for the phase shift difference , associated with the d-wave and s-wave continua, respectively. In the comparison, new experimental results for , , and , measured for laser-excited, polarized ³⁹K(4p _3/2) atoms, have been included. Received 21 July 1999 and Received in final form 14 October 1999     

Optical properties from time-dependent current-density-functional theory: the case of the alkali metals Na,K, Rb,and Cs

We present the optical conductivity as well as the electron-energy loss spectra of the alkali metals Na, K, Rb, and Cs calculated within time-dependent current-density functional theory. Our ab initio formulation describes from first principles both the Drude-tail and the interband absorption of these metals as well as the most dominant relativistic effects. We show that by using a recently derived current functional [Berger, Phys. Rev. Lett. 115, 137402 (2015)] we obtain an overall good agreement with experiment at a computational cost that is equivalent to the random-phase approximation. We also highlight the importance of the choice of the exchange-correlation potential of the ground state.     

Common crystalline and magnetic structure of superconducting A2Fe4Se5 (A=K,Rb,Cs,Tl) single crystals measured using neutron diffraction

Single-crystal neutron diffraction studies on superconductors A(2)Fe(4)Se(5), where A=Rb, Cs, (Tl, Rb), and (Tl, K) (T(c) ～ 30 K), uncover the same Fe vacancy ordered crystal structure and the same block antiferromagnetic order as in K(2)Fe(4)Se(5). The Fe order-disorder transition occurs at T(S)=500-578 K, and the antiferromagnetic transition at T(N) = 471-559 K with an ordered magnetic moment ～3.3μ(B)/Fe at 10 K. Thus, all recently discovered A intercalated iron selenide superconductors share the common crystalline and magnetic structure, which are very different from previous families of Fe-based superconductors, and constitute a distinct new 245 family.