RESUMEN

The antiferromagnetic molecular wheel Fe18 of 18 exchange-coupled Fe;{III} ions has been studied by magnetic torque, magnetization, and inelastic neutron scattering. The combined data show that the low-temperature magnetism of Fe18 is very accurately described by the Néel-vector tunneling (NVT) scenario, as unfolded by semiclassical theory. In addition, the magnetic torque as a function of applied field exhibits oscillations that reflect the oscillations in the NVT splitting with field due to quantum phase interference.

RESUMEN

We measure by inelastic neutron scattering the spin excitation spectra as a function of applied magnetic field in the quantum spin-ladder material (C5H12N)2CuBr4. Discrete magnon modes at low fields in the quantum disordered phase and at high fields in the saturated phase contrast sharply with a spinon continuum at intermediate fields characteristic of the Luttinger-liquid phase. By tuning the magnetic field, we drive the fractionalization of magnons into spinons and, in this deconfined regime, observe both commensurate and incommensurate continua.

RESUMEN

α-MnMoO(4) is a tetrameric magnetic cluster system which undergoes a transition to three-dimensional antiferromagnetic order at T(N) = 10.7 K. In the ordered state the Mn(2+) spins (s(ν) = 5/2) are ferromagnetically aligned within the tetramer, resulting in a total cluster spin S = 10. The magnetic excitation spectrum consists of eight excitation modes of tetramer origin propagating in the whole reciprocal space. From single-crystal inelastic neutron scattering investigations the three-dimensional coupling scheme is rationalized in the framework of a tetramer-based dispersion model. The tetramer states and the energy dispersion of all the magnetic excitations are described by Heisenberg-like intra- and intercluster exchange interactions, respectively, thus α-MnMoO(4) is a suitable tetrameric model system to study the interplay of these interactions.

RESUMEN

We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl3 through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point.

RESUMEN

The 1H NMR spectrum and nuclear relaxation rate T(1)(-1) in the antiferromagnetic wheel CsFe8 were measured to characterize the previously observed magnetic field-induced low-temperature phase around the level crossing at 8 T. The data show that the phase is characterized by a huge staggered transverse polarization of the electronic Fe spins, and the opening of a gap, providing microscopic evidence for the interpretation of the phase as a field-induced magnetoelastic instability.

Asunto(s)

RESUMEN

Near-infrared to visible upconversion luminescence in CsCaCl3:Tm2+, CsCaBr3:Tm2+ and CsCaI3:Tm2+ is presented and analysed. The upconversion process involves exclusively the 4f-5d excited states of Tm2+, which is a novelty among upconversion materials. The presence of more than one long-lived 4f-5d excited state is the prerequisite for this. Multiple emissions from Tm2+ are observed in the title compounds. This is made possible by the favourable energy structure within the 4f-5d states and the low phonon energies of the materials. The energy positions of the relevant 4f-5d states, and thus the photophysical and light emission properties, are affected by the chemical variation along the series. The upconversion efficiency increases from chloride to iodide and the mechanism is found to be a combination of absorption and energy-transfer steps.

RESUMEN

The magnetic torque of the antiferromagnetic molecular wheel CsFe8 was studied down to 50 mK and up to 28 T. Below about 0.5 K phase transitions were observed at the field-induced level crossings (LCs). Intermolecular magnetic interactions are very weak excluding field-induced magnetic ordering. A magnetoelastic coupling was considered. A generic model shows that the wheel structure is unconditionally unstable at the LCs, and the predicted torque curves explain the essential features of the data well.

RESUMEN

We report the optical and scintillation properties of the Ce(3+)-doped bromoelpasolites Cs(2)NaREBr(6) (RE = La,Y,Lu). The γ-ray scintillation light yield of these materials varies from 6000 to 17 000 photons per MeV absorbed γ-ray energy. At room temperature (RT), the γ-ray scintillation decay curves for all compounds show a fast component of 61 ns, whereas the intrinsic Ce(3+) decay time is 30 ns. The scintillation mechanism in elpasolites is addressed. In Cs(2)NaLuBr(6):Ce(3+) and Cs(2)NaYBr(6):Ce(3+), we observe for the first time the so-called Ce(3+) anomalous emission in bromide compounds. This emission previously observed for chloride compounds is an ultrafast Ce(3+) emission with a selective excitation mechanism. The decay time of the anomalous emission at 10 K in bromide compounds (∼7.80 ns) is faster than that in chloride compounds (∼9.90 ns). Two bands of the anomalous emission are resolved for the first time. The mechanism behind this emission is discussed.

RESUMEN

From inelastic neutron scattering experiments, we demonstrate quantum tunneling of the Néel vector in the antiferromagnetic molecular ferric wheel CsFe8. Analysis of the linewidth of the tunneling transition evidences coherent tunneling.

RESUMEN

Metal (4f)-ligand (Cl 3p) bonding in LnCl(6)(3-) (Ln = Ce to Yb) complexes has been studied on the basis of 4f-->4f and Cl,3p-->4f charge-transfer spectra and on the analysis of these spectra within the valence bond configuration interaction model to show that mixing of Cl 3p into the Ln 4f ligand field orbitals does not exceed 1%. Contrary to this, Kohn-Sham formalism of density functional theory using currently available approximations to the exchange-correlation functional tends to strongly overestimate 4f-3p covalency, yielding, for YbCl(6)(3-), a much larger mixing of Cl 3p-->4f charge transfer into the f(13) ionic ground-state wave function. Thus, ligand field density functional theory, which was recently developed and applied with success to complexes of 3d metals in our group, yields anomalously large ligand field splittings for Ln, the discrepancy with experiment increasing from left to the right of the Ln 4f series. It is shown that eliminating artificial ligand-to-metal charge transfer in Kohn-Sham calculations by a procedure described in this work leads to energies of 4f-4f transitions in good agreement with experiment. We recall an earlier concept of Ballhausen and Dahl which describes ligand field in terms of a pseudopotential and give a thorough analysis of the contributions to the ligand field from electrostatics (crystal field) and exchange (Pauli) repulsion. The close relation of the present results with those obtained using the first-principles based and electron density dependent effective embedding potential is pointed out along with implications for applications to other systems.

RESUMEN

The compound TlCuCl(3) represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Würtz [Phys. Rev. B 50, 13 515 (1994)10.1103/Phys. Rev. B 50, 13515] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important.

RESUMEN

Elastic and inelastic neutron scattering experiments have been performed on the dimer spin system NH4CuCl3, which shows plateaus in the magnetization curve at m=1/4 and m=3/4 of the saturation value. Two structural phase transitions at T1 approximately 156 K and at T(2)=70 K lead to a doubling of the crystallographic unit cell along the b direction and as a consequence a segregation into different dimer subsystems. Long-range magnetic ordering is reported below T(N)=1.3 K. The magnetic field dependence of the excitation spectrum identifies successive quantum phase transitions of the dimer subsystems as the driving mechanism for the unconventional magnetization process in agreement with a recent theoretical model.

RESUMEN

The origin of higher-order exchange interactions in localized S-state systems has been the subject of intensive investigations in the past. In particular, it has been suggested that a biquadratic exchange term may arise from the magnetoelastic energy. Here we report on the pressure and temperature dependence of the excitation spectra of magnetic Mn2+ dimers in CsMn0.28Mg0.72Br3 probed by inelastic neutron scattering. Biquadratic exchange and a strong distance dependence of the bilinear exchange are observed. It is shown that the mechanism of local exchange striction may explain the occurrence of biquadratic exchange in accordance with the elastic properties of the compound.

RESUMEN

The condensation of magnetic quasiparticles into the nonmagnetic ground state has been used to explain novel magnetic ordering phenomena observed in quantum spin systems. We present neutron scattering results across the pressure-induced quantum phase transition and for the novel ordered phase of the magnetic insulator TlCuCl3, which are consistent with the theoretically predicted two degenerate gapless Goldstone modes, similar to the low-energy spin excitations in the field-induced case. These novel experimental findings complete the field-induced Bose-Einstein condensate picture and support the recently proposed field-pressure phase diagram common for quantum spin systems with an energy gap of singlet-triplet nature.

RESUMEN

Bose-Einstein condensation denotes the formation of a collective quantum ground state of identical particles with integer spin or intrinsic angular momentum. In magnetic insulators, the magnetic properties are due to the unpaired shell electrons that have half-integer spin. However, in some such compounds (KCuCl3 and TlCuCl3), two Cu2+ ions are antiferromagnetically coupled to form a dimer in a crystalline network: the dimer ground state is a spin singlet (total spin zero), separated by an energy gap from the excited triplet state (total spin one). In these dimer compounds, Bose-Einstein condensation becomes theoretically possible. At a critical external magnetic field, the energy of one of the Zeeman split triplet components (a type of boson) intersects the ground-state singlet, resulting in long-range magnetic order; this transition represents a quantum critical point at which Bose-Einstein condensation occurs. Here we report an experimental investigation of the excitation spectrum in such a field-induced magnetically ordered state, using inelastic neutron scattering measurements of TlCuCl3 single crystals. We verify unambiguously the theoretically predicted gapless Goldstone mode characteristic of the Bose-Einstein condensation of the triplet states.

RESUMEN

Magnetization and electronic Raman data are presented for salts of the type Cs[Ga:Ti](SO(4))(2) x 12H(2)O, which enable a very precise definition of the electronic structure of the [Ti(OH(2))(6)](3+) cation. The magnetization data exhibit a spectacular deviation from Brillouin behavior, with the magnetic moment highly dependent on the strength of the applied field at a given ratio of B/T. This arises from unprecedented higher-order contributions to the magnetization, and these measurements afford the determination of the ground-state Zeeman coefficients to third-order. The anomalous magnetic behavior is a manifestation of Jahn-Teller coupling, giving rise to low-lying vibronic states, which mix into the ground state through the magnetic field. Electronic Raman measurements of the 1%-titanium(III)-doped sample identify the first vibronic excitation at approximately 18 cm(-1), which betokens a substantial quenching of spin-orbit coupling by the vibronic interaction. The ground-state Zeeman coefficients are strongly dependent on the concentration of titanium(III) in the crystals, and this can be modeled as a function of one parameter, representing the degree of strain induced by the cooperative Jahn-Teller effect. This study clearly demonstrates the importance that the Jahn-Teller effect can have in governing the magnetic properties of transition metal complexes with orbital triplet ground terms.

RESUMEN

The photophysical properties of Ti(2+)-doped NaCl and MgCl(2) at 15 K are compared. At this temperature both materials emit luminescence from their respective lowest excited states and from the (3)T(1g)(t(2g)e(g)) higher excited state. In Ti(2+):MgCl(2) the ligand field is significantly stronger than in Ti(2+):NaCl, leading to (1)T(2g)(t(2g)(2)) and (3)T(2g)(t(2g)e(g)) lowest excited states, respectively, in these materials. The near-infrared to visible upconversion mechanisms of these two materials are identified. The upconversion efficiency calculated for Ti(2+):MgCl(2) is approximately 2 orders of magnitude higher than for Ti(2+):NaCl. This is mainly due to the more efficient energy storage in the intermediate upconversion level in Ti(2+):MgCl(2), and its higher (3)T(1g)(t(2g)e(g)) --> (3)T(1g)(t(2g)(2)) luminescence quantum yield relative to Ti(2+):NaCl.

RESUMEN

The lowest energy spin-allowed crystal field band of (Et(4)N)(3)Cr(2)F(9) shows a distinct interference dip at 15,000 cm(-1) with an approximate width of 200 cm(-1). This spectroscopic feature is due to spin-orbit coupling between the (2)E and (4)T(2) excited states and is analyzed with a set of two coupled potential energy surfaces. The minimum of the (4)T(2) potential surface is displaced along at least two normal coordinates. The modes involved cannot be directly determined from the unresolved absorption spectrum, but are obtained from Raman spectra and from the well-resolved spin-forbidden crystal field transition to the (2)A(1) state. The first mode with a frequency of 415 cm(-1) has predominant Cr-F stretching character; the second mode has a frequency of 90 cm(-1) and involves the entire Cr(2)F(9)(3-) dimer.

RESUMEN

The synthesis, crystal growth, magnetic susceptibility, and polarized optical absorption spectra in the visible and near UV of (Et(4)N)(3)Fe(2)F(9) are reported. From single-crystal magnetic susceptibility data and high-resolution absorption spectra in the region of the (6)A(1) --> (4)A(1) spin-flip transition, exchange splittings in the ground and excited states are derived. Ferromagnetic ground and excited state exchange parameters J(GS) = -1.55 cm(-1) and J(ES) = -0.53 cm(-1) are determined, respectively, and the relevant orbital contributions to the net exchange are derived from the spectra. The results are compared with those reported earlier for the structurally and electronically analogous [Mn(2)X(9)](5-) pairs in CsMgX(3):Mn(2+) (X = Cl(-), Br(-)), in which the splittings are antiferromagnetic. This major difference is found to be due to the increased metal charge of Fe(3+) compared to Mn(2+), leading to orbital contraction and thus to a strong decrease of the orbital overlaps and hence the antiferromagnetic interactions.

RESUMEN

Direct near-IR excitation of Yb(3+) 2F(7/2)-->(2)F(5/2) levels at 10126, 10138, and 10596 cm(-1) in CsMnBr3:0.5%Yb(3+) leads to three types of luminescence at cryogenic temperatures: near-IR Yb(3+) emission and green and red upconverted luminescence. The green luminescence around 20 000 cm(-1) is identified as cooperative Yb(3+) pair upconversion. The broad red upconversion luminescence band centered at 14 700 cm(-1) is ascribed to the 4T(1g)-->6A(1g) transition of Mn(2+). Pulsed measurements indicate a sequence of ground-state absorption and excited-state absorption steps for the red upconversion process. One- and two-color excitation experiments support this, and we conclude that the red upconversion occurs by an exchange mechanism involving Yb(3+) and Mn(2+). The Yb(3+) 2F(5/2)-->(2)F(7/2) near-IR emission around 10 000 cm(-1) is also observed after Mn(2+) excitation at 21 838 cm(-1). This is indicative of a Mn(2+) 4T(1g)--> Yb(3+) 2F(5/2) relaxation process, which is a potential loss process for upconversion efficiency.