arXiv subjects

Roser Valenti

Publications and source records attributed to Roser Valenti.

At least 145 records · Page 8Linked to original sources

arXiv2008

Microscopic modeling of a spin crossover transition

In spin crossover materials, an abrupt phase transition between a low spin state and a high spin state can be driven by temperature, pressure or by light irradiation. Of a special relevance are Fe(II) based coordination polymers where, in contrast to molecular systems, the phase transition between a spin S=0 and a spin S=2 state shows a pronounced hysteresis which is desirable for technical applications. A satisfactory microscopic explanation of this large cooperative phenomenon has been sought for a long time. The lack of X-ray data has been one of the reasons for the absence of microscopic studies. In this work, we present an efficient route to prepare reliable model structures and within an ab initio density functional theory analysis and effective model considerations we show that in polymeric spin crossover compounds magnetic exchange between high spin Fe(II) centers is as important as elastic couplings for understanding the phase transition. We discuss the relevance of these interactions for the cooperative behavior in these materials.

cond-mat.mtrl-sci↗

arXiv2008

Proposed low energy model Hamiltonian for spin-gapped system CuTe2O5

Using first-principles electronic structure calculations based on the Nth order muffin tin orbital (NMTO)-downfolding technique, we derived the low-energy spin model for CuTe2O5. Our study reveals that this compound is a 2D coupled spin-dimer system with the strongest Cu-Cu interaction mediated by two O-Te-O bridges. We checked the goodness of our model by computing the magnetic susceptibility with the Quantum Monte Carlo technique and by comparing it with available experimental data. We also present magnetization and specific heat results which may be compared with future experimental investigations. Our derived model is in disagreement with a recently proposed model for this compound [J. Deisenhofer et al, Phys. Rev. B,74 (2006) 174421]. The situation needs to be settled in terms of further experimental investigations.

cond-mat.mtrl-sci↗

arXiv2007

Orbital order in ZnV$_2$O$_4$

In view of recent controversy regarding the orbital order in the frustrated spinel ZnV2O4, we analyze the orbital and magnetic groundstate of this system within an ab initio density functional theory approach. While LDA+U calculations in the presence of a cooperative Jahn-Teller distortion stabilize an A-type staggered orbital order, the consideration of relativistic spin-orbit effects unquenches the orbital moment and leads to a uniform orbital order with a net magnetic moment close to the experimental one. Our results show that ab initio calculations are able to resolve the existing discrepancies in previous theories and that it is the spin-orbit coupling alongwith electronic correlations which play a significant role in determining the orbital structure in these materials.

cond-mat.str-el↗

arXiv2007

Classical and ab initio preparation of reliable structures for polymeric coordination compounds

The detailed investigation of electronic and magnetic properties of polymeric coordination materials with accurate ab initio quantum mechanical methods is often computationally extremely demanding because of the large number of atoms in the unit cell. Moreover, usually the available structural data are insufficient or poorly determined, especially when the structure contains hydrogen atoms. In order to be able to perform controlled ab initio calculations on reliable structures, we use a two-step approach to systematically prepare model structures for polymeric coordination compound systems and to relax them to their equilibrium configuration. First, a structure is constructed on the basis of a crystallographic database and optimized by force field methods; in the second step, the structure is relaxed by ab initio quantum mechanical molecular dynamics. With this structure, we perform accurate electronic structure calculations. We will apply this procedure to a Fe(II) triazole compound and to a coordination polymer of Cu(II) ions with 2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene.

cond-mat.mtrl-sci↗

arXiv2007

Cu-based metalorganic systems: an ab initio study of the electronic structure

Within a first principles framework, we study the electronic structure of the recently synthesized polymeric coordination compound Cu(II)-2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene (CuCCP), which has been suggested to be a good realization of a Heisenberg spin-1/2 chain with antiferromagnetic coupling. By using a combination of classical with ab initio quantum mechanical methods, we design on the computer reliable modified structures of CuCCP aimed at studying effects of Cu-Cu coupling strength variations on this spin-1/2 system. For this purpose, we performed two types of modifications on CuCCP. In one case, we replaced H in the linker by i) an electron donating group (NH2) and ii) an electron withdrawing group (CN), while the other modification consisted in adding H2O and NH3 molecules in the structure which change the local coordination of the Cu(II) ions. With the NMTO-downfolding method we provide a quantitative analysis of the modified electronic structure and the nature of the Cu-Cu interaction paths in these new structures and discuss its implications for the underlying microscopic model.

cond-mat.mtrl-sci↗

arXiv2006

Microscopic model for the novel frustrated Cu(II)-spin tetrahedron-based Cu4Te5O12X4 (X=Cl, Br) systems

We present a microscopic study of the electronic and magnetic properties of the newly synthesized Cu(II)-spin tetrahedron-based Cu4Te5O12Cl4 compound based on Density Functional calculations and on ab initio-derived effective models. In view of these results, we discuss the origin of the observed differences in behavior between this system and the structurally similar and much studied Cu2Te2O5Cl2. Since the Br analog of the title compound has not been synthesized yet, we derive the crystal structure of Cu4Te5O12Br4 by geometry optimization in an ab initio molecular dynamics calculation and investigate the effect of substituting Cl by Br as well as the possible magnetic behavior of this system and compare with the recently studied sister compound, Cu2Te2O5Br2.

cond-mat.str-el↗

arXiv2006

The Mott-Hubbard Transition on the D=infinity Bethe Lattice

In view of a recent controversy we investigated the Mott-Hubbard transition in D=infinity with a novel cluster approach. i) We show that any truncated Bethe lattice of order n can be mapped exactly to a finite Hubbard-like cluster. ii) We evaluate the self-energy numerically for n=0,1,2 and compare with a series of self-consistent equation-of-motion solutions. iii) We find the gap to open continously at the critical U_c~2.5t* (t = t* / sqrt{4d}). iv) A low-energy theory for the Mott-Hubbard transition is developed and relations between critical exponents are presented.

cond-mat.str-el↗

arXiv2006

Effects of Fe substitution on the electronic, transport, and magnetic properties of ZnGa$_2$O$_4$: A systematic ab-initio study

We present a density functional study of Fe doped into the tetrahedral and octahedral cation sites of the wide band gap spinel ZnGa$_2$O$_4$. We calculate the electronic structure for different substitutions and discuss the magnetic and transport properties for each case considering different approximations for the exchange-correlation potential. We show that for certain doped cases, significant differences in the predicted behavior are obtained depending on the exchange correlation potential adopted. Possible applications of the doped systems as magnetic semiconductors are outlined.

cond-mat.mtrl-sci↗

arXiv2005

Na2V3O7, a frustrated nanotubular system with spin-1/2 diamond rings

Following the recent discussion on the puzzling nature of the interactions in the nanotubular system Na2V3O7, we present a detailed ab-initio microscopic analysis of its electronic and magnetic properties. By means of a non-trivial downfolding study we propose an effective model in terms of tubes of nine-site rings with the geometry of a spin-diamond necklace with frustrated inter-ring interactions. We show that this model provides a quantitative account of the observed magnetic behavior.

cond-mat.mtrl-sci↗

arXiv2005

The antiferromagnetic/paramagnetic transition in mixed-spin compounds R2BaNiO5

We present an extensive Quantum Monte Carlo study of the magnetic properties of the mixed-spin quantum systems R2BaNiO5 (R= magnetic rare earth) which show coexistence of 3-dimensional magnetic long-range order with 1-dimensional quantum gap excitations. We discuss the validity of the performed simulations in the critical region and show the excellent agreement with experimental results. We emphasize the importance of quantum fluctuations contained in our study which is absent in previous mean-field-like treatments.

cond-mat.str-el↗

arXiv2005

Ab-initio Phonon Calculations for the layered compound TiOCl

We present first-principles frozen-phonon calculations for the three Raman-active $A_g$ modes in the spin-1/2 layered TiOCl system within two different well-known approaches: the local density approximation (LDA) and the so-called LDA+U approximation. We observe that the inclusion of electron correlation in a mean-field level as implemented in the LDA+U leads to a better overall agreement with experimental results. We also discuss the implications of the two approaches on the physics of TiOCl.

cond-mat.str-el↗

arXiv2004

TiOCl, an orbital-ordered system?

We present first principles density functional calculations and downfolding studies of the electronic and magnetic properties of the layered quantum spin system TiOCl. We discuss explicitely the nature of the exchange pathes and attempt to clarify the concept of orbital ordering in this material. An analysis of the electronic structure of slightly distorted structures according to the phononic modes allowed in this material suggests that this system is subject to large orbital fluctuations driven by the electron-phonon coupling. Based on these results, we propose a microscopic explanation of the behavior of TiOCl near the phase transition to a spin-gapped system.

cond-mat.str-el↗

arXiv2004

First principles electronic structure of spinel LiCr2O4: A possible half-metal?

We have employed first-principles electronic structure calculations to examine the hypothetical (but plausible) oxide spinel, LiCr2O4 with the d^{2.5} electronic configuration. The cell (cubic) and internal (oxygen position) structural parameters have been obtained for this compound through structural relaxation in the first-principles framework. Within the one-electron band picture, we find that LiCr2O4 is magnetic, and a candidate half-metal. The electronic structure is substantially different from the closely related and well known rutile half-metal CrO2. In particular, we find a smaller conduction band width in the spinel compound, perhaps as a result of the distinct topology of the spinel crystal structure, and the reduced oxidation state. The magnetism and half-metallicity of LiCr2O4 has been mapped in the parameter space of its cubic crystal structure. Comparisons with superconducting LiTi2O4 (d^{0.5}), heavy-fermion LiV2O4 (d^{1.5}) and charge-ordering LiMn2O4 (d^{3.5}) suggest the effectiveness of a nearly-rigid band picture involving simple shifts of the position of E_F in these very different materials. Comparisons are also made with the electronic structure of ZnV2O4 (d^{2}), a correlated insulator that undergoes a structural and antiferromagnetic phase transition.

cond-mat.mtrl-sci↗

arXiv2003

Halogen-mediated exchange in the coupled-tetrahedra quantum spin systems Cu2Te2O5X2 (X=Br,Cl)

Motivated by recent discussion on possible quantum critical behavior in the coupled Cu-tetrahedra system Cu2Te2O5Br2, we present a comparative ab initio study of the electronic properties of Cu2Te2O5Br2 and the isostructural Cu2Te2O5Cl2. A detailed investigation of the copper-copper interaction pathes reveals that the halogen-ions play an important role in the inter-tetrahedral couplings via X_4-rings (X=Br, Cl). We find that, contrary to initial indications, both systems show a similar electronic behavior with long range exchange pathes mediated by the X_4-rings.

cond-mat↗

arXiv2002

Ab initio investigation of VOSeO3, a spin gap system with coupled spin dimers

Motivated by an early experimental study of VOSeO3, which suggested that it is a quasi-2D system of weakly coupled spin dimers with a small spin gap, we have investigated the electronic structure of this material via density-functional calculations. These ab initio results indicate that the system is better thought of as an alternating spin-1/2 chain with moderate interchain interactions, an analog of (VO)2P2O7. The potential interest of this system for studies in high magnetic field given the presumably small value of the spin gap is emphasized.

cond-mat↗

arXiv2002

Nature of the spin-singlet ground state in CaCuGe2O6

We investigate by means of ab initio electronic structure analysis and Quantum Monte Carlo calculations the scenario where longer-ranged magnetic interactions dominate over short-ranged interactions in the physical description of compounds. This question is discussed, in particular, for the case of CaCuGe$_2$O$_6$ which shows a spin-singlet behavior induced by third nearest neighbor copper pairs.

cond-mat↗

arXiv2002

A microscopic model for a class of mixed-spin quantum antiferromagnets

We propose a microscopic model that describes the magnetic behavior of the mixed-spin quantum systems R$_2$BaNiO$_5$ (R= magnetic rare earth). An evaluation of the properties of this model by Quantum Monte Carlo simulations shows remarkable good agreement with the experimental data and provides new insight into the physics of mixed-spin quantum magnets.

cond-mat.str-el↗

arXiv2002

Electronic and magnetic structure of CsV$_2$O$_5$

We have studied the electronic structure of the spin-gapped system CsV$_2$O$_5$ by means of an ab initio calculation. Our analysis and a re-examination of the susceptibility data indicate that the behavior of this system is much closer to that of an alternating spin-1/2 antiferromagnetic chain with significant inter-dimer coupling and weaker inter-chain couplings than that of isolated dimers as was initially proposed. Comparison to the vanadate family members $α'$-NaV$_2$O$_5$, $γ$-LiV$_2$O$_5$ and isostructural compounds like (VO)$_{2}$P$_{2}$O$_{7}$ (VOPO) is discussed.

cond-mat↗