103 research outputs found
Magnetic pyroxenes LiCrGe2O6 and LiCrSi2O6: dimensionality crossover in a non-frustrated S=3/2 Heisenberg model
The magnetism of magnetoelectric = 3/2 pyroxenes LiCrSiO and
LiCrGeO is studied by density functional theory (DFT) calculations,
quantum Monte Carlo (QMC) simulations, neutron diffraction, as well as
low-field and high-field magnetization measurements. In contrast with earlier
reports, we find that the two compounds feature remarkably different, albeit
non-frustrated magnetic models. In LiCrSiO, two relevant exchange
integrals, 9 K along the structural chains and
2 K between the chains, form a 2D anisotropic honeycomb lattice. In
contrast, the spin model of LiCrGeO is constituted of three different
exchange couplings. Surprisingly, the leading exchange
2.3 K operates between the chains, while 1.2 K is about
two times smaller. The additional interlayer coupling
renders this model 3D. QMC simulations reveal excellent agreement between
our magnetic models and the available experimental data. Underlying mechanisms
of the exchange couplings, magnetostructural correlations, as well as
implications for other pyroxene systems are discussed.Comment: 11 pages, 8 figures, 3 tables + Supplementary informatio
Frustration of square cupola in Sr(TiO)Cu(PO)
The structural and magnetic properties of the square-cupola antiferromagnet
Sr(TiO)Cu(PO) are investigated via x-ray diffraction,
magnetization, heat capacity, and P nuclear magnetic resonance
experiments on polycrystalline samples, as well as density-functional
band-structure calculations. The temperature-dependent unit cell volume could
be described well using the Debye approximation with the Debye temperature of
550~K. Magnetic response reveals a pronounced
two-dimensionality with a magnetic long-range-order below ~K. High-field magnetization exhibits a kink at of the saturation
magnetization. Asymmetric P NMR spectra clearly suggest strong in-plane
anisotropy in the magnetic susceptibility, as anticipated from the crystal
structure. From the P NMR shift vs bulk susceptibility plot, the
isotropic and axial parts of the hyperfine coupling between P nuclei and
the Cu spins are calculated to be and
~Oe/, respectively. The
low-temperature and low-field P NMR spectra indicate a commensurate
antiferromagnetic ordering. Frustrated nature of the compound is inferred from
the temperature-dependent P NMR spin-lattice relaxation rate and
confirmed by our microscopic analysis that reveals strong frustration of the
square cupola by next-nearest-neighbor exchange couplings.Comment: 11 pages, 13 figures, 1 table, Phys. Rev. B (Accepted, 2018
Collinear order in a frustrated three-dimensional spin- antiferromagnet LiCuWO
Magnetic frustration in three dimensions (3D) manifests itself in the
spin- insulator LiCuWO. Density-functional band-structure
calculations reveal a peculiar spin lattice built of triangular planes with
frustrated interplane couplings. The saturation field of 29 T contrasts with
the susceptibility maximum at 8.5 K and a relatively low N\'eel temperature
K. Magnetic order below is collinear with the propagation
vector and an ordered moment of 0.65(4) according to
neutron diffraction data. This reduced ordered moment together with the low
maximum of the magnetic specific heat () pinpoint strong
magnetic frustration in 3D. Collinear magnetic order suggests that quantum
fluctuations play crucial role in this system, where a non-collinear spiral
state would be stabilized classically.Comment: published version with supplemental material merged into the tex
Singlet ground state in the alternating spin- chain compound NaVOAsO
We present the synthesis and a detailed investigation of structural and
magnetic properties of polycrystalline NaVOAsO by means of x-ray
diffraction, magnetization, electron spin resonance (ESR), and As
nuclear magnetic resonance (NMR) measurements as well as density-functional
band structure calculations. Temperature-dependent magnetic susceptibility, ESR
intensity, and NMR line shift could be described well using an alternating
spin- chain model with the exchange coupling K and
an alternation parameter . From the high-field magnetic
isotherm measured at K, the critical field of the gap closing is found
to be T, which corresponds to the zero-field spin gap of
K. Both NMR shift and spin-lattice relaxation
rate show an activated behavior at low temperatures, further confirming the
singlet ground state. The spin chains do not coincide with the structural
chains, whereas the couplings between the spin chains are frustrated. Because
of a relatively small spin gap, NaVOAsO is a promising compound for further
experimental studies under high magnetic fields.Comment: 14 pages, 10 figures, 2 table
Antiferromagnetism of ZnVO(PO and the dilution with Ti
We report static and dynamic properties of the antiferromagnetic compound
Zn(VO)(PO), and the consequences of non-magnetic Ti
doping at the V site. P nuclear magnetic resonance (NMR) spectra
and spin-lattice relaxation rate () consistently show the formation of
the long-range antiferromagnetic order below \,K. The critical
exponent estimated from the temperature dependence of the
sublattice magnetization measured by P NMR at 9.4\,MHz is consistent
with universality classes of three-dimensional spin models. The isotropic and
axial hyperfine couplings between the P nuclei and V spins are
Oe/ and Oe/, respectively. Magnetic susceptibility
data above 6.5\,K and heat capacity data above 4.5\,K are well described by
quantum Monte-Carlo simulations for the Heisenberg model on the square lattice
with \,K. This value of is consistent with the values obtained
from the NMR shift, and electron spin resonance (ESR) intensity
analysis. Doping ZnVO(PO with non-magnetic Ti leads to a
marginal increase in the value and the overall dilution of the spin
lattice. In contrast to the recent \textit{ab initio} results, we find neither
evidence for the monoclinic structural distortion nor signatures of the
magnetic one-dimensionality for doped samples with up to 15\% of Ti. The
N\'eel temperature decreases linearly with increasing the amount of
the non-magnetic dopant.Comment: 13 pages, 12 figures, 2 table
Peculiar long-range superexchange in Cu2A2O7 (A = P, As, V) as a key element of the microscopic magnetic model
A microscopic magnetic model for alpha-Cu2P2O7 is evaluated in a combined
theoretical and experimental study. Despite a dominant intradimer coupling J1,
sizable interdimer couplings enforce long-range magnetic ordering at T_N=27 K.
The spin model for alpha-Cu2P2O7 is compared to the models of the isostructural
beta-Cu2V2O7 and alpha-Cu2As2O7 systems. As a surprise, coupled dimers in
alpha-Cu2P2O7 and alternating chains in alpha-Cu2As2O7 contrast with a
honeycomb lattice in beta-Cu2V2O7. We find that the qualitative difference in
the coupling regime of these isostructural compounds is governed by the nature
of AO4 side groups: d-elements (A = V) hybridize with nearby O atoms forming a
Cu-O-A-O-Cu superexchange path, while for p-elements (A = P, As) the
superexchange is realized via O-O edges of the tetrahedron. Implications for a
broad range of systems are discussed.Comment: 8 pages, 5 figures, 1 table; discussion extende
Frustrated magnet for adiabatic demagnetization cooling to milli-Kelvin temperatures
Generation of very low temperatures has been crucially important for
applications and fundamental research, as low-temperature quantum coherence
enables operation of quantum computers and formation of exotic quantum states,
such as superfluidity and superconductivity. One of the major techniques to
reach milli-Kelvin temperatures is adiabatic demagnetization refrigeration
(ADR). This method uses almost non-interacting magnetic moments of paramagnetic
salts where large distances suppress interactions between the magnetic ions.
The large spatial separations are facilitated by water molecules, with a
drawback of reduced stability of the material. Here, we show that an
HO-free frustrated magnet KBaYb(BO) can be ideal refrigerant for
ADR, achieving at least 22\,mK upon demagnetization under adiabatic conditions.
Compared to conventional refrigerants, KBaYb(BO does not degrade even
under high temperatures and ultra-high vacuum conditions. Further, its
frustrated magnetic network and structural randomness enable cooling to
temperatures several times lower than the energy scale of magnetic
interactions, which is the main limiting factor for the base temperature of
conventional refrigerants.Comment: accepted for publication in Communications Material
Signatures of van Hove singularities in the anisotropic in-plane optical conductivity of the topological semimetal NbSiTe
We present a temperature-dependent infrared spectroscopy study on the layered
topological semimetal NbSiTe combined with density-functional theory
(DFT) calculations of the electronic band structure and optical conductivity.
Our results reveal an anisotropic behavior of the in-plane (-plane) optical
conductivity, with three pronounced excitations located at around 0.15, 0.28,
and 0.41~eV for the polarization of the incident radiation along the axis.
These excitations are well reproduced in the theoretical spectra. Based on the
\textit{ab initio} results, the excitations around 0.15 eV and 0.28 eV are
interpreted as fingerprints of van Hove singularities in the electronic band
structure and compared to the findings for other topological semimetals.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
Two types of alternating spin- chains and their field-induced transitions in -LiVOPO
Thermodynamic properties, P nuclear magnetic resonance (NMR)
measurements, and density-functional band-structure calculations for
-LiVOPO are reported. This quantum magnet features a singlet
ground state and comprises two types of alternating spin- chains that
manifest themselves by the double maxima in the susceptibility and magnetic
specific heat, and by the two-step magnetization process with an intermediate
-plateau. From thermodynamic data and band-structure calculations, we
estimate the leading couplings of K and K and the
alternation ratios of and
within the two chains, respectively. The
zero-field spin gap K probed by thermodynamic
and NMR measurements is caused by the - spin chains and can be
closed in the applied field of T, giving rise to
a field-induced long-range order. The NMR data reveal predominant
three-dimensional spin-spin correlations at low temperatures. Field-induced
magnetic ordering transition observed above is attributed to the
Bose-Einstein condensation of triplons in the sublattice formed by the
- chains with weaker exchange couplings.Comment: 14 pages, 14 figure
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