168 research outputs found
Evolution of magnetic fluctuations through the Fe-induced paramagnetic to ferromagnetic transition in CrB
In itinerant ferromagnets, the quenched disorder is predicted to dramatically
affect the ferromagnetic to paramagnetic quantum phase transition driven by
external control parameters at zero temperature. Here we report a study on
Fe-doped CrB, which, starting from the paramagnetic parent, orders
ferromagnetically for Fe-doping concentrations larger than \%. In parent CrB, B nuclear magnetic resonance data reveal
the presence of both ferromagnetic and antiferromagnetic fluctuations. The
latter are suppressed with Fe-doping, before the ferromagnetic ones finally
prevail for . Indications for non-Fermi liquid behavior, usually
associated with the proximity of a quantum critical point, were found for all
samples, including undoped CrB. The sharpness of the ferromagnetic-like
transition changes on moving away from , indicating significant
changes in the nature of the magnetic transitions in the vicinity of the
quantum critical point. Our data provide constraints for understanding quantum
phase transitions in itinerant ferromagnets in the limit of weak quenched
disorder.Comment: 8 pages, 7 figure
Phonon-modulated magnetic interactions and spin Tomonaga-Luttinger liquid in the p-orbital antiferromagnet CsO2
The magnetic response of antiferromagnetic CsO2, coming from the p-orbital
S=1/2 spins of anionic O2- molecules, is followed by 133Cs nuclear magnetic
resonance across the structural phase transition occuring at Ts1=61 K on
cooling. Above Ts1, where spins form a square magnetic lattice, we observe a
huge, nonmonotonic temperature dependence of the exchange coupling originating
from thermal librations of O2- molecules. Below Ts1, where antiferromagnetic
spin chains are formed as a result of p-orbital ordering, we observe a spin
Tomonaga-Luttinger-liquid behavior of spin dynamics. These two interesting
phenomena, which provide rare simple manifestations of the coupling between
spin, lattice and orbital degrees of freedom, establish CsO2 as a model system
for molecular solids.Comment: 9 pages, 5 figures (with Supplemental Material), to appear in
Physical Review Letter
One-dimensional quantum antiferromagnetism in the orbital CsO compound revealed by electron paramagnetic resonance
Recently it was proposed that the orbital ordering of molecular
orbitals in the superoxide CsO compound leads to the formation of spin-1/2
chains below the structural phase transition occuring at ~K on
cooling. Here we report a detailed X-band electron paramagnetic resonance (EPR)
study of this phase in CsO powder. The EPR signal appears as a broad line
below , which is replaced by the antiferromagnetic resonance below
the N\'{e}el temperature ~K. The temperature dependence of the
EPR linewidth between and agrees with the
predictions for the one-dimensional Heisenberg antiferromagnetic chain of
spins in the presence of symmetric anisotropic exchange interaction.
Complementary analysis of the EPR lineshape, linewidth and the signal intensity
within the Tomonaga-Luttinger liquid (TLL) framework allows for a determination
of the TLL exponent . Present EPR data thus fully comply with the
quantum antiferromagnetic state of spin-1/2 chains in the orbitally ordered
phase of CsO, which is, therefore, a unique orbital system where such a
state could be studied.Comment: 6 pages, 3 figure
Recovering Metallicity in A4C60: The Case of Monomeric Li4C60
The restoration of metallicity in the high-temperature, cubic phase of Li4C60
represents a remarkable feature for a member of the A4C60 family (A = alkali
metal), invariably found to be insulators. Structural and resonance technique
investigations on Li4C60 at T > 600 K, show that its fcc structure is
associated with a complete (4e) charge transfer to C60 and a sparsely populated
Fermi level. These findings not only emphasize the crucial role played by
lattice symmetry in fulleride transport properties, but also re-dimension the
role of Jahn-Teller effects in band structure determination. Moreover, they
suggest the present system as a potential precursor to a new class of
superconducting fullerides.Comment: 4 pages, 3 figure
Symmetric and antisymmetric exchange anisotropies in quasi-one-dimensional CuSeO as revealed by ESR
We present an electron spin resonance (ESR) study of single-crystalline spin
chain-system CuSeO in the frequency range between 9 GHz and 450 GHz. In
a wide temperature range above the N\'{e}el temperature K we observe
strong and anisotropic frequency dependence of a resonance linewidth. Although
sizeable interchain interaction ( is the intrachain
interaction) is present in this system, the ESR results agree well with the
Oshikawa-Affleck theory for one-dimensional Heisenberg antiferromagnet.
This theory is used to extract the anisotropies present in CuSeO. We
find that the symmetric anisotropic exchange and the
antisymmetric Dzyaloshinskii-Moriya (DM) interaction are
very similar in size in this system. Staggered-field susceptibility induced by
the presence of the DM interaction is witnessed in the macroscopic
susceptibility anisotropy.Comment: 8 pages, 7 figures, 2 tables, published in Phys. Rev.
Origin of magnetic moments in carbon nanofoam
A range of carbon nanofoam samples was prepared by using a high-repetition-rate laser ablation technique under various Ar pressures. Their magnetic properties were systematically investigated by dc magnetization measurements and continuous wave (cw) as well as pulsed EPR techniques. In all samples we found very large zero-field cooled-field-cooled thermal hysteresis in the susceptibility measurements extending up to room temperature. Zero-field cooled (ZFC) susceptibility measurements also display very complex behavior with a susceptibility maximum that strongly varies in temperature from sample to sample. Low-temperature magnetization curves indicate a saturation magnetization MS ≈0.35 emu g at 2 K and can be well fitted with a classical Langevin function. MS is more than an order of magnitude larger than any possible iron impurity, proving that the observed magnetic phenomena are an intrinsic effect of the carbon nanofoam. Magnetization measurements are consistent with a spin-glass type ground state. The cusps in the ZFC susceptibility curves imply spin freezing temperatures that range from 50 K to the extremely high value of >300 K. Further EPR measurements revealed three different centers that coexist in all samples, distinguished on the basis of g -factor and relaxation time. Their possible origin and the role in the magnetic phenomena are discussed
Two-electronic component behavior in the multiband FeSeTe superconductor
We report X-band EPR and Te and Se NMR measurements on
single-crystalline superconducting FeSeTe ( = 11.5(1)
K). The data provide evidence for the coexistence of intrinsic localized and
itinerant electronic states. In the normal state, localized moments couple to
itinerant electrons in the Fe(Se,Te) layers and affect the local spin
susceptibility and spin fluctuations. Below , spin fluctuations become
rapidly suppressed and an unconventional superconducting state emerges in which
is reduced at a much faster rate than expected for conventional - or
-wave symmetry. We suggest that the localized states arise from the
strong electronic correlations within one of the Fe-derived bands. The
multiband electronic structure together with the electronic correlations thus
determine the normal and superconducting states of the FeSeTe
family, which appears much closer to other high- superconductors than
previously anticipated.Comment: 5 pages, 4 figure
Jahn-Teller orbital glass state in the expanded fcc Cs3C60 fulleride
The most expanded fcc-structured alkali fulleride, Cs3C60, is a Mott insulator at ambient pressure because of the weak overlap between the frontier t1u molecular orbitals of the C603− anions. It has a severely disordered antiferromagnetic ground state that becomes a superconductor with a high critical temperature, Tc of 35 K upon compression. The effect of the localised t1u3 electronic configuration on the properties of the material is not well-understood. Here we study the relationship between the intrinsic crystallographic C603− orientational disorder and the molecular Jahn–Teller (JT) effect dynamics in the Mott insulating state. The high-resolution 13C magic-angle-spinning (MAS) NMR spectrum at room temperature comprises three peaks in the intensity ratio 1:2:2 consistent with the presence of three crystallographically-inequivalent carbon sites in the fcc unit cell and revealing that the JT-effect dynamics are fast on the NMR time-scale of 10−5 s despite the presence of the frozen-in C603− merohedral disorder disclosed by the 133Cs MAS NMR fine splitting of the tetrahedral and octahedral 133Cs resonances. Cooling to sub-liquid-nitrogen temperatures leads to severe broadening of both the 13C and 133Cs MAS NMR multiplets, which provides the signature of an increased number of inequivalent 13C and 133Cs sites. This is attributed to the freezing out of the C603− JT dynamics and the development of a t1u electronic orbital glass state guided by the merohedral disorder of the fcc structure. The observation of the dynamic and static JT effect in the Mott insulating state of the metrically cubic but merohedrally disordered Cs3C60 fulleride in different temperature ranges reveals the intimate relation between charge localization, magnetic ground state, lifting of electronic degeneracy, and orientational disorder in these strongly-correlated systems
Antiferromagnetic fluctuations in the normal state of LiFeAs
We present a detailed study of 75As NMR Knight shift and spin-lattice
relaxation rate in the normal state of stoichiometric polycrystalline LiFeAs.
Our analysis of the Korringa relation suggests that LiFeAs exhibits strong
antiferromagnetic fluctuations, if transferred hyperfine coupling is a dominant
interaction between 75As nuclei and Fe electronic spins, whereas for an on-site
hyperfine coupling scenario, these are weaker, but still present to account for
our experimental observations. Density-functional calculations of electric
field gradient correctly reproduce the experimental values for both 75As and
7Li sites.Comment: 5 pages, 3 figures, thoroughly revised version with refined
experimental data, accepted for publication as a Rapid Communication in
Physical Review B
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