2,097 research outputs found
First Measurement of Collectivity of Coexisting Shapes based on Type II Shell Evolution: The Case of Zr
Background: Type II shell evolution has recently been identified as a
microscopic cause for nuclear shape coexistence. Purpose: Establish a low-lying
rotational band in 96-Zr. Methods: High-resolution inelastic electron
scattering and a relative analysis of transition strengths are used. Results:
The B(E2; 0_1^+ -> 2_2^+) value is measured and electromagnetic decay strengths
of the secdond 2^+ state are deduced. Conclusions: Shape coexistence is
established for 96-Zr. Type II shell evolution provides a systematic and
quantitative mechanism to understand deformation at low excitation energies.Comment: 5 pages, 4 figure
Spin fluctuations in the quasi-two dimensional Heisenberg ferromagnet GdI_2 studied by Electron Spin Resonance
The spin dynamics of GdI_2 have been investigated by ESR spectroscopy. The
temperature dependences of the resonance field and ESR intensity are well
described by the model for the spin susceptibility proposed by Eremin et al.
[Phys. Rev. B 64, 064425 (2001)]. The temperature dependence of the resonance
linewidth shows a maximum similar to the electrical resistance and is discussed
in terms of scattering processes between conduction electrons and localized
spins.Comment: to be published in PR
Experimental observation of Aharonov-Bohm caging using orbital angular momentum modes in optical waveguides
The discovery of artificial gauge fields, controlling the dynamics of
uncharged particles that otherwise elude the influence of standard electric or
magnetic fields, has revolutionized the field of quantum simulation. Hence,
developing new techniques to induce those fields is essential to boost quantum
simulation in photonic structures. Here, we experimentally demonstrate in a
photonic lattice the generation of an artificial gauge field by modifying the
input state, overcoming the need to modify the geometry along the evolution or
imposing the presence of external fields. In particular, we show that an
effective magnetic flux naturally appears when light beams carrying orbital
angular momentum are injected into waveguide lattices with certain
configurations. To demonstrate the existence of that flux, we measure the
resulting Aharonov-Bohm caging effect. Therefore, we prove the possibility of
switching on and off artificial gauge fields by changing the topological charge
of the input state, paving the way to access different topological regimes in
one single structure, which represents an important step forward for optical
quantum simulation
Optical properties and electronic structure of Ca-doped alpha'-NaV2O5
The dielectric function of alpha'-Na(1-x)Ca(x)V2O5 (0 < x < 20%) was measured
for the a and b axes in the photon energy range 0.8-4.5 eV at room temperature.
By varying the Ca-concentration we control the relative abundancy of V4+ and
V5+. We observe that the intensity of the main optical absorption peak at 1 eV
is proportional to the number of V5+ ions. This rules out the interpretation as
a V4+ d-d excitation, and it establishes that this is the on-rung
bonding-antibonding transition.Comment: 6 pages, ReVTeX, 5 figures in encapsulated postscript forma
Low temperature ellipsometry of NaV2O5
The dielectric function of alpha'NaV2O5 was measured with electric field
along the a and b axes in the photon energy range 0.8-4.5 eV for temperatures
down to 4K. We observe a pronounced decrease of the intensity of the 1 eV peak
upon increasing temperature with an activation energy of about 25meV,
indicating that a finite fraction of the rungs becomes occupied with two
electrons while others are emptied as temperature increases. No appreciable
shifts of peaks were found s in the valence state of individual V atoms at the
phase transition is very small. A remarkable inflection of this temperature
dependence at the phase transition at 34 K indicates that charge ordering is
associated with the low temperature phase.Comment: Revisions in style and order of presentation. One new figure. In
press in Physical Review B. REVTeX, 4 pages with 4 postscript figure
Orthorhombic versus monoclinic symmetry of the charge-ordered state of NaV2O5
High-resolution X-ray diffraction data show that the low-temperature
superstructure of alpha-NaV2O5 has an F-centered orthorhombic 2a x 2b x 4c
superlattice. A structure model is proposed, that is characterized by layers
with zigzag charge order on all ladders and stacking disorder, such that the
averaged structure has space group Fmm2. This model is in accordance with both
X-ray scattering and NMR data. Variations in the stacking order and disorder
offer an explanation for the recently observed devils staircase of the
superlattice period along c.Comment: REVTEX, 4 pages including 2 figures, shortened, submitted to PR
Charge Order Driven spin-Peierls Transition in NaV2O5
We conclude from 23Na and 51V NMR measurements in NaxV2O5(x=0.996) a charge
ordering transition starting at T=37 K and preceding the lattice distortion and
the formation of a spin gap Delta=106 K at Tc=34.7 K. Above Tc, only a single
Na site is observed in agreement with the Pmmn space group of this first
1/4-filled ladder system. Below Tc=34.7 K, this line evolves into eight
distinct 23Na quadrupolar split lines, which evidences a lattice distortion
with, at least, a doubling of the unit cell in the (a,b) plane. A model for
this unique transition implying both charge density wave and spin-Peierls order
is discussed.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Magnetic correlations in infinite-layer nickelates: an experimental and theoretical multi-method study
We report a comprehensive study of magnetic correlations in LaNiO, a
parent compound of the recently discovered family of infinite-layer (IL)
nickelate superconductors, using multiple experimental and theoretical methods.
Our specific heat, muon-spin rotation (SR), and magnetic susceptibility
measurements on polycrystalline LaNiO show that long-range magnetic order
remains absent down to 2 K. Nevertheless, we detect residual entropy in the
low-temperature specific heat, which is compatible with a model fit that
includes paramagnon excitations. The SR and low-field static and dynamic
magnetic susceptibility measurements indicate the presence of short-range
magnetic correlations and glassy spin dynamics, which we attribute to local
oxygen non-stoichiometry in the average infinite-layer crystal structure. This
glassy behavior can be suppressed in strong external fields, allowing us to
extract the intrinsic paramagnetic susceptibility. Remarkably, we find that the
intrinsic susceptibility shows non-Curie-Weiss behavior at high temperatures,
in analogy to doped cuprates that possess robust non-local spin fluctuations.
The distinct temperature dependence of the intrinsic susceptibility of
LaNiO can be theoretically understood by a multi-method study of the
single-band Hubbard model in which we apply complementary cutting-edge quantum
many-body techniques (dynamical mean-field theory, cellular dynamical
mean-field theory and the dynamical vertex approximation) to investigate the
influence of both short- and long-ranged correlations. Our results suggest a
profound analogy between the magnetic correlations in parent (undoped) IL
nickelates and doped cuprates.Comment: 18 pages, 14 figure
Examining the surface phase diagram of IrTe with photoemission
In the transition metal dichalcogenide IrTe, low-temperature
charge-ordered phase transitions involving Ir dimers lead to the occurrence of
stripe phases of different periodicities, and nearly degenerate energies.
Bulk-sensitive measurements have shown that, upon cooling, IrTe undergoes
two such first-order transitions to and
reconstructed phases at ~K and ~K,
respectively. Here, using surface sensitive probes of the electronic structure
of IrTe, we reveal the first-order phase transition at ~K to
the stripes phase, previously proposed to be the surface ground
state. This is achieved by combining x-ray photoemission spectroscopy and
angle-resolved photoemission spectroscopy, which give access to the evolution
of stripe domains and a particular surface state, the energy of which is
dependent on the Ir dimer length. By performing measurements over a full
thermal cycle, we also report the complete hysteresis of all these phases
Thermodynamics of Spin S = 1/2 Antiferromagnetic Uniform and Alternating-Exchange Heisenberg Chains
The magnetic susceptibility chi and specific heat C versus temperature T of
the spin-1/2 antiferromagnetic alternating-exchange (J1 and J2) Heisenberg
chain are studied for the entire range 0 \leq alpha \leq 1 of the alternation
parameter alpha = J2/J1. For the uniform chain (alpha = 1), detailed
comparisons of the high-accuracy chi(T) and C(T) Bethe ansatz data of Kluemper
and Johnston are made with the asymptotically exact low-T field theory
predictions of Lukyanov. QMC simulations and TMRG calculations of chi(alpha,T)
are presented. From the low-T TMRG data, the spin gap Delta(alpha)/J1 is
extracted for 0.8 \leq alpha \leq 0.995. High accuracy fits to all of the above
numerical data are obtained. We examine in detail the theoretical predictions
of Bulaevskii for chi(alpha,T) and compare them with our results. Our
experimental chi(T) and C(T) data for NaV2O5 single crystals are modeled in
detail. The chi(T) data above the spin dimerization temperature Tc = 34 K are
not in agreement with the prediction for the uniform Heisenberg chain, but can
be explained if there is a moderate ferromagnetic interchain coupling and/or if
J changes with T. By fitting the chi(T) data, we obtain Delta(T = 0) = 103(2)
K, alternation parameter delta(0) = (1 - alpha)/(1 + alpha) = 0.034(6) and
average exchange constant J(0) = 640(80) K. The delta(T) and Delta(T) are
derived from the data. A spin pseudogap with a large magnitude \approx 0.4
Delta(0) is consistently found just above Tc, which decreases with increasing
T. Analysis of our C(T) data indicates that at Tc, at least 77% of the entropy
change due to the transition at Tc and associated order parameter fluctuations
arise from the lattice and/or charge degrees of freedom and less than 23% from
the spin degrees of freedom.Comment: 53 two-column REVTeX pages, 50 embedded figures, 7 tables. Revisions
required due to incorrect Eq. (39) in Ref. 51 which gives the low-T
approximation for the specific heat of a S = 1/2 1D system with a spin gap;
no conclusions were changed. Additional minor revisions made. Phys. Rev. B
(in press
- …