222 research outputs found
Reconstructing the spatial structure of quantum correlations
Quantum correlations are a fundamental property of quantum many-body states.
Yet they remain experimentally elusive, hindering certification of genuine
quantum behavior, especially in quantum materials. Here we show that the
momentum-dependent dynamical susceptibility measured via inelastic neutron
scattering enables the systematic reconstruction of quantum correlation
functions, which express the degree of quantum coherence in the fluctuations of
two spins at arbitrary mutual distance. Using neutron scattering data on the
compound KCuF \unicode{x2014} a system of weakly coupled
Heisenberg chains \unicode{x2014} and of numerically exact quantum Monte
Carlo data, we show that quantum correlations possess a radically different
spatial structure with respect to conventional correlations. Indeed, they
exhibit a new emergent length of quantum-mechanical origin \unicode{x2014}
the quantum coherence length \unicode{x2014} which is finite at any finite
temperature (including when long-range magnetic order develops). Moreover, we
show theoretically that coupled Heisenberg spin chains exhibit a form of
quantum monogamy, with a trade-off between quantum correlations along and
transverse to the spin chains. These results highlight real-space quantum
correlators as an informative, model-independent means of probing the
underlying quantum state of real quantum materials.Comment: Main text: 8 pages, 5 figures. Supplementary information: 4 pages, 5
figure
Multipartite entanglement in the 1-D spin- Heisenberg Antiferromagnet
Multipartite entanglement refers to the simultaneous entanglement between
multiple subsystems of a many-body quantum system. While multipartite
entanglement can be difficult to quantify analytically, it is known that it can
be witnessed through the Quantum Fisher information (QFI), a quantity that can
also be related to dynamical Kubo response functions. In this work, we first
show that the finite temperature QFI can generally be expressed in terms of a
static structure factor of the system, plus a correction that vanishes as
. We argue that this implies that the static structure factor
witnesses multipartite entanglement near quantum critical points at
temperatures below a characteristic energy scale that is determined by
universal properties, up to a non-universal amplitude. Therefore, in systems
with a known static structure factor, we can deduce finite temperature scaling
of multipartite entanglement and low temperature entanglement depth without
knowledge of the full dynamical response function of the system. This is
particularly useful to study 1D quantum critical systems in which sub-power-law
divergences can dominate entanglement growth, where the conventional scaling
theory of the QFI breaks down. The 1D spin- antiferromagnetic
Heisenberg model is an important example of such a system, and we show that
multipartite entanglement in the Heisenberg chain diverges non-trivially as
. We verify these predictions with calculations of the
QFI using conformal field theory and matrix product state simulations. Finally
we discuss the implications of our results for experiments to probe
entanglement in quantum materials, comparing to neutron scattering data in
KCuF, a material well-described by the Heisenberg chain.Comment: 8 pages and 3 figures; 1 page and 1 figure of the appendix; typos
corrected; references adde
WHO grade I meningiomas: classification-tree for prognostic factors of survival
World Health Organization (WHO) grade I meningiomas are intracranial extracerebral tumors, in which microsurgery as a stand-alone therapy provides high rates of disease control and low recurrence rates. Our aim was to identify prognostic factors of overall survival and time-to-retreat (OS; TTR) in a cohort of patients with surgically managed WHO grade I meningioma. Patients with WHO grade I meningiomas from a retrospectively (1990 to 2002) and prospectively managed (2003 to 2010) databank of Oslo University Hospital, Norway, were included. The mean follow-up was 9.2 ± 5.7 years, with a total of 11,414 patient-years. One thousand three hundred fifty-five patients were included. The mean age was 58 ± 13.2, mean Karnofsky Performance Status (KPS) 92.6 ± 26.1 and female-to-male ratio 2.5:1. The 1-year, 5-year, 10-year, 15-year, and 20-year probabilities were 0.98, 0.91, 0.87, 0.84, and 0.8 for TTR. Patient age (OR 0.92 [0.91, 0.94]), male sex (OR 0.59 [0.45, 0.76]), preoperative KPS ≥ 70 (OR 2.22 [1.59, 3.13]), skull base location (OR 0.77 [0.60, 1]), and the occurrence of a postoperative hematoma (OR 0.44 [0.26, 0.76]) were identified as independent prognostic factors of OS. Patient age (OR 1.02 [1.01, 1.03]) and skull base location (OR 0.30 [0.21, 0.45]) were independent predictors of decreased PFS. Using a recursive partitioning analysis, we suggest a classification tree for the prediction of 5-year PFS based on patient and tumor characteristics. The findings from this cohort of meningioma WHO I patients helps to identify patients at risk of recurrence and tailor the therapeutic management
Excess heat capacity in magnetically ordered Ce heavy fermion metals
We study the magnetic heat capacity of a series of magnetically ordered
Ce-based heavy fermion materials, which show an anomalous heat capacity
in excess of the phonon contribution in many materials. For compounds for which
magnon models have been worked out, we show that the local-moment magnon heat
capacity derived from the measured magnon spectra underestimates the
experimental specific heat. The excess heat capacity reveals increasing density
of states with increasing energy, akin to a pseudogap. We show that this
anomalous temperature-dependent term is not associated with proximity to a
quantum critical point (QCP), but is strongly correlated with , indicating
the anomalous excitations are governed by the magnetic exchange interaction.
This insight may hold key information for understanding magnetically ordered
heavy fermions.Comment: 5 pages, 4 figure
Quantifying and controlling entanglement in the quantum magnet CsCoCl
The lack of methods to experimentally detect and quantify entanglement in
quantum matter impedes our ability to identify materials hosting highly
entangled phases, such as quantum spin liquids. We thus investigate the
feasibility of using inelastic neutron scattering (INS) to implement a
model-independent measurement protocol for entanglement based on three
entanglement witnesses: one-tangle, two-tangle, and quantum Fisher information
(QFI). We perform high-resolution INS measurements on CsCoCl, a close
realization of the transverse-field XXZ spin chain, where we can
control entanglement using the magnetic field, and compare with density-matrix
renormalization group calculations for validation. The three witnesses allow us
to infer entanglement properties and make deductions about the quantum state in
the material. We find QFI to be a particularly robust experimental probe of
entanglement, whereas the one- and two-tangles require more careful analysis.
Our results lay the foundation for a general entanglement detection protocol
for quantum spin systems.Comment: Main text: 7 pages, 4 figures. Supplementary Information: 15 pages,
15 figure
Spin wave Hamiltonian and anomalous scattering in NiPS
We report a comprehensive spin wave analysis of the semiconducting honeycomb
van der Waal antiferromagnet NiPS. Using single crystal inelastic neutron
scattering, we map out the full Brillouin zone and fit the observed modes to a
spin wave model with rigorously defined uncertainty. We find that the third
neighbor exchange dominates the Hamiltonian, a feature which we fully
account for by ab-initio density functional theory calculations. We also
quantify the degree to which the three-fold rotation symmetry is broken and
account for the excitations observed in other measurements, yielding a
spin exchange model which is consistent across multiple experimental probes. We
also identify a strongly reduced static ordered moment and reduced low-energy
intensity relative to the linear spin wave calculations, signaling unexplained
features in the magnetism which requires going beyond the linear spin wave
approximation.Comment: 7 pages, 8 figures; 5 pages and 6 additional figures of appendice
Non-linear magnons and exchange Hamiltonians of delafossite proximate quantum spin liquids
Quantum spin liquids (QSL) are theoretical states of matter with long-range
entanglement and exotic quasiparticles. However, they generally elude
quantitative theory, rendering their underlying phases mysterious and hampering
efforts to identify experimental QSL states. Here we study triangular lattice
resonating valence bond QSL candidate materials KYbSe and NaYbSe. We
measure the magnon modes in their 1/3 plateau phase, where quantitative theory
is tractable, using inelastic neutron scattering and fit them using nonlinear
spin wave theory. We also fit the KYbSe heat capacity using high
temperature series expansion. Both KYbSe fits yield the same magnetic
Hamiltonian to within uncertainty, confirming previous estimates and showing
the Heisenberg to be an accurate model for these materials. Most
importantly, comparing KYbSe and NaYbSe shows that smaller -site
Na ion has a larger ratio. However, hydrostatic pressure applied
to KYbSe increases the ordering temperature (a result consistent with
density functional theory calculations), indicating that pressure decreases
. These results show how periodic table and hydrostatic pressure can
tune the YbSe materials in a controlled way.Comment: 7 pages, 7 figures; 4 pages and 7 additional figures of supplemental
informatio
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