TY - BOOK
T1 - Neutron Scattering Investigations of Correlated Electron Systems and Neutron Instrumentation
AU - Holm, Sonja Lindahl
PY - 2016
Y1 - 2016
N2 - This PhD work has two main topics; one on neutron instrumentations, and one on correlated
electron systems. There have been a total of ten different subprojects. Common to
all the projects is the neutron scattering technique that is presented in the first chapters
of the thesis.
Neutrons are a unique probe for studying the atomic and molecular structure and dynamics
of materials. Even though neutrons are very expensive to produce, the advantages
neutrons provide overshadow the price. As neutrons interact weakly with materials compared
to many other probes, e.g. electrons or photons, it is possible to make a neutron
scattering experiment through sample environment equipment like cryostats or pressure
cells. Another advantage of neutron experiments is that the wavelength and energy of the
neutron match the inter-atomic distances and basic excitations of solid materials. The
scattering cross section varies through the periodic table in a seemingly random fashion.
Neutron scattering offers a unique possibility to study light elements that have relatively
high cross sections.
The first main topic is on neutron instrumentation for the European Spallation Source
(ESS). ESS is currently under construction in Lund, Sweden, and is going to be the first
high powered long pulsed spallation source. The long source pulse calls for long instruments,
and hence new guide designs are being developed. The new instrument designs are
tested using the Monte Carlo ray tracing tool McStas.
Two papers describing the impact of the time structure (pulse length and repetition
frequency) choice for ESS are appended. McStas simulations of a low resolution cold
powder diffractometer and high resolution thermal powder diffractometer with wavelength
frame multiplication have been carried out for 20 different settings of the time structure.
The instrument designs were changed to fit each setting with pulse lengths between 1
ms and 2 ms and repetition frequencies between 10 Hz and 25 Hz. The cold powder
diffractometer was found to perform well with all the different source settings. The thermal
powder diffractometer, however, has a performance five times better for 1 ms pulse length
and 10 Hz than for 2 ms and 25 Hz. For the full suite of 15 generic neutron instruments
a discussion of the figure-of-merit for a full facility is given. In addition, it is noted that
for the simple guide systems used in the simulations, most neutrons reaching the sample
originate from the central spot on the moderator with a radius of 3 cm to 5 cm.
The simulation work on the "guide-split" concept is appended as part of the thesis.
With this concept it is possible to feed several cold neutron instruments with low divergence
on the sample, from the same beam port without notably compromising the neutron flux
at any of the sample positions. Three guide split set-ups have been tested via computer
xiv
simulations in McStas; with two, four, and eight secondary guides with a total length of 150
m. During the optimization, the parameter space was found to have several plateaus with
high brilliance transfer (>90%). A smooth divergence distribution at the sample position
can be obtained by choosing parameters that further this.
A description of the multipurpose machine, HEIMDAL, accepted for construction at
ESS, is given in full length in the appended published paper. A short summary is given
in the main text of the thesis. HEIMDAL will be a multi length scale neutron scattering
instrument for the study of structures covering almost nine orders of magnitude from
0.01 nm to 50 mm. The instrument features a variable resolution thermal neutron powder
diffractometer, combined with small angle neutron scattering and neutron imaging. The
instrument uses a novel combination of a cold and a thermal guide to fulfill the diverse
requirements for diffraction and small angle neutron scattering. With an instrument length
of 170 m, HEIMDAL will take utilize the high neutron flux of the long pulse at ESS, whilst
maintaining a high q-resolution due to the long flight path. Full virtual experiments for
the powder diffraction and small angle scattering have been performed to assess the performance
of the instrument design.
The second main topic is on correlated electron systems. Here the magnetism of six
different compounds have been studied with neutron scattering, including three different
hole-doped cuprate high-temperature superconductors (HTSC), an electron-doped iron
pnictide HTSC, a mineral with small clusters of geometrically frustrated magnetism, and
a multiferroic manganite.
A study of the inter-planar correlations in the HTSC La1:88Sr0:12CuO4 (Tc = 27 K)
is appended as a published paper. The correlations are studied both in a magnetic field
applied perpendicularly to the CuO2 planes, and in zero field under different cooling conditions.
We find that the effect of the magnetic field increases the magnetic scattering signal
at all values of the out-of-plane wave vector L, indicating an overall increase of the magnetic
moments. In addition, weak correlations between the copper oxide planes develop in
the presence of a magnetic field. This effect is not taken into account in previous reports
on the field effect of magnetic scattering, since usually only L 0 is probed.
A paper draft submitted for publication describing the results of elastic and inelastic
neutron scattering experiments performed on the oxygen-doped La2CuO4+y HTSC is appended
(Tc 40 K). The work is also included as a section in the thesis where the magnetic
incommensurate stripe phase and fluctuations are described. A neutron scattering study
with high flux and high resolution reveals that the dynamic stripes in a cuprate are not the
Goldstone modes of the static stripes, but rather that the signal originates from different
phases in the samples. As a consequence, future models of the stripes in these materials
will need to include phase separation.
The work on the co-doped HTSC La1:94Sr0:06CuO4+0:035 (Tc = 37 K) is described in a
chapter 7 of the thesis. Neutron scattering and muon spin rotation experiments show that
xv
the system has no static magnetism in zero applied field. A magnetic volume fraction can
be field induced and has a linear dependence with no onset field. This behavior indicates
that the system is at a quantum critical point. This co-doped sample with a Sr content
of x = 0:06 has a balance between the quenched disorder from the Sr impurities and
annealed order of the excess oxygen where no pinning of the magnetic order is present.
With competing order parameters of the superconducting and magnetic phases, one would
expect a boost of Tc in this sample without static magnetism. This is in contrast to what
is observed as the critical temperature is slightly lower for this system compared to other
co-doped systems, suggesting that the magnetic and superconducting phases co-exist.
A published manuscript describes the study of magnetic and superconducting properties
of Ba(Fe1xCox)2As2. A combined neutron scattering and muon spin rotation study of
the system as a function of temperature and external magnetic fields was performed. Below
the superconducting transition temperature, the magnetic and superconducting order
parameters coexist and compete. A magnetic field can significantly enhance the magnetic
scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5 T.
A microscopic modeling of the data using a five-band Hamiltonian relevant to iron pnictides
was performed. In the superconducting state, vortices can slow down and freeze
spin fluctuations locally. When such regions couple, they result in a long-range ordered
antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement
with experiments.
Appended is a study of the magnetic frustration in the mineral boleite. The crystal
structure of boleite contains antiferromagnetically coupled Cu2+ S = 1=2 ions forming
truncated 24-spin cube clusters of linked triangles. The clusters in boleite afford a situation
intermediate between molecular and bulk magnetism, accessible to both experiment and
numerical theory, in which a spin liquid can be studied. Susceptibility, neutron scattering
and exact diagonalization calculations suggest that effective S = 1=2 degrees of freedom
emerge on the triangles. Weaker inter-triangle interactions, in turn, lead to a condensation
of these effective spins into a singlet state at lower temperature. The behavior of the 24
spin system resembles a spin liquid behavior, and due to the small size the boleite system
is defined as a quantum spin droplet.
The investigation of the type-I multiferroic material h-YMnO3 is described in the main
text of the thesis, and a manuscript on the work is appended. With inelastic neutron scattering
an avoided crossing has been found between magnon and phonon modes close to the
boundary of the Brillouin zone. This is further confirmed by use of neutron polarization
analysis. A theoretical description of this magnon-phonon hybridization using a Heisenberg
model of localized spins, acoustic phonon modes and a coupling via the single-ion
magnetostriction allows to calculate the spectra and the measured cross-section. An external
magnetic field along the c-axis reveals a linear splitting of one spin wave branch which
allows an exclusion of several proposed magnetic ground states based on the theoretical
model and a comparison to the measurements.
AB - This PhD work has two main topics; one on neutron instrumentations, and one on correlated
electron systems. There have been a total of ten different subprojects. Common to
all the projects is the neutron scattering technique that is presented in the first chapters
of the thesis.
Neutrons are a unique probe for studying the atomic and molecular structure and dynamics
of materials. Even though neutrons are very expensive to produce, the advantages
neutrons provide overshadow the price. As neutrons interact weakly with materials compared
to many other probes, e.g. electrons or photons, it is possible to make a neutron
scattering experiment through sample environment equipment like cryostats or pressure
cells. Another advantage of neutron experiments is that the wavelength and energy of the
neutron match the inter-atomic distances and basic excitations of solid materials. The
scattering cross section varies through the periodic table in a seemingly random fashion.
Neutron scattering offers a unique possibility to study light elements that have relatively
high cross sections.
The first main topic is on neutron instrumentation for the European Spallation Source
(ESS). ESS is currently under construction in Lund, Sweden, and is going to be the first
high powered long pulsed spallation source. The long source pulse calls for long instruments,
and hence new guide designs are being developed. The new instrument designs are
tested using the Monte Carlo ray tracing tool McStas.
Two papers describing the impact of the time structure (pulse length and repetition
frequency) choice for ESS are appended. McStas simulations of a low resolution cold
powder diffractometer and high resolution thermal powder diffractometer with wavelength
frame multiplication have been carried out for 20 different settings of the time structure.
The instrument designs were changed to fit each setting with pulse lengths between 1
ms and 2 ms and repetition frequencies between 10 Hz and 25 Hz. The cold powder
diffractometer was found to perform well with all the different source settings. The thermal
powder diffractometer, however, has a performance five times better for 1 ms pulse length
and 10 Hz than for 2 ms and 25 Hz. For the full suite of 15 generic neutron instruments
a discussion of the figure-of-merit for a full facility is given. In addition, it is noted that
for the simple guide systems used in the simulations, most neutrons reaching the sample
originate from the central spot on the moderator with a radius of 3 cm to 5 cm.
The simulation work on the "guide-split" concept is appended as part of the thesis.
With this concept it is possible to feed several cold neutron instruments with low divergence
on the sample, from the same beam port without notably compromising the neutron flux
at any of the sample positions. Three guide split set-ups have been tested via computer
xiv
simulations in McStas; with two, four, and eight secondary guides with a total length of 150
m. During the optimization, the parameter space was found to have several plateaus with
high brilliance transfer (>90%). A smooth divergence distribution at the sample position
can be obtained by choosing parameters that further this.
A description of the multipurpose machine, HEIMDAL, accepted for construction at
ESS, is given in full length in the appended published paper. A short summary is given
in the main text of the thesis. HEIMDAL will be a multi length scale neutron scattering
instrument for the study of structures covering almost nine orders of magnitude from
0.01 nm to 50 mm. The instrument features a variable resolution thermal neutron powder
diffractometer, combined with small angle neutron scattering and neutron imaging. The
instrument uses a novel combination of a cold and a thermal guide to fulfill the diverse
requirements for diffraction and small angle neutron scattering. With an instrument length
of 170 m, HEIMDAL will take utilize the high neutron flux of the long pulse at ESS, whilst
maintaining a high q-resolution due to the long flight path. Full virtual experiments for
the powder diffraction and small angle scattering have been performed to assess the performance
of the instrument design.
The second main topic is on correlated electron systems. Here the magnetism of six
different compounds have been studied with neutron scattering, including three different
hole-doped cuprate high-temperature superconductors (HTSC), an electron-doped iron
pnictide HTSC, a mineral with small clusters of geometrically frustrated magnetism, and
a multiferroic manganite.
A study of the inter-planar correlations in the HTSC La1:88Sr0:12CuO4 (Tc = 27 K)
is appended as a published paper. The correlations are studied both in a magnetic field
applied perpendicularly to the CuO2 planes, and in zero field under different cooling conditions.
We find that the effect of the magnetic field increases the magnetic scattering signal
at all values of the out-of-plane wave vector L, indicating an overall increase of the magnetic
moments. In addition, weak correlations between the copper oxide planes develop in
the presence of a magnetic field. This effect is not taken into account in previous reports
on the field effect of magnetic scattering, since usually only L 0 is probed.
A paper draft submitted for publication describing the results of elastic and inelastic
neutron scattering experiments performed on the oxygen-doped La2CuO4+y HTSC is appended
(Tc 40 K). The work is also included as a section in the thesis where the magnetic
incommensurate stripe phase and fluctuations are described. A neutron scattering study
with high flux and high resolution reveals that the dynamic stripes in a cuprate are not the
Goldstone modes of the static stripes, but rather that the signal originates from different
phases in the samples. As a consequence, future models of the stripes in these materials
will need to include phase separation.
The work on the co-doped HTSC La1:94Sr0:06CuO4+0:035 (Tc = 37 K) is described in a
chapter 7 of the thesis. Neutron scattering and muon spin rotation experiments show that
xv
the system has no static magnetism in zero applied field. A magnetic volume fraction can
be field induced and has a linear dependence with no onset field. This behavior indicates
that the system is at a quantum critical point. This co-doped sample with a Sr content
of x = 0:06 has a balance between the quenched disorder from the Sr impurities and
annealed order of the excess oxygen where no pinning of the magnetic order is present.
With competing order parameters of the superconducting and magnetic phases, one would
expect a boost of Tc in this sample without static magnetism. This is in contrast to what
is observed as the critical temperature is slightly lower for this system compared to other
co-doped systems, suggesting that the magnetic and superconducting phases co-exist.
A published manuscript describes the study of magnetic and superconducting properties
of Ba(Fe1xCox)2As2. A combined neutron scattering and muon spin rotation study of
the system as a function of temperature and external magnetic fields was performed. Below
the superconducting transition temperature, the magnetic and superconducting order
parameters coexist and compete. A magnetic field can significantly enhance the magnetic
scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5 T.
A microscopic modeling of the data using a five-band Hamiltonian relevant to iron pnictides
was performed. In the superconducting state, vortices can slow down and freeze
spin fluctuations locally. When such regions couple, they result in a long-range ordered
antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement
with experiments.
Appended is a study of the magnetic frustration in the mineral boleite. The crystal
structure of boleite contains antiferromagnetically coupled Cu2+ S = 1=2 ions forming
truncated 24-spin cube clusters of linked triangles. The clusters in boleite afford a situation
intermediate between molecular and bulk magnetism, accessible to both experiment and
numerical theory, in which a spin liquid can be studied. Susceptibility, neutron scattering
and exact diagonalization calculations suggest that effective S = 1=2 degrees of freedom
emerge on the triangles. Weaker inter-triangle interactions, in turn, lead to a condensation
of these effective spins into a singlet state at lower temperature. The behavior of the 24
spin system resembles a spin liquid behavior, and due to the small size the boleite system
is defined as a quantum spin droplet.
The investigation of the type-I multiferroic material h-YMnO3 is described in the main
text of the thesis, and a manuscript on the work is appended. With inelastic neutron scattering
an avoided crossing has been found between magnon and phonon modes close to the
boundary of the Brillouin zone. This is further confirmed by use of neutron polarization
analysis. A theoretical description of this magnon-phonon hybridization using a Heisenberg
model of localized spins, acoustic phonon modes and a coupling via the single-ion
magnetostriction allows to calculate the spectra and the measured cross-section. An external
magnetic field along the c-axis reveals a linear splitting of one spin wave branch which
allows an exclusion of several proposed magnetic ground states based on the theoretical
model and a comparison to the measurements.
UR - https://rex.kb.dk:443/KGL:KGL:KGL01009273363
M3 - Ph.D. thesis
BT - Neutron Scattering Investigations of Correlated Electron Systems and Neutron Instrumentation
PB - The Niels Bohr Institute, Faculty of Science, University of Copenhagen
ER -