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"Singh, D. J."
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Bipolar Conduction as the Possible Origin of the Electronic Transition in Pentatellurides: Metallic vs Semiconducting Behavior
The pentatelluridesZrTe5andHfTe5are layered compounds with one-dimensional transition-metal chains that show a not-yet-understood temperature-dependent transition in transport properties as well as recently discovered properties suggesting topological semimetallic behavior. Here, we report magnetotransport properties for two kinds ofZrTe5single crystals grown with the chemical vapor transport (CVT) and the flux method (Flux), respectively. They show distinct transport properties at zero field: The CVT crystal displays a metallic behavior with a pronounced resistance peak and a sudden sign reversal in thermopower at approximately 130 K, consistent with previous observations of the electronic transition; in striking contrast, the Flux crystal exhibits a semiconducting-like behavior at low temperatures and a positive thermopower over the whole temperature range. For both samples, strong effects on the transport properties are observed when the magnetic field is applied along the orthorhombicbandcaxes, i.e., perpendicular to the chain direction. Refinements on the single-crystal x-ray diffraction and the measurements of energy dispersive spectroscopy reveal the presence of noticeable Te vacancies in the CVT samples, while the Flux samples are close to the stoichiometry. Analyses on the magnetotransport properties confirm that the carrier densities of the CVT sample are about two orders higher than those of the Flux sample. Our results thus indicate that the widely observed anomalous transport behaviors in pentatellurides actually take place in the Te-deficient samples. For the stoichiometric pentatellurides, our electronic structure calculations show narrow-gap semiconducting behavior, with different transport anisotropies for holes and electrons. For the degenerately dopedn-type samples, our transport calculations can result in a resistivity peak and crossover in thermopower from negative to positive at temperatures close to those observed experimentally due to a combination of bipolar effects and different anisotropies of electrons and holes. Our present work resolves the long-standing puzzle regarding the anomalous transport behaviors of pentatellurides, as well as the electronic structure in favor of a semiconducting state.
Journal Article
Electronic structure of the iron-based superconductor LaOFeP
Superconductivity: 'itinerant' oxypnictides
The discovery of superconductivity in the iron-based layered compounds known as iron oxypnictides has renewed interest in high-temperature superconductivity. Two distinct classes of theories about the nature of the ground state of the oxypnictides have been put forward, characterized by contrasting underlying band structures. Such a controversy is partly due to the lack of conclusive experimental information on the electronic structures. Now Lu
et al
. report angle-resolved photoemission spectroscopy (ARPES) of an iron oxypnictide, LaOFeP, with a pretty high critical temperature of
T
c
= 5.9 K. Their results favour an 'itinerant' ground state, over one resembling the 'Mott insulator' state found in copper oxide superconductors.
Angle-resolved photoemission spectroscopy (ARPES) of LaOFeP (
T
c
= 5.9 K) is reported. These results favour the itinerant ground state, albeit with band renormalization. In addition, the data reveal important differences between these and copper-based superconductors.
The recent discovery of superconductivity in the iron oxypnictide family of compounds
1
,
2
,
3
,
4
,
5
,
6
,
7
,
8
,
9
has generated intense interest. The layered crystal structure with transition-metal ions in planar square-lattice form and the discovery of spin-density-wave order near 130 K (refs
10
,
11
) seem to hint at a strong similarity with the copper oxide superconductors. An important current issue is the nature of the ground state of the parent compounds. Two distinct classes of theories, distinguished by the underlying band structure, have been put forward: a local-moment antiferromagnetic ground state in the strong-coupling approach
12
,
13
,
14
,
15
,
16
,
17
, and an itinerant ground state in the weak-coupling approach
18
,
19
,
20
,
21
,
22
. The first approach stresses on-site correlations, proximity to a Mott-insulating state and, thus, the resemblance to the high-transition-temperature copper oxides, whereas the second approach emphasizes the itinerant-electron physics and the interplay between the competing ferromagnetic and antiferromagnetic fluctuations. The debate over the two approaches is partly due to the lack of conclusive experimental information on the electronic structures. Here we report angle-resolved photoemission spectroscopy (ARPES) of LaOFeP (superconducting transition temperature,
T
c
= 5.9 K), the first-reported iron-based superconductor
2
. Our results favour the itinerant ground state, albeit with band renormalization. In addition, our data reveal important differences between these and copper-based superconductors.
Journal Article
Reemergence of high-Tc superconductivity in the (Li1-xFex)OHFe1-ySe under high pressure
In order to elucidate pressure-induced second superconducting phase (SC-II) in A
x
Fe
2−
y
Se
2
(A = K, Rb, Cs, and Tl) having an intrinsic phase separation, we perform a detailed high-pressure magnetotransport study on the isoelectronic, phase-pure (Li
1−
x
Fe
x
)OHFe
1−
y
Se single crystals. Here we show that its ambient-pressure superconducting phase (SC-I) with a critical temperature
T
c
≈ 40 K is suppressed gradually to below 2 K and an SC-II phase emerges above
P
c
≈ 5 GPa with
T
c
increasing progressively to above 50 K up to 12.5 GPa. Our high-precision resistivity data uncover a sharp transition of the normal state from Fermi liquid for SC-I to non-Fermi liquid for SC-II phase. In addition, the reemergence of high-
T
c
SC-II is found to accompany with a concurrent enhancement of electron carrier density. Without structural transition below 10 GPa, the observed SC-II with enhanced carrier density should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction.
The understanding of the reemergence of pressure induced superconductivity in alkali-metal intercalated FeSe is hampered by sample complexities. Here, Sun et al. report the electronic properties of (Li
1–
x
Fe
x
)OHFe
1–
y
Se single crystal not only in the reemerged superconducting state but also in the normal state.
Journal Article
Spin-orbit coupling control of anisotropy, ground state and frustration in 5d2 Sr2MgOsO6
The influence of spin-orbit coupling (SOC) on the physical properties of the 5
d
2
system Sr
2
MgOsO
6
is probed via a combination of magnetometry, specific heat measurements, elastic and inelastic neutron scattering and density functional theory calculations. Although a significant degree of frustration is expected, we find that Sr
2
MgOsO
6
orders in a type I antiferromagnetic structure at the remarkably high temperature of 108 K. The measurements presented allow for the first accurate quantification of the size of the magnetic moment in a 5
d
2
system of 0.60(2) μ
B
–a significantly reduced moment from the expected value for such a system. Furthermore, significant anisotropy is identified via a spin excitation gap and we confirm by first principles calculations that SOC not only provides the magnetocrystalline anisotropy, but also plays a crucial role in determining both the ground state magnetic order and the size of the local moment in this compound. Through comparison to Sr
2
ScOsO
6
, it is demonstrated that SOC-induced anisotropy has the ability to relieve frustration in 5
d
2
systems relative to their 5
d
3
counterparts, providing an explanation of the high
T
N
found in Sr
2
MgOsO
6
.
Journal Article
Giant anharmonic phonon scattering in PbTe
Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic mode and the longitudinal acoustic modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying longitudinal acoustic phonons. The longitudinal acoustic–transverse optic anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.
Neutron scattering and first-principles calculations show that the small thermal conductivity of PbTe is due to anharmonic coupling between the acoustic phonon modes and the optical ferroelectric ones. The results provide a microscopic picture of why many good thermoelectrics are found near a ferroelectric lattice instability.
Journal Article
Electronic correlations in the iron pnictides
When electrons experience Coulomb repulsion, their kinetic energy becomes significantly reduced. This effect has now been measured in the pnictide superconductor LaFePO, and shows that correlations between electrons in these materials are just as strong as in some copper oxide and ruthenate superconductors.
In correlated metals derived from Mott insulators, the motion of an electron is impeded by Coulomb repulsion due to other electrons. This phenomenon causes a substantial reduction in the electron’s kinetic energy, leading to remarkable experimental manifestations in optical spectroscopy
1
. The high-transition-temperature (
T
c
) superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-
T
c
superconductivity in the iron pnictides
2
,
3
,
4
puts a spotlight on the relevance of correlation effects in these materials
5
. Here, we present an infrared and optical study on single crystals of the iron pnictide superconductor LaFePO. We find clear evidence of electronic correlations in metallic LaFePO with the kinetic energy of the electrons reduced to half of that predicted by band theory of nearly free electrons. We deduce that electronic many-body effects are important in the iron pnictides despite the absence of a Mott transition.
Journal Article
Reemergence of high-T c superconductivity in the (Li1-x Fe x )OHFe1-y Se under high pressure
In order to elucidate pressure-induced second superconducting phase (SC-II) in AxFe2−ySe2 (A = K, Rb, Cs, and Tl) having an intrinsic phase separation, we perform a detailed high-pressure magnetotransport study on the isoelectronic, phase-pure (Li1−xFex)OHFe1−ySe single crystals. Here we show that its ambient-pressure superconducting phase (SC-I) with a critical temperature Tc ≈ 40 K is suppressed gradually to below 2 K and an SC-II phase emerges above Pc ≈ 5 GPa with Tc increasing progressively to above 50 K up to 12.5 GPa. Our high-precision resistivity data uncover a sharp transition of the normal state from Fermi liquid for SC-I to non-Fermi liquid for SC-II phase. In addition, the reemergence of high-Tc SC-II is found to accompany with a concurrent enhancement of electron carrier density. Without structural transition below 10 GPa, the observed SC-II with enhanced carrier density should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction.
Journal Article
Comparison of rheological properties of graphene / carbon nanotube hydrogenated oil based biodegradable drilling fluid
An experimental investigation has been carried out to investigate the rheological properties of graphene / carbon nanotube hydrogenated oil based biodegradable drilling fluid at different nanoparticle loadings. The rheological behaviours of interest in this investigation are the viscosity and shear stresses of two different nanofluids respectively. The limiting parameters in this study are 25 ppm, 50 ppm and 100 ppm weight concentration at operating temperature ranging from 30°C to 50°C. Both nanofluids are subjected to shear rate ranging from 0 - 140 s-1 for comparison of rheological behaviours. Both samples' viscosity reduces to base fluid's viscosity value at higher shear rate with carbon nanotube-hydrogenated oil yielding higher viscosity compared to graphene-hydrogenated oil for all nanoparticle loadings at lower shear rate. Shear stress analysis also shows similar results with carbon nanotube based samples showing higher stress between the two at all particle loadings. Both samples show Newtonian behaviour that is similar to base fluid even at higher particle loadings. Analysis revealed both nanofluids yields close to zero yield stress even with the presence of graphene or carbon nanotube particles. The significance of this study shows that addition of low nanomaterials for enhancement of drilling fluids can improve its thermophysical properties without compromising the quality of drilling fluids such as viscosity and shear stress properties.
Journal Article