608 research outputs found
A Model for the Genesis of Arterial Pressure Mayer Waves from Heart Rate and Sympathetic Activity
Both theoretic models and cross-spectral analyses suggest that an oscillating sympathetic nervous outflow generates the low frequency arterial pressure fluctuations termed Mayer waves. Fluctuations in heart rate also have been suggested to relate closely to Mayer waves, but empiric models have not assessed the joint causative influences of hemt rate and sympathetic activity. Therefore, we constructed a model based simply upon the hemodynamic equation deriving from Ohm's Law. With this model, we determined time relations and relative contributions of heart rate and sympathetic activity to the genesis of arterial pressure Mayer waves. We assessed data from eight healthy young volunteers in the basal state and in a high sympathetic state known to produce concurrent increases in sympathetic nervous outflow and Mayer wave amplitude. We fit the Mayer waves (0.05-0.20 Hz) in mean arterial pressure by the weighted sum ofleading oscillations in heart rate and sympathetic nerve activity. This model of our data showed heart rate oscillations leading by 2-3.75 seconds were responsible for almost half of the variance in arterial pressure (basal R^2=0.435Ā±0.140, high sympathetic R^2=0.438Ā±0.180). Surprisingly, sympathetic activity (lead 0-5 seconds) contributed only modestly to the explained variance in Mayer waves during either sympathetic state (basal: āR^2=0.046Ā±0.026; heightened: āR^2=0.085Ā±0.036). Thus, it appears that heart rate oscillations contribute to Mayer waves in a simple linear fashion, whereas sympathetic fluctuations contribute little to Mayer waves in this way. Although these results do not exclude an important vascular sympathetic role, they do suggest that additional Ji1ctors, such as sympathetic transduction into vascular resistance, modulate its influence.Binda and Fred Shuman Foundation; National Institute on Aging (AG14376)
A Model for the Genesis of Arterial Pressure Mayer Waves from Heart Rate and Sympathetic Activity
Both theoretic models and cross-spectral analyses suggest that an oscillating sympathetic nervous outflow generates the low frequency arterial pressure fluctuations termed Mayer waves. Fluctuations in heart rate also have been suggested to relate closely to Mayer waves, but empiric models have not assessed the joint causative influences of hemt rate and sympathetic activity. Therefore, we constructed a model based simply upon the hemodynamic equation deriving from Ohm's Law. With this model, we determined time relations and relative contributions of heart rate and sympathetic activity to the genesis of arterial pressure Mayer waves. We assessed data from eight healthy young volunteers in the basal state and in a high sympathetic state known to produce concurrent increases in sympathetic nervous outflow and Mayer wave amplitude. We fit the Mayer waves (0.05-0.20 Hz) in mean arterial pressure by the weighted sum ofleading oscillations in heart rate and sympathetic nerve activity. This model of our data showed heart rate oscillations leading by 2-3.75 seconds were responsible for almost half of the variance in arterial pressure (basal R^2=0.435Ā±0.140, high sympathetic R^2=0.438Ā±0.180). Surprisingly, sympathetic activity (lead 0-5 seconds) contributed only modestly to the explained variance in Mayer waves during either sympathetic state (basal: āR^2=0.046Ā±0.026; heightened: āR^2=0.085Ā±0.036). Thus, it appears that heart rate oscillations contribute to Mayer waves in a simple linear fashion, whereas sympathetic fluctuations contribute little to Mayer waves in this way. Although these results do not exclude an important vascular sympathetic role, they do suggest that additional Ji1ctors, such as sympathetic transduction into vascular resistance, modulate its influence.Binda and Fred Shuman Foundation; National Institute on Aging (AG14376)
Tunable electronic anisotropy in single-crystal A2Cr3As3 (A = K, Rb) quasi-one-dimensional superconductors
Single crystals of A2Cr3As3 (A = K, Rb) were successfully grown using a
self-flux method and studied via structural, transport and thermodynamic
measurement techniques. The superconducting state properties between the two
species are similar, with critical temperatures of 6.1 K and 4.8 K in K2Cr3As3
and Rb2Cr3As3, respectively. However, the emergence of a strong normal state
electronic anisotropy in Rb2Cr3As3 suggests a unique electronic tuning
parameter is coupled to the inter-chain spacing in the A2Cr3As3 structure,
which increases with alkali metal ionic size while the one-dimensional
[(Cr3As3)^{2-}]_{\infty} chain structure itself remains essentially unchanged.
Together with dramatic enhancements in both conductivity and magnetoresistance
(MR), the appearance of a strong anisotropy in the MR of Rb2Cr3As3 is
consistent with the proposed quasi-one-dimensional character of band structure
and its evolution with alkali metal species in this new family of
superconductors.Comment: 6 pages, 8 figures; to appear in Phys. Rev.
Intrinsic Insulating Ground State in Transition Metal Dichalcogenide TiSe2
The transition metal dichalcogenide TiSe has received significant
research attention over the past four decades. Different studies have presented
ways to suppress the 200~K charge density wave transition, vary low temperature
resistivity by several orders of magnitude, and stabilize magnetism or
superconductivity. Here we give the results of a new synthesis technique
whereby samples were grown in a high pressure environment with up to 180~bar of
argon gas. Above 100~K, properties are nearly unchanged from previous reports,
but a hysteretic resistance region that begins around 80~K, accompanied by
insulating low temperature behavior, is distinct from anything previously
observed. An accompanying decrease in carrier concentration is seen in Hall
effect measurements, and photoemission data show a removal of an electron
pocket from the Fermi surface in an insulating sample. We conclude that high
inert gas pressure synthesis accesses an underlying nonmetallic ground state in
a material long speculated to be an excitonic insulator.Comment: 11 pages, 7 figure
CoAs: The line of 3d demarcation
Transition metal-pnictide compounds have received attention for their
tendency to combine magnetism and unconventional superconductivity. Binary CoAs
lies on the border of paramagnetism and the more complex behavior seen in
isostructural CrAs, MnP, FeAs, and FeP. Here we report the properties of CoAs
single crystals grown with two distinct techniques along with density
functional theory calculations of its electronic structure and magnetic ground
state. While all indications are that CoAs is paramagnetic, both experiment and
theory suggest proximity to a ferromagnetic instability. Quantum oscillations
are seen in torque measurements up to 31.5~T, and support the calculated
paramagnetic Fermiology.Comment: 10 pages, 6 figure
CoAs: The Line of 3d Demarcation
Transition metal-pnictide compounds have received attention for their tendency to combine magnetism and unconventional superconductivity. Binary CoAs lies on the border of paramagnetism and the more complex behavior seen in isostructural CrAs, MnP, FeAs, and FeP. Here we report the properties of CoAs single crystals grown with two distinct techniques along with density functional theory calculations of its electronic structure and magnetic ground state. While all indications are that CoAs is paramagnetic, both experiment and theory suggest proximity to a ferromagnetic instability. Quantum oscillations are seen in torque measurements up to 31.5 T and support the calculated paramagnetic Fermiology
Extreme magnetic field-boosted superconductivity
Applied magnetic fields underlie exotic quantum states, such as the
fractional quantum Hall effect and Bose-Einstein condensation of spin
excitations. Superconductivity, on the other hand, is inherently antagonistic
towards magnetic fields. Only in rare cases can these effects be mitigated over
limited fields, leading to reentrant superconductivity. Here, we report the
unprecedented coexistence of multiple high-field reentrant superconducting
phases in the spin-triplet superconductor UTe2. Strikingly, we observe
superconductivity in the highest magnetic field range identified for any
reentrant superconductor, beyond 65 T. These extreme properties reflect a new
kind of exotic superconductivity rooted in magnetic fluctuations and boosted by
a quantum dimensional crossover
Single crystal investigation of proposed type-II Weyl semimetal CeAlGe
We present details of materials synthesis, crystal structure, and anisotropic
magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl
semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in
noncentrosymmetric I4md space group, in line with predictions of
non-trivial topology. Magnetization, specific heat and electrical transport
measurements were used to confirm antiferromagnetic order below 5 K, with an
estimated magnon excitation gap of = 9.11 K from heat capacity and
hole-like carrier density of 1.44 10 cm from Hall effect
measurements. The easy magnetic axis is along the [100] crystallographic
direction, indicating that the moment lies in the tetragonal -plane
below 7 K. A spin-flop transition to less than 1 /Ce is observed to
occur below 30 kOe at 1.8 K in the () data. Small
magnetic fields of 3 kOe and 30 kOe are sufficient to suppress magnetic order
when applied along the - and -axes, respectively, resulting in
a complex phase diagram for and a simpler one for
Evolution of Structure and Superconductivity in Ba(NiāāāCoā)āAsā
The effects of Co substitution on Ba(Ni1-xCox)2As2 (0 ā¤ x ā¤ 0.251) single crystals grown out of Pb flux are investigated via transport, magnetic, and thermodynamic measurements. BaNi2As2 exhibits a first-order tetragonal to triclinic structural phase transition at Ts = 137 K upon cooling, and enters a superconducting phase below Tc = 0.7 K. The structural phase transition is sensitive to cobalt content and is suppressed completely by x ā„ 0.133. The superconducting critical temperature, Tc, increases continuously with x, reaching a maximum of Tc = 2.3 K at x = 0.083 and then decreases monotonically until superconductivity is no longer observable well into the tetragonal phase. In contrast to similar BaNi2As2 substitutional studies, which show an abrupt change in Tc at the triclinic-tetragonal boundary that extends far into the tetragonal phase, Ba(Ni1-xCox)2As2 exhibits a domelike phase diagram centered around the zero-temperature tetragonal-triclinic boundary. Together with an anomalously large heat capacity jump ĪCe/Ī³T ā¼ 2.2 near optimal doping, the smooth evolution of Tc in the Ba(Ni1-xCox)2As2 system suggests a mechanism for pairing enhancement other than phonon softening
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