The transition temperature Tc of iron-based superconductors has a dome-shaped dependence on chemical doping, and the superconductivity that develops underneath may obscure a potential quantum ...critical point (QCP) residing at absolute zero. With the aim of detecting signatures of this quantum criticality, Hashimoto et al. (p 1554; see the Perspective by Sachdev ) measured the penetration depth of the pnictide series BaFe2(As1-x Px )2 as a function of x. A sharp peak right around the point where Tc has a maximum (x = 0.30) was observed, implying that the superfluid density diminishes sharply where one would expect it to be the most robust. This unusual finding is interpreted as a sign of a QCP at x = 0.30. In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λL. In two-dimensional systems, this ratio n/m* (~1/λL2) determines the effective Fermi temperature, TF. We report a sharp peak in the x-dependence of λL at zero temperature in clean samples of BaFe2(As1-xPx)2 at the optimum composition x = 0.30, where the superconducting transition temperature Tc reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of Tc/TF at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality. PUBLICATION ABSTRACT
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Since the 1985 discovery of the phase transition at THO=17.5 K in the heavy-fermion metal URu2Si2, neither symmetry change in the crystal structure nor large magnetic moment that can account for the ...entropy change has been observed, which makes this hidden order enigmatic. Recent high-field experiments have suggested electronic nematicity that breaks fourfold rotational symmetry, but direct evidence has been lacking for its ground state in the absence of magnetic field. Here we report on the observation of lattice symmetry breaking from the fourfold tetragonal to twofold orthorhombic structure by high-resolution synchrotron X-ray diffraction measurements at zero field, which pins down the space symmetry of the order. Small orthorhombic symmetry-breaking distortion sets in at THO with a jump, uncovering the weakly first-order nature of the hidden-order transition. This distortion is observed only in ultrapure samples, implying a highly unusual coupling nature between the electronic nematicity and underlying lattice.
In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration ...depth, λ L . In two-dimensional systems, this ratio n/m* (~1/λ L ²) determines the effective Fermi temperature, T F . We report a sharp peak in the x-dependence of λ L at zero temperature in clean samples of BaFe₂(As 1-x P x )₂ at the optimum composition x = 0.30, where the superconducting transition temperature T c reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of T c /T F at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.
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1. Whole-cell voltage clamp and cell-attached patch-clamp techniques were applied to single atrial myocytes enzymatically
dissociated from adult guinea-pig hearts. 2. In whole-cell clamp conditions, ...external applications, of ATP activated the muscarinic
K+ (KACh) current, identified by its inward rectification, its reversal potential near the calculated K+ equilibrium potential
(EK) and its relaxation properties during step changes of whole-cell membrane potential. Theophylline, an antagonist for Pi-purinoceptors,
did not affect the action of ATP on the KACh current, indicating that the response was evoked through P2-purinoceptors. 3.
The concentration-response relationship for ATP was well described by a Hill equation with a half-maximal concentration of
1.84 microM and a Hill coefficient of 0.94. ATP (100 microM) produced a maximal increase of the KACh current to 10.92 microA
microF-1, which corresponds to 44.9 and 80.9% of the maximal increases evoked by ACh (10 microM) and adenosine (100 microM),
respectively. 4. The activation of KACh current gradually declined to a steady level despite the continuous presence of ATP
(desensitization). Recovery from the desensitization was relatively rapid with a half-time of approximately 1.5 min. 5. The
activation of KACh current by ATP was completely abolished by pre-incubating myocytes with pertussis toxin (PTX, 5 micrograms
ml-1), indicating that P2-purinoceptors are coupled to PTX-sensitive G proteins to activate the KACh channel. 6. In the cell-attached
patch recording, ATP (5 microM) applied to the pipette solution enhanced the activity of a channel with single-channel conductance
of 52.7 +/- 0.9 pS (mean +/- S.E.M., n = 10), reversal potential near EK and mean open time of 1.1 +/- 0.1 ms. These conductance
and kinetic properties are identical to those of the KACh channel in the heart. In contrast, ATP applied to the bath solution
did not significantly affect the basal activity of KACh channel openings. These observations suggest that the mechanism coupling
the P2-purinoceptor to the activation of the KACh channel involves membrane-delimited component(s) rather than soluble second
messenger(s). 7. These results strongly suggest a direct coupling of the P2-purinoceptor to the KACh channel through PTX-sensitive
G proteins, analogous to the coupling mechanism of the muscarinic ACh receptor and Pi-purinoceptor to this channel.
We examined influences of a blocker (glibenclamide) and an opener (nicorandil) of the ATP-sensitive potassium (KATP) channel on extracellular K concentration ( K+e), as well as the myocardial ...function and metabolites during global ischemia and reperfusion in Langendorff-perfused rat heart preparation. In control hearts, K+e began to rise 20 s after the onset of ischemia up to an initial peak (8.3 +/- 0.3 mM) at 2.5 +/- 0.7 min, then fell to 6.0 +/- 0.8 mM after 8.2 +/- 0.7 min, and then rose progressively to 14.6 +/- 0.8 mM at the end of 30 min of ischemia. Glibenclamide (50 microM) reduced the initial peak of K+e to 7.2 +/- 0.3 mM (P less than 0.01), and nicorandil (200 microM) increased it to 9.4 +/- 0.6 mM (P less than 0.01). There were no significant differences in K+e values among all groups at the end of ischemia. During ischemia, nicorandil decreased the time to mechanical arrest from 1.9 +/- 0.1 min to 1.5 +/- 0.1 min, whereas it was increased by glibenclamide to 2.7 +/- 0.4 min. In control hearts, the time to onset of ischemic contracture was 14.7 +/- 1.8 min. Nicorandil delayed onset of contracture and glibenclamide accelerated it. Thus we have confirmed that some part of the early increase in K+e during ischemia is attributable to K+ efflux through the KATP channel in our model, and opening of the KATP channel may contribute to a rapid reduction of the contractility of the ischemic myocardium that subsequently protects the myocardium against further ischemic injury.
1. We studied the effects of P2-purinoceptor stimulation on the delayed rectifier K+ current (IK) in guinea-pig atrial myocytes
using a whole-cell voltage-clamp technique. 2. External application of ...ATP increased IK, evoked by a 500 ms depolarizing pulse
from a holding potential of -40 mV, under conditions in which the L-type Ca2+ channel was blocked; the effect was dose dependent
with a half-maximal concentration (K1/2) of 0.95 microM. ATP (50 microM) produced a maximal increase of IK of about a factor
of 2. 3. External ADP also enhanced IK in a dose-dependent manner with a K1/2 of 3.65 microM, whereas adenosine (100 microM)
failed to evoke this response. Theophylline (500 microM), a blocker of the Pi-purinoceptor, did not antagonize the stimulating
action of ATP on IK. These results indicate that IK was enhanced via P2-purinoceptors. 4. External ATP or ADP did not produce
a significant change in the current kinetics of IK. 5. Pre-incubation of the atrial myocytes with pertussis toxin (PTX, 5
micrograms ml-1) did not affect the stimulating action of ATP on IK, indicating that PTX-sensitive G proteins did not mediate
the ATP action. 6. The enhancement of IK by ATP developed slowly; the effects usually reached a maximum approximately 30-60
s after the application of ATP. This suggests the involvement of a diffusible cytosolic second messenger(s) in the response.
ATP could further increase IK after maximal enhancement by isoprenaline (0.5-1.0 microM), suggesting that the intermediate
steps were independent of cyclic AMP-dependent protein kinase (protein kinase A). 7. Potentiation of IK by ATP was not attenuated
by either (i) pretreatment of the cells with 5 microM 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine dihydrochloride (H-7)
or (ii) intracellular perfusion of 20 mM 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), suggesting that
protein kinase C and intracellular Ca2+ did not mediate the response. 8. It is concluded that the activation of P2-purinoceptors
increases IK through intracellular mechanisms independent of protein kinase A, protein kinase C or intracellular free Ca2+
in guinea-pig atrial myocytes.
A Spike Inside the Dome
The transition temperature
T
c
of iron-based superconductors has a dome-shaped dependence on chemical doping, and the superconductivity that develops underneath may obscure a ...potential quantum critical point (QCP) residing at absolute zero. With the aim of detecting signatures of this quantum criticality,
Hashimoto
et al.
(p
1554
; see the Perspective by
Sachdev
) measured the penetration depth of the pnictide series BaFe
2
(As
1−
x
P
x
)
2
as a function of
x.
A sharp peak right around the point where
T
c
has a maximum (
x
= 0.30) was observed, implying that the superfluid density diminishes sharply where one would expect it to be the most robust. This unusual finding is interpreted as a sign of a QCP at
x
= 0.30.
A quantum critical point may be lurking inside the superconducting dome of a pnictide series.
In a superconductor, the ratio of the carrier density,
n
, to its effective mass,
m
*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λ
L
. In two-dimensional systems, this ratio
n
/
m
* (~1/λ
L
2
) determines the effective Fermi temperature,
T
F
. We report a sharp peak in the
x
-dependence of λ
L
at zero temperature in clean samples of BaFe
2
(As
1
–x
P
x
)
2
at the optimum composition
x
= 0.30, where the superconducting transition temperature
T
c
reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of
T
c
/
T
F
at
x
= 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.
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