673 research outputs found
High pressure transport study of non-Fermi liquid behaviour in U2Pt2In and U3Ni3Sn4
The strongly correlated metals U2Pt2In and U3Ni3Sn4 show pronounced non-Fermi
liquid (NFL) phenomena at ambient pressure. Here we review single-crystal
electrical resistivity measurements under pressure (p <= 1.8 GPa) conducted to
investigate the stability of the NFL phase. For tetragonal U2Pt2In (I||a) we
observe a rapid recovery of the Fermi-liquid T^2-term with pressure. The
Fermi-liquid temperature varies as T_FL ~ p-p_c, where p_c= 0 is a critical
pressure. The analysis within the magnetotransport theory of Rosch provides
evidence for the location of U2Pt2In at a zero pressure antiferromagnetic
quantum critical point (QCP). In the case of cubic U3Ni3Sn4 we find T_FL ~
(p-p_c)^1/2. The analysis provides evidence for an antiferromagnetic QCP in
U3Ni3Sn4 at a negative pressure p_c= -0.04+-0.04 GPa.Comment: 6 pages (4 figures); to appear in Proc. of Int. Conf. PPHMF-IV (20-25
Oct. 2001, Santa Fe
Resistivity of non-Fermi liquid U2Pt2In under pressure
Non-Fermi liquid behaviour in single-crystalline U2Pt2In has been studied by
means of resistivity experiments (I||c) under hydrostatic pressure (P<1.5 GPa).
At ambient pressure the resistivity rho(T) follows a power law rho~T^alpha with
alpha~0.5. Upon applying pressure alpha increases. For P>1 GPa a minimum
develops in rho(T). A study of the field dependence of the minimum confirms its
magnetic origin. The ratio c/a is proposed as the effective control parameter,
rather than the unit cell volume.Comment: 5 pages (incl. 2 figures), submitted to SCES'99, Nagan
Sign change of the Grueneisen parameter and magnetocaloric effect near quantum critical points
We consider the Grueneisen parameter and the magnetocaloric effect near a
pressure and magnetic field controlled quantum critical point, respectively.
Generically, the Grueneisen parameter (and the thermal expansion) displays a
characteristic sign change close to the quantum-critical point signaling an
accumulation of entropy. If the quantum critical point is the endpoint of a
line of finite temperature phase transitions, T_c \propto (p_c-p)^Psi, then we
obtain for p<p_c, (1) a characteristic increase \Gamma \sim T^{-1/(\nu z)} of
the Grueneisen parameter Gamma for T>T_c, (2) a sign change in the Ginzburg
regime of the classical transition, (3) possibly a peak at T_c, (4) a second
increase Gamma \sim -T^{-1/(nu z)} below T_c for systems above the upper
critical dimension and (5) a saturation of Gamma \propto 1/(p_c-p). We argue
that due to the characteristic divergencies and sign changes the thermal
expansion, the Grueneisen parameter and magnetocaloric effect are excellent
tools to detect and identify putative quantum critical points.Comment: 10 pages, 7 figures; final version, only minor change
Wavelets: a powerful tool for studying rotation, activity, and pulsation in Kepler and CoRoT stellar light curves
Aims. The wavelet transform has been used as a powerful tool for treating
several problems in astrophysics. In this work, we show that the time-frequency
analysis of stellar light curves using the wavelet transform is a practical
tool for identifying rotation, magnetic activity, and pulsation signatures. We
present the wavelet spectral composition and multiscale variations of the time
series for four classes of stars: targets dominated by magnetic activity, stars
with transiting planets, those with binary transits, and pulsating stars.
Methods. We applied the Morlet wavelet (6th order), which offers high time and
frequency resolution. By applying the wavelet transform to the signal, we
obtain the wavelet local and global power spectra. The first is interpreted as
energy distribution of the signal in time-frequency space, and the second is
obtained by time integration of the local map. Results. Since the wavelet
transform is a useful mathematical tool for nonstationary signals, this
technique applied to Kepler and CoRoT light curves allows us to clearly
identify particular signatures for different phenomena. In particular, patterns
were identified for the temporal evolution of the rotation period and other
periodicity due to active regions affecting these light curves. In addition, a
beat-pattern signature in the local wavelet map of pulsating stars over the
entire time span was also detected.Comment: Accepted for publication on A&
High-pressure study of the non-Fermi liquid material U_2Pt_2In
The effect of hydrostatic pressure (p<= 1.8 GPa) on the non-Fermi liquid
state of U_2Pt_2In is investigated by electrical resistivity measurements in
the temperature interval 0.3-300 K. The experiments were carried out on
single-crystals with the current along (I||c) and perpendicular (I||a) to the
tetragonal axis. The pressure effect is strongly current-direction dependent.
For I||a we observe a rapid recovery of the Fermi-liquid T^2-term with
pressure. The low-temperature resistivity can be analysed satisfactorily within
the magnetotransport theory of Rosch, which provides strong evidence for the
location of U_2Pt_2In at an antiferromagnetic quantum critical point. For I||c
the resistivity increases under pressure, indicating the enhancement of an
additional scattering mechanism. In addition, we have measured the pressure
dependence of the antiferromagnetic ordering temperature (T_N= 37.6 K) of the
related compound U_2Pd_2In. A simple Doniach-type diagram for U_2Pt_2In and
U_2Pd_2In under pressure is presented.Comment: 21 pages (including 5 figures); pdf forma
From Metabolism to Ecology:Cross-Feeding Interactions Shape the Balance between Polymicrobial Conflict and Mutualism
Polymicrobial interactions are widespread in nature, and play a major role in maintaining human health and ecosystems. Whenever one organism uses metabolites produced by another organism as energy or nutrient sources, this is called cross-feeding. The ecological outcomes of cross-feeding interactions are poorly understood and potentially diverse: mutualism, competition, exploitation or commensalism. A major reason for this uncertainty is the lack of theoretical approaches linking microbial metabolism to microbial ecology. To address this issue, we explore the dynamics of a one-way interspecific cross-feeding interaction, in which food can be traded for a service (detoxification). Our results show that diverse ecological interactions (competition, mutualism, exploitation) can emerge from this simple cross-feeding interaction, and can be predicted by the metabolic, demographic and environmental parameters that govern the balance of the costs and benefits of association. In particular, our model predicts stronger mutualism for intermediate by-product toxicity because the resource-service exchange is constrained to the service being neither too vital (high toxicity impairs resource provision) nor dispensable (low toxicity reduces need for service). These results support the idea that bridging microbial ecology and metabolism is a critical step towards a better understanding of the factors governing the emergence and dynamics of polymicrobial interactions
Magnetic quantum critical point and superconductivity in UPt3 doped with Pd
Transverse-field muon spin relaxation measurements have been carried out on
the heavy-fermion superconductor UPt3 doped with small amounts of Pd. We find
that the critical Pd concentration for the emergence of the large-moment
antiferromagnetic phase is ~0.6 at.%Pd. At the same Pd content,
superconductivity is completely suppressed. The existence of a magnetic quantum
critical point in the phase diagram, which coincides with the critical point
for superconductivity, provides evidence for ferromagnetic spin-fluctuation
mediated odd-parity superconductivity, which competes with antiferromagnetic
order.Comment: 4 pages (includes 3 figures); postscript fil
Magnetic quantum critical point and superconductivity in UPt3 doped with Pd
Transverse-field muon spin relaxation measurements have been carried out on
the heavy-fermion superconductor UPt3 doped with small amounts of Pd. We find
that the critical Pd concentration for the emergence of the large-moment
antiferromagnetic phase is ~0.6 at.%Pd. At the same Pd content,
superconductivity is completely suppressed. The existence of a magnetic quantum
critical point in the phase diagram, which coincides with the critical point
for superconductivity, provides evidence for ferromagnetic spin-fluctuation
mediated odd-parity superconductivity, which competes with antiferromagnetic
order.Comment: 4 pages (includes 3 figures); postscript fil
Fabrication and electrical transport properties of embedded graphite microwires in a diamond matrix
Micrometer width and nanometer thick wires with different shapes were
produced \approx 3~\upmum below the surface of a diamond crystal using a
microbeam of He ions with 1.8~MeV energy. Initial samples are amorphous and
after annealing at ~K, the wires crystallized into a
graphite-like structures, according to confocal Raman spectroscopy
measurements. The electrical resistivity at room temperature is only one order
of magnitude larger than the in-plane resistivity of highly oriented pyrolytic
bulk graphite and shows a small resistivity ratio(). A small negative magnetoresistance below ~K was
measured and can be well understood taking spin-dependent scattering processes
into account. The used method provides the means to design and produce
millimeter to micrometer sized conducting circuits with arbitrary shape
embedded in a diamond matrix.Comment: 12 pages, 5 figures, to be published in Journal of Physics D: Applied
Physics (Feb. 2017
Fabrication of a Horizontal and a Vertical Large Surface Area Nanogap Electrochemical Sensor
Nanogap sensors have a wide range of applications as they can provide accurate direct detection of biomolecules through impedimetric or amperometric signals. Signal response from nanogap sensors is dependent on both the electrode spacing and surface area. However, creating large surface area nanogap sensors presents several challenges during fabrication. We show two different approaches to achieve both horizontal and vertical coplanar nanogap geometries. In the first method we use electron-beam lithography (EBL) to pattern an 11 mm long serpentine nanogap (215 nm) between two electrodes. For the second method we use inductively-coupled plasma (ICP) reactive ion etching (RIE) to create a channel in a silicon substrate, optically pattern a buried 1.0 mm × 1.5 mm electrode before anodically bonding a second identical electrode, patterned on glass, directly above. The devices have a wide range of applicability in different sensing techniques with the large area nanogaps presenting advantages over other devices of the same family. As a case study we explore the detection of peptide nucleic acid (PNA)−DNA binding events using dielectric spectroscopy with the horizontal coplanar device
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