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Temperature-pressure phase diagram of the Kitaev hyperhoneycomb iridate β-Li_{2}IrO_{3} is explored using magnetization, thermal expansion, magnetostriction, and muon spin rotation measurements, as well as single-crystal x-ray diffraction under pressure and ab initio calculations. The Neel temperature of β-Li_{2}IrO_{3} increases with the slope of 0.9 K/GPa upon initial compression, but the reduction in the polarization field H_{c} reflects a growing instability of the incommensurate order. At 1.4 GPa, the ordered state breaks down upon a first-order transition, giving way to a new ground state marked by the coexistence of dynamically correlated and frozen spins. This partial freezing in the absence of any conspicuous structural defects may indicate the classical nature of the resulting pressure-induced spin liquid, an observation paralleled to the increase in the nearest-neighbor off-diagonal exchange Γ under pressure.

In the heavy-fermion metal CePdAl long-range antiferromagnetic order coexists with geometric frustration of one third of the Ce moments. At low temperatures the Kondo effect tends to screen the frustrated moments. We use magnetic fields $B$ to suppress the Kondo screening and study the magnetic phase diagram and the evolution of the entropy with $B$ employing thermodynamic probes. We estimate the frustration by introducing a definition of the frustration parameter based on the enhanced entropy, a fundamental feature of frustrated systems. In the field range where the Kondo screening is suppressed the liberated moments tend to maximize the magnetic entropy and strongly enhance the frustration. Based on our experiments, this field range may be a promising candidate to search for a quantum spin liquid., Comment: 5+2 pages with 4 figures

Muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) experiments on single crystals of the structurally perfect triangular antiferromagnet ${\mathrm{YbMgGaO}}_{4}$ indicate the absence of both static long-range magnetic order and spin freezing down to 0.048 K in a zero field. Below 0.4 K, the ${\ensuremath{\mu}}^{+}$ spin relaxation rates, which are proportional to the dynamic correlation function of the ${\mathrm{Yb}}^{3+}$ spins, exhibit temperature-independent plateaus. All these $\ensuremath{\mu}\mathrm{SR}$ results unequivocally support the formation of a gapless U(1) quantum spin liquid ground state in the triangular antiferromagnet ${\mathrm{YbMgGaO}}_{4}$.

Muon spin relaxation (μSR) experiments on single crystals of the structurally perfect triangular antiferromagnet YbMgGaO_{4} indicate the absence of both static long-range magnetic order and spin freezing down to 0.048 K in a zero field. Below 0.4 K, the μ^{+} spin relaxation rates, which are proportional to the dynamic correlation function of the Yb^{3+} spins, exhibit temperature-independent plateaus. All these μSR results unequivocally support the formation of a gapless U(1) quantum spin liquid ground state in the triangular antiferromagnet YbMgGaO_{4}.

Bilayer strontium ruthenate Sr_{3}Ru_{2}O_{7} displays pronounced non-Fermi liquid behavior at magnetic fields around 8 T, applied perpendicular to the ruthenate planes, which previously has been associated with an itinerant metamagnetic quantum critical end point (QCEP). We focus on the magnetic Gruneisen parameter Γ_{H}, which is the most direct probe to characterize field-induced quantum criticality. We confirm quantum critical scaling due to a putative two-dimensional QCEP near 7.845(5) T, which is masked by two ordered phases A and B, identified previously by neutron scattering. In addition, we find evidence for a QCEP at 7.53(2) T and determine the quantum critical regimes of both instabilities and the effect of their superposition.

Our comprehensive study on ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ reveals a dramatic reduction of magnetic detwinning fields compared to other $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A=\mathrm{Ba}$, Sr, Ca) iron pnictides by indirect magnetoelastic coupling of the ${\mathrm{Eu}}^{2+}$ ions. We find that only $\ensuremath{\sim}0.1\text{ }\text{ }\mathrm{T}$ are sufficient for persistent detwinning below the local ${\mathrm{Eu}}^{2+}$ ordering; above ${T}_{\mathrm{Eu}}=19\text{ }\text{ }\mathrm{K}$, higher fields are necessary. Even after the field is switched off, a significant imbalance of twin domains remains constant up to the structural and electronic phase transition (190 K). This persistent detwinning provides the unique possibility to study the low temperature electronic in-plane anisotropy of iron pnictides without applying any symmetry-breaking external force.

We have prepared high-quality epitaxial thin films of CaRuO$_3$ with residual resistivity ratios up to 55. Shubnikov-de Haas oscillations in the magnetoresistance and a $T^2$ temperature dependence in the electrical resistivity only below 1.5 K, whose coefficient is substantially suppressed in large magnetic fields, establish CaRuO$_3$ as a Fermi liquid (FL) with anomalously low coherence scale. Non-Fermi liquid (NFL) $T^{3/2}$ dependence is found between 2 and 25 K. The high sample quality allows access to the intrinsic electronic properties via THz spectroscopy. For frequencies below 0.6 THz, the conductivity is Drude-like and can be modeled by FL concepts, while for higher frequencies non-Drude behavior, inconsistent with FL predictions, is found. This establishes CaRuO$_3$ as a prime example of optical NFL behavior in the THz range.

Inelastic neutron scattering measurements of magnetic excitations in the charge-ordered state of ${\mathrm{Yb}}_{4}{\mathrm{As}}_{3}$ have been performed under magnetic field up to about 6 T. By applying a magnetic field, the spectrum at the one-dimensional wave vector $q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ [ $\ensuremath{\pi}/d$] changes drastically from a broad one corresponding to the spinon excitation continuum of the one-dimensional $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\frac{1}{2}$ spin system to a sharp one at a finite energy, indicating the opening of an energy gap in the system. The magnetic field dependence of the gap is well fitted by the power law ${H}^{2/3}$. The experimental result gives strong evidence for the existence of a staggered field alternating along ${\mathrm{Yb}}^{3+}$ chains induced by the Dzyaloshinsky-Moriya interaction.

We report the first observation of non-Fermi-liquid (NFL) effects in a clean Yb compound at ambient pressure and zero magnetic field. The electrical resistivity and the specific-heat coefficient of high-quality single crystals of YbRh(2)Si(2) present a linear and a logarithmic temperature dependence, respectively, in more than a decade in temperature. We ascribe this NFL behavior to the presence of (presumably) quasi-2D antiferromagnetic spin fluctuations related to a very weak magnetic phase transition at T(N) approximately 65 mK. Application of hydrostatic pressure induces anomalies in the electrical resistivity, indicating the stabilization of magnetic order.

Pronounced non-Fermi-liquid (NFL) effects in thermodynamic $(C,\ensuremath{\alpha})$ and transport $(\ensuremath{\rho})$ properties at ambient pressure $(p)$ and zero field are reported for high-purity samples of the undoped heavy-fermion compound ${\mathrm{CeNi}}_{2}{\mathrm{Ge}}_{2}$. The origin of the NFL effects is not yet clear. Novel phase transitions are observed at both finite and ambient pressure. Under $p\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.7\mathrm{GPa}$, a novel, perhaps spin-density-wave-type, phase transition occurs at ${T}_{m}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.9\mathrm{K}$. At $p\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$, incipient superconductivity below $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}100\mathrm{mK}$ is found in the two samples with lowest residual resistivities, ${\ensuremath{\rho}}_{0}l0.45\ensuremath{\mu}\ensuremath{\Omega}\mathrm{cm}$.

We report resistivity, $\ensuremath{\rho}(T)$, and specific-heat, $C(T)$, results on near stoichiometric $\mathrm{CeCu}{}_{2}{\mathrm{Si}}_{2}$ samples, in the vicinity of a quantum critical point (QCP). The latter is defined by ${T}_{A}\ensuremath{\rightarrow}0$, where ${T}_{A}\ensuremath{\lesssim}0.8\mathrm{K}$ marks the transition into a spin-density--wave-type ``phase $A$'' which competes with heavy-fermion superconductivity below ${T}_{c}\ensuremath{\approx}0.65\mathrm{K}$. Upon approaching the QCP, $\ensuremath{\rho}(T)$ and $C(T)$ behave very disparately, suggesting a breakup of the heavy quasiparticles. Very surprising observations are being made for samples with ${T}_{A}g0$ also.

We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity, and Hall effect in the heavy fermion metal ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}$. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands.

By systematic investigations of the magnetic, transport, and thermodynamic properties of single crystals of ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{)}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}1$), we explore the complex interplay of superconductivity and ${\mathrm{Eu}}^{2+}$ magnetism. Below 30 K, two magnetic transitions are observed for all P substituted crystals, suggesting a revision of the phase diagram. In addition to the canted $A$-type antiferromagnetic order of ${\mathrm{Eu}}^{2+}$ at $\ensuremath{\sim}20\text{ }\text{ }\mathrm{K}$, a spin glass transition is discovered at lower temperatures. Most remarkably, the reentrant spin glass state of ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{)}_{2}$ coexists with superconductivity around $x\ensuremath{\approx}0.2$.

The electronic structure of the honeycomb lattice iridates ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ and ${\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split $d\mathrm{\text{\ensuremath{-}}}d$ excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to noncubic crystal fields, derived from the splitting of ${j}_{\mathrm{eff}}=3/2$ states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of ${j}_{\mathrm{eff}}$ physics in ${A}_{2}{\mathrm{IrO}}_{3}$. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ and ${\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ can be described as spin-orbit Mott insulators, similar to the square lattice iridate ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$.

We report measurements of the electrical and thermal transport on the hexagonal heavy-fermion metal YbAgGe for temperatures $T\geq$ 40 mK and in magnetic fields $H\parallel ab$ up to 14 T. This distorted Kagome-lattice system displays a series of magnetic states and a quantum critical point at $H_c =4.5$ T. The Lorenz ratio $L(T)/L_0$ displays a marked reduction only close to $H_c$. A $T$-linear contribution below 120 mK, present at all different fields, allows to extrapolate the Lorenz ratio towards T=0. At the critical field this yields $L/L_0=0.92\pm 0.03$, suggesting a violation of the Wiedemann-Franz law due to strong inelastic scattering., 7 pages including supplemental material

We study ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ by angle-resolved photoemission spectroscopy, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir $5d$-${t}_{2g}$ manifold highlights the importance of structural distortions and spin-orbit (SO) coupling in driving the system closer to a Mott transition. We detect an insulating gap ${\ensuremath{\Delta}}_{\mathrm{gap}}\ensuremath{\simeq}340\text{ }\text{ }\mathrm{meV}$ which, at variance with a Slater-type description, is already open at 300 K and does not show significant temperature dependence even across ${T}_{N}\ensuremath{\simeq}15\text{ }\text{ }\mathrm{K}$. An LDA analysis with the inclusion of SO and Coulomb repulsion $U$ reveals that, while the prodromes of an underlying insulating state are already found in $\mathrm{LDA}+\mathrm{SO}$, the correct gap magnitude can only be reproduced by $\mathrm{LDA}+\mathrm{SO}+U$, with $U=3\text{ }\text{ }\mathrm{eV}$. This establishes ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ as a novel type of Mott-like correlated insulator in which Coulomb and relativistic effects have to be treated on an equal footing.

The heavy-fermion superconductor CeCoIn$_5$ displays an additional transition within its superconducting (SC) state, whose nature is characterized by high-precision studies of the isothermal field dependence of the entropy, derived from combined specific heat and magnetocaloric effect measurements at temperatures $T\geq 100$ mK and fields $H\leq 12$ T aligned parallel, perpendicular and $18^\circ$ off the tetragonal [100] direction. For any of these conditions, we do not observe an additional entropy contribution upon tuning at constant temperature by magnetic field from the homogeneous SC into the presumed Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) SC state. By contrast, for $H\parallel [100]$ a negative isothermal entropy contribution, compatible with spin-density-wave (SDW) ordering, is found. Our data exclude the formation of a FFLO state in CeCoIn$_5$ for out-of-plane field directions, where no SDW order exists.

We report inelastic neutron scattering measurements on ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below ${T}_{N}=15.3\text{ }\text{ }\mathrm{K}$ is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90\ifmmode^\circ\else\textdegree\fi{} Ir-O-Ir bonds required for dominant Kitaev exchange.

Combining thermodynamic measurements with theoretical density functional and thermodynamic calculations we demonstrate that the honeycomb lattice iridates A2IrO3 (A = Na, Li) are magnetically ordered Mott insulators where the magnetism of the effective spin-orbital S = 1/2 moments can be captured by a Heisenberg-Kitaev (HK) model with Heisenberg interactions beyond nearest-neighbor exchange. Experimentally, we observe an increase of the Curie-Weiss temperature from \theta = -125 K for Na2IrO3 to \theta = -33 K for Li2IrO3, while the antiferromagnetic ordering temperature remains roughly the same T_N = 15 K for both materials. Using finite-temperature functional renormalization group calculations we show that this evolution of \theta, T_N, the frustration parameter f = \theta/T_N, and the zig-zag magnetic ordering structure suggested for both materials by density functional theory can be captured within this extended HK model. Combining our experimental and theoretical results, we estimate that Na2IrO3 is deep in the magnetically ordered regime of the HK model (\alpha \approx 0.25), while Li2IrO3 appears to be close to a spin-liquid regime (0.6 < \alpha < 0.7)., Comment: Version accepted for publication in PRL. Additional DFT and thermodynamic calculations have been included. 6 pages of supplementary material included

Employing the momentum sensitivity of time- and angle-resolved photoemission spectroscopy we demonstrate the analysis of ultrafast single- and many-particle dynamics in antiferromagnetic ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$. Their separation is based on a temperature-dependent difference of photoexcited hole and electron relaxation times probing the single-particle band and the spin density wave gap, respectively. Reformation of the magnetic order occurs at 800 fs, which is 4 times slower compared to electron-phonon equilibration due to a smaller spin-dependent relaxation phase space.

The electronic nematic phase of Sr₃Ru₂O₇ is investigated by high-resolution in-plane thermal expansion measurements in magnetic fields close to 8 T applied at various angles Θ off the c axis. At Θ < 10° we observe a very small (10⁻⁷) lattice distortion which breaks the fourfold in-plane symmetry, resulting in nematic domains with interchanged a and b axis. At Θ ≳ 10° the domains are almost fully aligned and thermal expansion indicates an area-preserving lattice distortion of order 2 × 10⁻⁶ which is likely related to orbital ordering. Since the system is located in the immediate vicinity of a metamagnetic quantum critical end point, the results represent the first observation of a structural relaxation driven by quantum criticality.

We report measurements of quantum oscillations detected in the putative nematic phase of ${\mathrm{Sr}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$. Improvements in sample purity enabled the resolution of small amplitude de Haas--van Alphen (dHvA) oscillations between two first order metamagnetic transitions delimiting the phase. Two distinct frequencies were observed, whose amplitudes follow the normal Lifshitz-Kosevich profile. Variations of the dHvA frequencies are explained in terms of a chemical potential shift produced by reaching a peak in the density of states, and an anomalous field dependence of the oscillatory amplitude provides information on domains.

We report a comprehensive study of CeIn$_{3-x}$Sn$_x$ $(0.55 \leq x \leq 0.8)$ single crystals close to the antiferromagnetic (AF) quantum critical point (QCP) at $x_c\approx 0.67$ by means of the low-temperature thermal expansion and Gr\"uneisen parameter. This system represents the first example for a {\it cubic} heavy fermion (HF) in which $T_{\rm N}$ can be suppressed {\it continuously} down to T=0. A characteristic sign change of the Gr\"uneisen parameter between the AF and paramagnetic state indicates the accumulation of entropy close to the QCP. The observed quantum critical behavior is compatible with the predictions of the itinerant theory for three-dimensional critical spinfluctuations. This has important implications for the role of the dimensionality in HF QCPs., Comment: Physical Review Letters, to be published

We report low-temperature thermal expansion measurements on the bilayer ruthenate ${\mathrm{Sr}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ as a function of magnetic field applied perpendicular to the ruthenium-oxide planes. The field dependence of the $c$-axis expansion coefficient indicates the accumulation of entropy close to 8 T, related to an underlying quantum critical point. The latter is masked by two first-order metamagnetic transitions which bound a regime of enhanced entropy. Outside this region the singular thermal expansion behavior is compatible with the predictions of the itinerant theory for a two-dimensional metamagnetic quantum critical end point.

The bulk magnetic susceptibility chi(T,B) of YbRh(2)(Si(0.95)Ge(0.05))(2) has been investigated close to the field-induced quantum critical point at B(c) = 0.027 T. For B < or= 0.05 T a Curie-Weiss law with a negative Weiss temperature is observed at temperatures below 0.3 K. Outside this region, the susceptibility indicates ferromagnetic quantum critical fluctuations, chi(T) proportional, variantT-0.6 above 0.3 K. At low temperatures the Pauli susceptibility follows chi(0) proportional, variant(B-B(c))(-0.6) and scales with the coefficient of the T(2) term in the electrical resistivity. The Sommerfeld-Wilson ratio is highly enhanced and increases up to 30 close to the critical field.

We report dc-magnetization measurements on YbRh2Si2 at temperatures down to 0.04 K, magnetic fields B< or =11.5 T, and under hydrostatic pressure P< or =1.3 GPa. At ambient pressure a kink at B* =9.9 T indicates a new type of field-induced transition from an itinerant to a localized 4f state. This transition is different from the metamagnetic transition observed in other heavy-fermion compounds, as here ferromagnetic rather than antiferromagnetic correlations dominate below B*. Hydrostatic pressure experiments reveal a clear correspondence of B* to the characteristic spin fluctuation temperature determined from specific heat.

The heavy-fermion system ${\mathrm{C}\mathrm{e}\mathrm{C}\mathrm{u}}_{6\ensuremath{-}x}{\mathrm{A}\mathrm{g}}_{x}$ is studied at its antiferromagnetic quantum critical point, ${x}_{c}=0.2$, by low-temperature ($T\ensuremath{\ge}50\text{ }\text{ }\mathrm{m}\mathrm{K}$) specific heat, $C(T)$, and volume thermal expansion, $\ensuremath{\beta}(T)$, measurements. Whereas $C/T\ensuremath{\propto}\mathrm{log} ({T}_{0}/T)$ would be compatible with the predictions of the itinerant spin-density-wave (SDW) theory for two-dimensional critical spin fluctuations, $\ensuremath{\beta}(T)/T$ and the Gr\"uneisen ratio, $\ensuremath{\Gamma}(T)\ensuremath{\propto}\ensuremath{\beta}/C$, diverge much weaker than expected, in strong contrast to this model. Both $C$ and $\ensuremath{\beta}$, plotted as a function of the reduced temperature $t=T/{T}_{0}$ with ${T}_{0}=4.6\text{ }\text{ }\mathrm{K}$, are similar to what was observed for ${\mathrm{Y}\mathrm{b}\mathrm{R}\mathrm{h}}_{2}({\mathrm{S}\mathrm{i}}_{0.95}{\mathrm{G}\mathrm{e}}_{0.05}{)}_{2}$ (${T}_{0}=23.3\text{ }\text{ }\mathrm{K}$), indicating a striking discrepancy to the SDW prediction in both systems.

We report low-temperature thermal expansion measurements on the tetragonal heavy-fermion superconductors $\mathrm{C}\mathrm{e}M{\mathrm{I}\mathrm{n}}_{5}$ ($M=\mathrm{I}\mathrm{r},\mathrm{C}\mathrm{o}$) in magnetic fields up to 8 T which allow for the analysis of the uniaxial pressure effects on both normal-state and superconducting properties. Our study reveals that ${T}_{c}$ is strongly affected by at least two factors, the lattice anisotropy and the $4f$-conduction-electron hybridization strength which is most sensitive to $c$-axis lattice distortions. Non-Fermi-liquid behavior caused by quantum-critical fluctuations is observed for both systems, most pronounced for ${\mathrm{C}\mathrm{e}\mathrm{C}\mathrm{o}\mathrm{I}\mathrm{n}}_{5}$.

The superconducting phase transition in heavy fermion CeCoIn5 (T(c)=2.3 K in zero field) becomes first order when the magnetic field H parallel [001] is greater than 4.7 T, and the transition temperature is below T0 approximately 0.31T(c). The change from second order at lower fields is reflected in strong sharpening of both specific heat and thermal expansion anomalies associated with the phase transition, a strong magnetocaloric effect, and a steplike change in the sample volume. This effect is due to Pauli limiting in a type-II superconductor, and was predicted theoretically in the mid-1960s.

We report low-temperature calorimetric, magnetic, and resistivity measurements on the antiferromagnetic (AF) heavy-fermion metal ${\mathrm{Y}\mathrm{b}\mathrm{R}\mathrm{h}}_{2}{\mathrm{S}\mathrm{i}}_{2}$ (${T}_{N}=70\text{ }\mathrm{m}\mathrm{K}$) as a function of magnetic field $B$. While for fields exceeding the critical value ${B}_{c0}$ at which ${T}_{N}\ensuremath{\rightarrow}0$ the low-temperature resistivity shows an $A{T}^{2}$ dependence, a $1/(B\ensuremath{-}{B}_{c0})$ divergence of $A(B)$ upon reducing $B$ to ${B}_{c0}$ suggests singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. The observations are interpreted in terms of a new type of quantum critical point separating a weakly AF ordered from a weakly polarized heavy Landau-Fermi liquid state.

Dilatometric and magnetic experiments on high-quality single crystals of the superconducting compounds UP${\mathrm{d}}_{2}$A${\mathrm{l}}_{3}$ and CeR${\mathrm{u}}_{2}$, which have enhanced spin susceptibilities, demonstrate a first-order transition between weak and strong pinning at $Tl{0.9T}_{c}$ ( $Hg10$ kOe). This is compatible with a staggered order parameter due to the formation of a ``Fulde-Ferrell-Larkin-Ovchinnikov'' state.