Xin Lu

Zhejiang University, Hang-hsien, Zhejiang Sheng, China

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Publications (11)16.2 Total impact

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    ABSTRACT: In four classes of materials, the layered copper-oxides, organics, iron-pnictides and heavy-fermion compounds, an unconventional superconducting state emerges as a magnetic transition is tuned toward absolute zero temperature, that is, toward a magnetic quantum-critical point (QCP). In most materials, the QCP is accessed by chemical substitutions or applied pressure. CeCoIn5 is one of the few materials that are born as a quantum-critical superconductor and, therefore, offers the opportunity to explore the consequences of chemical disorder. Cadmium-doped crystals of CeCoIn5 are a particularly interesting case where Cd substitution induces long-range magnetic order, as in Zn-doped copper-oxides. Applied pressure globally supresses the Cd-induced magnetic order and restores bulk superconductivity. Here we show, however, that local magnetic correlations, whose spatial extent decreases with applied pressure, persist at the extrapolated QCP. The residual droplets of impurity-induced magnetic moments prevent the reappearance of conventional signatures of quantum criticality, but induce a heterogeneous electronic state. These discoveries show that spin droplets can be a source of electronic heterogeneity in classes of strongly correlated electron systems and emphasize the need for caution when interpreting the effects of tuning a correlated system by chemical substitution.
    06/2014; 10(2).
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    ABSTRACT: We report magnetization M, heat capacity C, and electrical resistivity ρ for single crystals of the itinerant electron antiferromagnet CaCo2P2 (TN ≈ 110 K). Measurements at ambient pressure reveal rich magnetic behavior, where ferromagnetic correlations are present in the paramagnetic state and a subsequent feature is seen at T1 ≈ 22 K within the ordered state. Heat-capacity measurements additionally reveal moderately enhanced electronic correlations, as evidenced by the electronic coefficient of the specific heat γ = 23 mJ/mol·K2, which is large by comparison to closely related 122 analogs and the value predicted by electronic structure calculations. Upon the application of pressure, TN is suppressed toward zero. For P ≥ 0.89 GPa, another phase transition appears at T2 < TN which is also suppressed by P. At Pc ≈ 1.4-1.5 GPa, TN and T2 drop abruptly to zero at a putative quantum phase transition. For P > Pc, a broad shoulder in ρ (T) appears at T*, which moves to higher T and broadens with increasing P. We discuss possible scenarios to understand the phase diagram and compare to other compounds which show similar P-driven behavior.
    02/2014; 89(9).
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    ABSTRACT: We report the temperature-pressure (T-P) phase diagram of CePt2In7 single crystals, especially the pressure evolution of the antiferromagnetic order and the emergence of superconductivity, which have been studied by electrical resistivity and ac calorimetry under nearly hydrostatic environments. Compared with its polycrystalline counterpart, bulk superconductivity coexists with antiferromagnetism in a much narrower pressure region. The possible existence of textured superconductivity and local quantum criticality also are observed in CePt2In7, exhibiting a remarkable similarity with CeRhIn5.
    Physical review. B, Condensed matter 06/2013; 88(2). · 3.77 Impact Factor
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    ABSTRACT: We performed electrical resistivity measurements under pressures up to a maximum of ≈ 5 GPa for the d-electron antiferromagnet CaCo2P2,{ }where we find that the N'{e}el temperature (TN \quad = 106 K) is rapidly suppressed towards zero near 1.4 GPa. In the vicinity of the suppressed magnetic state, the Fermi liquid coefficient of the electrical resistivity A increases abruptly, suggesting a divergence in the effective mass of the charge carrier quasiparticles. In addition, we find that the residual resistivity ρ 0 increases abruptly at 1.4 GPa. For P > 1.4 GPa, we also observe a broad hump in ∂ ρ /∂ T at a temperature T*, which increases with increasing P. We will compare these measurements to expectations for prototypical f-electron quantum critical point (QCP) systems (e.g., CeRhIn5 and CeRh2Si2) and the iron arsenide high temperature superconductors (e.g., CaFe2As2, SrFe2As2, and BaFe2As2) and discuss implications for studying a possible d-electron QCP in the absence of superconductivity.
    03/2013;
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    ABSTRACT: We report results for high purity specimens of the heavy fermion antiferromagnets Ce2Rh3Ge5 and Ce2Pt3Si5, which have similar ordering temperatures: TN= 5.5 K and 6.3 K, respectively, and belong to the same family of materials that includes the pressure-induced superconductor Ce2Ni3Ge5. Our measurements show that the antiferromagnetic state is suppressed to zero temperature at similar magnetic fields (Hc= 23 T and 36 T, respectively), suggesting comparable magnetic energy scales in these compounds. In contrast, while the pressure needed to access a quantum critical point (QCP) in Ce2Rh3Ge5 is extremely low (Pc ˜ 5 kbar), the Néel temperature for Ce2Pt3Si5 is insensitive to pressures up to 15 kbar. This result implies that although these compounds are markedly similar, the mechanism that drives the QCP in Ce2Rh3Ge5 is not present in Ce2Pt3Si5. We discuss possible differences between these compounds and mechanisms for their quantum criticality with an emphasis on how the shape of the Fermi surface affects their physical properties.
    03/2013;
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    ABSTRACT: Sizable single crystals of the superconducting iron-pnictide system (Ca 1-x Lax)10(Pt3As8)(Fe2As2)5 (x=0 to 0.182) have been grown and characterized by X-ray, microscopic, resistivity, Hall coefficient, susceptibility and specific heat measurements. Features in magnetic susceptibility, specific heat and two kinks in the derivative of the electrical resistivity around 100 K in the x=0 compound support the existence of decoupled structural and magnetic phase transitions. With La doping, the structural/magnetic phase transitions are suppressed and a dome of superconductivity with a maximal Tc up to 23 K is observed in the temperature-concentration phase diagram. Soft point-contact spectroscopy was performed on the optimally doped sample of x=0.145. By fitting the multigap Blonder-Tinkham-Klapwijk(BTK) model to the data, three gaps with δ1=1 meV, δ2=8 meV and δ3=27 meV are revealed. Acknowledgement: Work at Los Alamos was performed under the auspices of the US DOE.
    03/2013;
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    ABSTRACT: Superconductivity in strongly correlated electron systems frequently emerges in proximity to another broken symmetry. In heavy-electron superconductors, the nearby ordered state most commonly is magnetism, and the so-called Ce115 heavy-electron compounds have been particularly instructive for revealing new relationships between magnetism and superconductivity. From measurements of the resistive and bulk transitions to superconductivity in these materials, we find that the resistive transition appears at a temperature considerably higher than the bulk transition when superconductivity and magnetic order coexist, but this temperature difference disappears in the absence of long-range magnetic order. Further, in the pressure-temperature region of coexistence in CeRhIn5, a new anisotropy in the resistive transition develops even though the tetragonal crystal structure apparently remains unchanged, implying a form of textured superconductivity. We suggest that this texture may be a generic response to coexisting order in these and other heavy-fermion superconductors.
    Physica C Superconductivity 08/2012; 481. · 0.72 Impact Factor
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    ABSTRACT: We have extended the soft point-contact spectroscopy technique under nearly hydrostatic pressure to make charge-spectroscopy measurements of URu2Si2 in both hidden order (HO) and large-moment antiferromagnetic (LMAF) states. In the HO state at ambient pressure, the spectroscopy shows two asymmetric peaks around the Fermi energy that emerge below the hidden order temperature THO˜ 17.5 K. In the LMAF state at higher pressures, the spectra are remarkably similar to those in the HO state, indicating a similar Fermi surface gapping in the HO and LMAF states. The energy scale of this gap is, within experimental uncertainty, consistent with that of the incommensurate spin resonance at Q1= (1±0.4, 0, 0), which also is present in both HO and LMAF states. Our results provide a new clue to unraveling the puzzling HO state.
    02/2012;
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    ABSTRACT: The relationship between quantum criticality (QC), non-Fermi-liquid (nFl) behavior and the emergence of unconventional superconductivity (SC) in the vicinity of an antiferromagnetic quantum critical point (QCP) is one of the important issues in strongly correlated electron physics. Here we report on the effect of electronic tuning on superconductivity and quantum criticality in CeCoIn5 driven by electron (Pt and Sn) and hole doping (Hg). We show that both Pt and Sn doping have similar strong effect on superconductivity and push the system slightly away from the QCP. The sub-linear power law exponent, even at a high doping level (where the superconductivity is suppressed) could point to the formation of electronic inhomogeneity. Moreover, hole doping by Hg can tune the system back to the QCP as demonstrated by an increase of Tc (and subsequently the onset of AFM), a decrease of the coherence temperature T^* and an increase of the power law coefficient n stressing the importance of the interplay of electronic tuning and pair breaking effects.
    02/2012;
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    ABSTRACT: We use heat-capacity measurements as a function of field rotation to identify the nodal gap structure of CeIrIn(5) at pressures to 2.05 GPa, deep inside its superconducting dome. A fourfold oscillation in the heat capacity at 0.3 K is observed for all pressures, but with its sign reversed between 1.50 and 0.90 GPa. On the basis of recent theoretical models for the field-angle-dependent specific heat, all data, including the sign reversal, imply a d(x(2)-y(2)) order parameter with nodes along [110], which constrains theoretical models of the pairing mechanism in CeIrIn(5).
    Physical Review Letters 01/2012; 108(2):027001. · 7.94 Impact Factor
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    ABSTRACT: We report soft point-contact spectroscopy studies of URu2Si2 both in the hidder order (HO) and the large-moment antiferromagnetic (LMAF) states accessed by pressure. In the HO state at ambient pressure, the spectroscopy shows two asymmetric peaks around the Fermi energy that emerge below the hidden order temperature T_{HO}. In the LMAF state at higher pressures, the spectra are remarkably similar to those in the HO state, indicating a similar Fermi surface gapping in the HO and LMAF states and providing a new clue to unraveling the puzzling HO state.
    Physical review. B, Condensed matter 12/2011; 85(2). · 3.77 Impact Factor