Correlated low temperature states of YFe2Ge2 and pressure metallised NiS2
Correlated low temperature states of YFe2Ge2 and pressure metallised NiS2
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DOI：
10.17863/cam.21468
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发表时间：
2018-04
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通讯作者：
K. Semeniuk
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作者：
K. Semeniuk
虽然自由电子模型在描述固体性质方面通常可以取得令人惊讶的成功，但在许多材料中，电子之间的相互作用非常重要，不容忽视。这些强关联系统有时会表现出相当意想不到的，不寻常的和有用的现象，理解这是凝聚态物理的目标之一。顺磁性YFe 2Ge 2的热容测量给出了约100 mJ mol−1 K−2的索末菲系数，这比能带结构计算预测的值高出约一个数量级。这表明化合物中存在强电子相关性，可能是由于接近反铁磁量子临界点（QCP）。低温电阻率的非费米液体T3/2行为也表明了后者的存在。低于1.8 K，超导相在材料中发展，使其成为具有1-2-2结构的非磷属元素化物和非硫属元素化物铁基超导体的罕见情况。本论文描述了新一代剩余电阻率达到200的高质量YFe_2Ge_2样品的生长和研究。电阻率，热容和磁化率的测量证实了超导性的内在和体积特性，这也被认为是一个非常规的性质。为了检验附近QCP的假设，在高达35 kbar的高压下进行了电阻测量。超导临界温度的压力依赖性已被发现是相当弱的。已经进行了μSR测量，但由于样品的不均匀性导致临界温度的广泛分布，因此提供的信息有限。虽然超导性是电子之间有效吸引的结果，但在不同的情况下，系统的电子性质可以由库仑排斥决定。这是另一种过渡金属基化合物NiS 2的情况，它是莫特绝缘体。施加约30 kbar的流体静压力使材料穿过Mott金属-绝缘体转变（MIT）进入金属相。我们已经使用的隧道二极管振荡器（TDO）技术来测量量子振荡的NiS 2的金属化状态，使其能够跟踪的演变的主要费米面和相关的有效质量作为压力的函数。新的研究结果，访问更广泛的压力范围比以前的研究，并提供有力的证据表明，有效的载体质量发散接近莫特麻省理工学院，预期内的布林克曼-赖斯的情况下，并预测在动态平均场理论计算。量子振荡已经在接近绝缘相的33 kbar和高达97 kbar的压力下测量。除了对Mott MIT的机制提供有价值的见解外，这项研究还证明了TDO技术在高压下研究材料的潜力。
While the free electron model can often be surprisingly successful in describing properties of solids, there are plenty of materials in which interactions between electrons are too significant to be neglected. These strongly correlated systems sometimes exhibit rather unexpected, unusual and useful phenomena, understanding of which is one of the aims of condensed matter physics. Heat capacity measurements of paramagnetic YFe2Ge2 give a Sommerfeld coefficient of about 100 mJ mol−1 K−2, which is about an order of magnitude higher than the value predicted by band structure calculations. This suggests the existence of strong electronic correlations in the compound, potentially due to proximity to an antiferromagnetic quantum critical point (QCP). Existence of the latter is also indicated by the non-Fermi liquid T 3/2 behaviour of the low temperature resistivity. Below 1.8 K a superconducting phase develops in the material, making it a rare case of a non-pnictide and non-chalcogenide iron based superconductor with the 1-2-2 structure. This thesis describes growth and study of a new generation of high quality YFe2Ge2 samples with residual resistance ratios reaching 200. Measurements of resistivity, heat capacity and magnetic susceptibility confirm the intrinsic and bulk character of the superconductivity, which is also argued to be of an unconventional nature. In order to test the hypothesis of the nearby QCP, resistance measurements under high pressure of up to 35 kbar have been conducted. Pressure dependence of the critical temperature of the superconductivity has been found to be rather weak. μSR measurements have been performed, but provided limited information due to sample inhomogeneity resulting in a broad distribution of the critical temperature. While the superconductivity is the result of an effective attraction between electrons, under different circumstances the electronic properties of a system can instead be dictated by the Coulomb repulsion. This is the case for another transition metal based compound NiS2, which is a Mott insulator. Applying hydrostatic pressure of about 30 kbar brings the material across the Mott metal-insulator transition (MIT) into the metallic phase. We have used the tunnel diode oscillator (TDO) technique to measure quantum oscillations in the metallised state of NiS2, making it possible to track the evolution of the principal Fermi surface and the associated effective mass as a function of pressure. New results are presented which access a wider pressure range than previous studies and provide strong evidence that the effective carrier mass diverges close to the Mott MIT, as expected within the Brinkman-Rice scenario and predicted in dynamical mean field theory calculations. Quantum oscillations have been measured at pressures as close to the insulating phase as 33 kbar and as high as 97 kbar. In addition to providing a valuable insight into the mechanism of the Mott MIT, this study has also demonstrated the potential of the TDO technique for studying materials at high pressures.