for studying a range of topological phenomena relevant to both condensed matter and particle physics.

Within the Landau paradigm of continuous phase transitions, ordered states of matter are characterized by a broken symmetry. Although the broken symmetry is usually evident, determining the driving force behind the phase transition is often a more subtle matter due to coupling between otherwise distinct order parameters. In this paper we show how measurement of the divergent nematic susceptibility of an iron pnictide superconductor unambiguously distinguishes an electronic nematic phase transition from a simple ferroelastic distortion. These measurements also reveal an electronic nematic quantum phase transition at the composition with optimal superconducting transition temperature. [6][7][8][9] and iron pnictides [10][11][12] have been proposed as candidate platforms that might harbour an electronic nematic phase, which opens up exciting new possibilities related to the interplay of nematic order with high temperature superconductivity. However, one of the key doubts accompanied by the experimental discoveries is that the crystal lattice of these two systems does not retain a fourfold symmetry. In particular, in iron pnictides there is an orthorhombic structural distortion accompanying the rapid increase of resistivity anisotropy, which puts the legitimacy of the term "electronic nematic" into question. Here we report measurements of the resistivity anisotropy of Ba(Fe 1−x Co x ) 2 As 2 induced by a tunable uni-axial strain, which exhibits a divergent behaviour as the system approaches the phase transition from the high temperature side. Our result explicitly shows that the structural phase transition in Ba(Fe 1−x Co x ) 2 As 2 is purely driven by the instability in the electronic part of the free energy, and furthermore reveals an electronic nematic quantum phase transition at the composition with optimal superconducting transition temperature.We apply a tuneable in-plane uniaxial strain to single crystal samples of Ba(Fe 1−x Co x ) 2 As 2 to probe the nematic response. As shown in Fig. 1(A), by gluing the sample on the side wall of a piezostack, strains can be applied by the deformation of the piezo, which is controlled by an applied voltage(V P ) [13]. The strain (i.e. the fractional change of length along the current direction, ǫ P = ∆L/L) was monitored via a strain gauge glued on the back side of the piezo stack. Both ǫ P and the fractional change of resistivity (η = ∆ρ/ρ 0 , where ρ 0 is the resistivity of the free standing sample before gluing on the piezo stack) were measured at constant temperature while the applied voltage was swept, as shown in Fig.

A key actor in the conventional theory of superconductivity is the induced interaction between electrons mediated by the exchange of virtual collective fluctuations (phonons in the case of conventional s-wave superconductors). Other collective modes that can play the same role, especially spin fluctuations, have been widely discussed in the context of high-temperature and heavy Fermion superconductors. The strength of such collective fluctuations is measured by the associated susceptibility. Here we use differential elastoresistance measurements from five optimally doped iron-based superconductors to show that divergent nematic susceptibility appears to be a generic feature in the optimal doping regime of these materials. This observation motivates consideration of the effects of nematic fluctuations on the superconducting pairing interaction in this family of compounds and possibly beyond.

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The physics of quantum critical phase transitions connects to some of the most difficult problems in condensed matter physics, including metal-insulator transitions, frustrated magnetism and high-temperature superconductivity. Near a quantum critical point, a new kind of metal emerges, the thermodynamic and transport properties of which do not fit into the unified phenomenology for conventional metals-the Landau Fermi-liquid theory-characterized by a low-temperature limiting T-linear specific heat and a T 2 resistivity 1 . Studying the evolution of the temperature dependence of these observables as a function of a control parameter leads to the identification of both the presence and the nature of the quantum phase transition in candidate systems. In this study we measure the transport properties of BaFe 2 (As 1−x P x ) 2 below the critical temperature T c by suppressing superconductivity with high magnetic fields. At sufficiently low temperatures, the resistivity of all compositions (x 0.31) crosses over from a linear to a quadratic temperature dependence, consistent with a low-temperature Fermi-liquid ground state. As compositions with optimal T c are approached from the overdoped side, this crossover becomes steeper, consistent with models of quantum criticality where the effective Fermi temperature T F goes to zero.The iron-based superconductors are part of a family of unconventional superconductors that exhibit several competing orders. The parent material BaFe 2 As 2 is a tetragonal paramagnet at high temperature and becomes an orthorhombic metallic antiferromagnet at ∼140 K (ref. 2). As the material is electron doped, hole doped or isovalently substituted this transition is rapidly suppressed, giving rise to superconductivity. In this work, we attempt to understand the nature of the low-T metallic state of the Fe-based superconductor BaFe 2 (As 1−x P x ) 2 by suppressing the superconductivity in a high magnetic field. Even though BaFe 2 (As 1−x P x ) 2 is isovalently substituted, we will describe the chemical composition-temperature (x-T ) phase diagram using language commonly applied to electron/hole-doped compounds, namely 'underdoped' refers to materials that exhibit a structural/magnetic instability, and 'overdoped' for paramagnetic compounds that do not. For this material, the maximum T c (optimal doping) occurs at x = 0.30.BaFe 2 (As 1−x P x ) 2 is a multi-band compound with both electronand hole-like carriers and the magnetoresistance is therefore a sum of contributions from all Fermi surfaces. In Fig. 1 we illustrate the magnetoresistance as a function of temperature and field for a range of compositions from x = 0.31 to x = 0.73 and T c spanning 29.5 K to 0 K. For all temperatures measured, a quadratic magnetoresistance fit captures most of the data and the intercept ρ 0,T is extrapolated (shown by black lines in Fig. 1). At low fields, this fit deviates from the quadratic dependence in the near-optimally doped samples, even at temperatures T > T c , although the deviation ostensibly disappears a...

Two-dimensional (2D) semimetals beyond graphene have been relatively unexplored in the atomicallythin limit. Here we introduce a facile growth mechanism for semimetallic WTe 2 crystals, then fabricate few-layer test structures while carefully avoiding degradation from exposure to air. Low-field electrical measurements of 80 nm to 2 µm long devices allow us to separate intrinsic and contact resistance, revealing metallic response in the thinnest encapsulated and stable WTe 2 devices studied to date (3 to 20 layers thick). High-field electrical measurements and electro-thermal modeling demonstrate that ultra-thin WTe 2 can carry remarkably high current density (approaching 50 MA/cm 2 , higher than most common interconnect metals) despite a very low thermal conductivity (of the order ~3 Wm -1 K -1 ). These results suggest several pathways for air-stable technological viability of this layered semimetal.Keywords: two-dimensional (2D) atomic layers; semimetals; transition metal dichalcogenides; current density; thermal conductivity; environmental stability * Contact: epop@stanford.edu 2 The preceding decade has seen much interest in two-dimensional (2D) nanomaterials, often exhibiting distinct evolution of chemical and physical properties as material thickness is scaled from layered bulk to individual atomic or molecular monolayers. [1][2][3] While semiconducting 2D materials have received much attention, layered 2D semimetals other than graphene have been relatively underexplored in the atomically thin limit. Materials like β-MoTe 2 and WTe 2 stabilize as semimetals in a distortion of the octahedral 1T (CdI 2 structure) geometry, with in-plane buckled chains formed by pairs of Mo/W atoms dimerizing in intermetallic charge-exchange, 4-6 while van der Waals bonding dominates interlayer interaction. Whereas MoTe 2 may be synthesized in both 2H and 1T' polytypes, or reversibly switched between the two as a function of temperature or strain, 7, 8 WTe 2 has been known since the 1960s to adopt an orthorhombic structure with space group Pmn2 1 (sometimes called "Td"), irrespective of growth conditions 4, 5, 6, 9, 10 or conventional strain, 8 as the heaviest of the Group VI dichalcogenides.Despite the inaccessibility of a semiconducting phase, semimetallic WTe 2 has received renewed attention from the experimental observation of non-saturating magnetoresistance in bulk samples, in excess of 13,000,000% at 60 T. 11 This behavior was attributed to perfect compensation between balanced electron and hole populations at the Fermi surface below 150 K, projected to persist down to individual monolayers. 12,13 Recent studies have also identified WTe 2 as a potential contact for 2D semiconductors, with a relatively low workfunction (Φ < 4.4 eV) amongst 2D metals, 14 recently applied in realizing unipolar n-type transport in the typically ambipolar semiconductor WSe 2 . 15 Layer-dependent experiments of any kind are nonetheless limited, [16][17][18][19] owing to a lack of geological sources, challenges in precursor purifica...

Atomically thin two-dimensional semiconductors feature silicon-like band gaps and native high-κ metal oxides.

We show that the strain-induced resistivity anisotropy in the tetragonal state of the representative underdoped Fe-arsenides BaFe2As2, Ba(Fe1−xCox)2As2 and Ba(Fe1−xNix)2As2 is independent of disorder over a wide range of defect and impurity concentrations. This result demonstrates that the anisotropy in the in-plane resistivity in the paramagnetic orthorhombic state of this material is not due to elastic scattering from anisotropic defects. Conversely, our result can be most easily understood if the resistivity anisotropy arises primarily from an intrinsic anisotropy in the electronic structure.PACS numbers: 74.70. Xa, Ongoing experimental investigations reveal that the underdoped regime of the cuprate high-temperature superconductors harbors a variety of poorly understood broken symmetry states. In the case of the ferro-pnictide and chalcogenide superconductors, the broken symmetries are much clearer [1], but the physical origin of the phase transitions is still a subject of debate [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Of particular interest, the ferropnictides suffer a tetragonalto-orthorhombic structural transition at a temperature T s that either precedes or accompanies the onset of long range antiferromagnetic magnetic order at T N (see Ref.[19] and references therein). From the perspective of symmetry, all physical properties develop a two-fold inplane anisotropy at such a phase transition. However, the magnitude depends on microscopic details, and therefore measurements that probe the anisotropy in the broken symmetry state can directly or indirectly inform our understanding of the mechanism that drives the phase transition. Quantities such as the in-plane resistivity anisotropy are therefore of considerable interest, and it is especially important to establish intrinsic versus extrinsic effects.In this paper, we show for several representative underdoped Fe-pnictides that the strain-induced resistivity anisotropy in the tetragonal state is independent of the degree of disorder for a given value of T N over a wide range of defect and impurity concentrations. This result can be directly compared to the anisotropy that develops spontaneously in the orthorhombic state (Appendix II) [19][20][21][22][23], and therefore demonstrates that the in-plane resistivity anisotropy observed for this family of compounds in the paramagnetic orthorhombic state is not an extrinsic effect associated with defect scattering. The result can be most easily understood if the resistivity anisotropy in this regime is primarily determined by the Fermi surface anisotropy rather than an anisotropy in the scattering rate.The structural phase transition that occurs in underdoped Fe-pnictides breaks a point symmetry of the original crystal lattice, and hence free-standing crystals naturally form structural twins in order to minimize the elastic energy [19]. The in-plane anisotropy can nevertheless be probed using uniaxial stress to detwin single crystals, as has now been done for several different families [19][20...