Doping dependence of antiferromagnetism in models of the pnictides

Schmiedt, Jacob; Brydon, P. M. R.; Timm, Carsten

doi:10.1103/physrevb.85.214425

Cited by 9 publications

(7 citation statements)

References 69 publications

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“…n opt = 5.91 is defined as the electron filling with the highest T N as deduced by the paramagnetic susceptibility, and the total filling is n = n opt − x. The fact that the optimal doping level for the magnetic order is offset from n = 6 for DFT-generated bands is well known, 25 and not important for the conclusions of this paper. As seen from both cases, the 2Q phases, OM and SCO, exist at the foot of the MS dome on the electron and holedoped side respectively, and whereas the transition between the MS and OM phases is sharp, a more gradual transition takes place between the MS and the SCO phases as indicated by the green intermediate regions.…”

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confidence: 95%

Competing magnetic double-Qphases and superconductivity-induced reentrance ofC2magnetic stripe order in iron pnictides

Gastiasoro¹,

Andersen²

2015

Phys. Rev. B

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We perform a microscopic theoretical study of the generic properties of competing magnetic phases in iron pnictides. As a function of electron filling and temperature, the magnetic stripe (single-Q) order forms a dome, but competing non-collinear and non-uniform double-Q phases exist at the foot of the dome in agreement with recent experiments. We compute and compare the electronic properties of the different magnetic phases, investigate the role of competing superconductivity, and show how disorder may stabilize double-Q order. Superconductivity is shown to compete more strongly with double-Q magnetic phases, which can lead to re-entrance of the C2 (single-Q) order in agreement with recent thermal expansion measurements on K-doped Ba-122 crystals.PACS numbers: 74.70.Xa, 74.62.En, In correlated materials in general, and unconventional superconductors in particular, a microscopic understanding of the magnetism is of paramount importance. Generally, this is because a proper description of the relevant exchange mechanism in these materials is intimately tied to their basic electronic properties. More specifically, it is additionally shown within a wide class of models that the nature of the magnetic fluctuations may be closely linked to the emergence of the superconducting condensate. 1-3Focussing on the iron-based superconductors, the prevalent magnetic structure consists of collinear magnetic stripe (MS) order with in-plane moments oriented antiferromagnetic (ferromagnetic) along the a (b) axis of the orthorhombic Fe lattice as shown in Fig. 1(a). Thus, this configuration of moments singles out the Q 1 = (π, 0) ordering vector (1Q), i.e. M(r) = M 1 exp(iQ 1 · r). An obvious question, however, is why the system does not take advantage of the enhanced susceptibility at both Q 1 and Q 2 = (0, π) to form other magnetic phases, e.g. double-Q (2Q) phases consisting of superpositions of ordering at Q 1 and Q 2 with M(r) = l=1,2 M l exp(iQ l ·r). This question has been studied theoretically mainly using various effective field theories restricted to the vicinity of the magnetic transition temperature T N .4-7 These works have identified two competing magnetic structures of the 2Q type: 1) an orthomagnetic (OM) non-collinear phase with nearest neighbor moments at right angles as shown in Fig. 1(b), and 2) a collinear non-uniform spin and charge ordered (SCO) phase as shown in Fig. 1(c). The favorable magnetic order depends delicately on the band structure, doping level, and interactions. 4-7Experimentally, the dominating magnetic order in the iron pnictides is the 1Q MS state. This phase lowers the C 4 symmetry of the high-T tetragonal phase to orthorhombic C 2 , and causes an associated splitting of the crystal Bragg peaks due to magneto-elastic coupling. Recently, several experiments have, however, reported the discovery of magnetic order without an associated structural splitting, i.e. in the tetragonal phase, 8,9 which has been taken as indirect evidence for a magnetic driven structural transition in the case of 1Q M...

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confidence: 95%

Competing magnetic double-Qphases and superconductivity-induced reentrance ofC2magnetic stripe order in iron pnictides

Gastiasoro¹,

Andersen²

2015

Phys. Rev. B

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“…The notion of nesting and related concepts were broadly employed in the recent studies of iron-based pnictides [16][17][18][19][20][21][22] . For example, Ref.…”

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confidence: 99%

Magnetic field effects in electron systems with imperfect nesting

et al. 2017

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We analyze the effects of an applied magnetic field on the phase diagram of a weakly-correlated electron system with imperfect nesting. The Hamiltonian under study describes two bands: electron and hole ones. Both bands have spherical Fermi surfaces, whose radii are slightly mismatched due to doping. These types of models are often used in the analysis of magnetic states in chromium and its alloys, superconducting iron pnictides, AA-type bilayer graphene, borides, etc. At zero magnetic field, the uniform ground state of the system turns out to be unstable against electronic phase separation. The applied magnetic field affects the phase diagram in several ways. In particular, the Zeeman term stabilizes new antiferromagnetic phases. It also significantly shifts the boundaries of inhomogeneous (phase-separated) states. At sufficiently high fields, the Landau quantization gives rise to oscillations of the order parameters and of the Néel temperature as a function of the magnetic field.

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“…We note that while several calculations of inhomogeneous states have been performed using realistic, five-orbital models [35][36][37][38][39][40][41], none have discussed the role of residual electronic correlations and induced local SDW order.…”

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Origin of electronic dimers in the spin-density wave phase of Fe-based superconductors

2014

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We investigate the emergent impurity-induced states arising from point-like scatterers in the spin-density wave phase of iron-based superconductors within a microscopic five-band model. Independent of the details of the band-structure and disorder potential, it is shown how stable magnetic (π, π) unidirectional nematogens are formed locally by the impurities. Interestingly, these nematogens exhibit a dimer structure in the electronic density, are directed along the antiferromagnetic a-axis, and have typical lengths of ∼ 10 lattice constants in excellent agreement with recent scanning tunnelling experiments. These electronic dimers provide a natural explanation of the dopant-induced transport anisotropy found e.g. in the 122 iron pnictides. [19][20][21][22][23][24]. Since impurities are known to nucleate magnetic order locally [25][26][27], it is reasonable to assume that incipient nematic order in a fluctuating state can be similarly condensed around a defect, to create a local nematic electronic state. Once present, such anisotropic defect structures can influence macroscopic anisotropy as well, and it has been suggested [3,23,28,29] that they are responsible for the resistivity anisotropy observed in detwinned Ba-122 crystals [5,6,29].Because impurities represent a well-defined perturbation which can be examined locally, studying the electronic states they create can yield important information on the background correlations present in the pure system [25]. In YBCO, for example, magnetic droplets formed around Zn impurities are known to have a size consistent with the pure antiferromagnetic (AF) correlation length.[25] At present, the microscopic mechanism responsible for the creation of local defect states in Febased materials and for breaking of rotational symmetry is unclear. Some clues are offered by STM experiments on defects in the underdoped, magnetically ordered phase, where the symmetry is already broken by the (π, 0) spindensity wave (SDW) order (the wave vector is given in the effective 1-Fe Brillouin zone). Fourier transform scanning tunneling spectroscopy (STS) deduced the existence of electronic defects with C 2 symmetry[3] nucleated by the Co dopants in Ca(Fe 1−x Co x ) 2 As 2 , while a more detailed analysis reported a dimer structure.[23] These dimers are approximately eight lattice constants (a) long and oriented along the AF a-axis, consistent with the larger resistivity found along the ferromagnetic b-axis.As a first step in understanding the origin of local C 4 symmetry breaking observed in several experiments on various materials, it seems useful to study a situation where a known chemical impurity substitutes at a known position, and ask why such a dimer-like structure (with charge or local density of states (LDOS) peaks located such a great distance from the impurity site) should be induced. As in the cuprates, this problem should be accessible to weak-coupling theories of these systems, provided they account for the electronic states of the system to which the impurity couples and tre...

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Doping dependence of antiferromagnetism in models of the pnictides

Cited by 9 publications

References 69 publications

Competing magnetic double-Qphases and superconductivity-induced reentrance ofC2magnetic stripe order in iron pnictides

Competing magnetic double-Qphases and superconductivity-induced reentrance ofC2magnetic stripe order in iron pnictides

Magnetic field effects in electron systems with imperfect nesting

Origin of electronic dimers in the spin-density wave phase of Fe-based superconductors

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