Engineering topics

Christensen, Morten H.

Publications and source records attributed to Christensen, Morten H..

9 records

DOE OSTI2023

Competing magnetic fluctuations and orders in a multiorbital model of doped SrCo 2 ⁢As 2

We revisit the intriguing magnetic behavior of the paradigmatic itinerant frustrated magnet Sr⁢Co 2 ⁢As 2 , which shows strong and competing magnetic fluctuations yet does not develop long-range magnetic order. By calculating the static spin susceptibility Χ⁡(q) within a realistic 16-orbital Hubbard-Hund model, we determine the leading instability to be ferromagnetic (FM). We then explore the effect of doping and calculate the critical Hubbard interaction strength U c that is required for the development of magnetic order. We find that U c decreases under electron doping and with increasing Hund's coupling J, but increases rapidly under hole doping. This suggests that magnetic order could possibly emerge under electron doping but not under hole doping, which agrees with experimental findings. We map out the leading magnetic instability as a function of doping and Hund's coupling and find several antiferromagnetic phases in addition to FM. We also quantify the degree of itinerant frustration in the model and resolve the contributions of different orbitals to the magnetic susceptibility. Lastly, we discuss the dynamic spin susceptibility Χ⁡(q,ω) at finite frequencies, where we recover the anisotropy of the peaks at Q π = (π,0) and (0,π) observed by inelastic neutron scattering that is associated with the phenomenon of itinerant magnetic frustration. By comparing results between theory and experiment, we conclude that the essential experimental features of doped SrCo 2 ⁢As 2 are well captured by an itinerant Hubbard-Hund multiorbital model if one considers a small shift of the chemical potential towards hole doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Nematic fluctuations in an orbital selective superconductor Fe 1+ y Te 1– x Se x

Fe 1+y Te 1–x Se x is characterized by its complex magnetic phase diagram and highly orbital-dependent band renormalization. Despite this, the behavior of nematicity and nematic fluctuations, especially for high tellurium concentrations, remains largely unknown. Here we present a study of both B 1 g and B 2 g nematic fluctuations in Fe 1+y Te 1–x Se x (0 ≤ x ≤ 0.53) using the technique of elastoresistivity measurement. We discovered that the nematic fluctuations in two symmetry channels are closely linked to the corresponding spin fluctuations, confirming the intertwined nature of these two degrees of freedom. We also revealed an unusual temperature dependence of the nematic susceptibility, which we attributed to a loss of coherence of the d xy orbital. Our results highlight the importance of orbital differentiation on the nematic properties of iron-based materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Hot-lines topology and the fate of the spin resonance mode in three-dimensional unconventional superconductors

In the quasi-two-dimensional (quasi-2D) copper- and iron-based superconductors, the onset of superconductivity is accompanied by a prominent peak in the magnetic spectrum at momenta close to the wave-vector of the nearby antiferromagnetic state. Such a peak is well described in terms of a spin resonance mode, i.e., a spin-1 exciton theoretically predicted for quasi-2D superconductors with a sign-changing gap. The same theories, however, indicate that such a resonance mode should be absent in a three-dimensional (3D) system with a spherical Fermi surface. This raises the question of the fate of the spin resonance mode in layered unconventional superconductors that are not strongly anisotropic, such as certain heavy-fermion compounds and potentially the newly discovered nickelate superconductor NdNiO 2 . Here, we use the random phase approximation to calculate the dynamical spin susceptibility of 3D superconductors with a d x 2 –y 2 -wave gap symmetry and corrugated cylindrical-like Fermi surfaces. By varying the out-of-plane hopping anisotropy t z /t, we demonstrate that the appearance of a spin resonance mode is determined by the topology of the hot lines, i.e., lines on the Fermi surface that are connected by the magnetic wave vector. For an in-plane antiferromagnetic wave vector, the hot lines undergo a topological transition from open lines to closed loops at a critical t z /t value. The closed hot lines cross the nodal superconducting lines, making the spin resonance mode overdamped and incoherent. In contrast, for an out-of-plane antiferromagnetic wave vector, the hot lines remain open and the spin resonance mode remains sharp. We discuss the experimental implications of our results for the out-of-plane dispersion of the spin resonance mode and, more generally, for inelastic neutron scattering experiments on unconventional superconductors.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Multiple magnetic orders in LaFeAs 1-x P x O uncover universality of iron-pnictide superconductors

The iron-pnictide superconductors have generated tremendous excitement as the competition between magnetism and superconductivity has allowed unique in-roads towards elucidating a microscopic theory of unconventional high-temperature superconductivity. In addition to the stripe spin density wave ($C^a_{2M}$) phase observed in the parent compounds of all iron-pnictide superconductors, two novel magnetic orders have recently been discovered in different parent structures: an out-of-plane collinear double-Q ($C^c_{4M}$) structure in the hole-doped (Ca, Sr, Ba) 1-x (Na) x Fe 2 As 2 and Ba 1-x K x Fe 2 As 2 families, and a spin vortex crystal “hedgehog” ($C^{ab}_{4M}$) structure in the CaKFe 4 As 4 family. Using neutron diffraction, we demonstrate that LaFeAs 1-x P x O contains all three magnetic orders within a single-phase diagram as a function of substitution, all of which compete strongly with superconductivity. Our experimental observations combined with theoretical modeling demonstrate how the reduction in electronic correlations by chemical substitution results in larger Fermi surfaces and the sequential stabilization of multiple magnetic anisotropies. Our work presents a unified narrative for the competing magnetic and superconducting phases observed in various iron-pnictide systems with different crystal structures and chemistry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Theory of the charge density wave in A V 3 Sb 5 kagome metals

The family of metallic kagome compounds AV 3 Sb 5 (A = K, Rb, Cs) was recently discovered to exhibit both superconductivity and charge order. The nature of the charge density wave (CDW) phase is presently unsettled, which complicates the interpretation of the superconducting ground state. In this paper, we use group theory and density functional theory (DFT) to derive and solve a phenomenological Landau model for this CDW state. The DFT results reveal three unstable phonon modes with the same in-plane momentum but different out-of-plane momenta, whose frequencies depend strongly on the electronic temperature. This is indicative of an electronically driven CDW, stabilized by features of the in-plane electronic dispersion. Motivated by the DFT analysis, we construct a Landau free-energy expansion for coupled CDW order parameters with wave vectors at the M and L points of the hexagonal Brillouin zone. We find an unusual trilinear term coupling these different order parameters, which can promote the simultaneous condensation of both CDWs even if the two modes are not nearly degenerate. We classify the different types of coupled multi-Q CDW orders, focusing on those that break the sixfold rotational symmetry and lead to a unit-cell doubling along all three crystallographic directions, as suggested by experiments. Here, we determine a region in parameter space, characterized by large nonlinear Landau coefficients, where these phases—dubbed staggered trihexagonal and staggered Star of David—are the leading instabilities of the system. Finally, we discuss the implications of our results for the kagome metals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Revealing the competition between charge density wave and superconductivity in CsV 3 Sb 5 through uniaxial strain

In this paper we report the effect of uniaxial strain ε applied along the crystalline a axis on the newly discovered kagome superconductor CsV 3 Sb 5 . At ambient conditions, CsV 3 Sb 5 shows a charge-density wave (CDW) transition at T CDW = 94.5K and superconducts below T c = 3.34K. In our paper, when the uniaxial strain ε is varied from –0.90% to 0.90%,T c monotonically increases by ~33% from 3.0 to 4.0 K, giving rise to the empirical relation T c (ε) = 3.4 + 0.56ε + 0.12ε 2 . On the other hand, for ε changing from –0.76% to 1.26%,TCDW decreases monotonically by ~10% from 97.5 to 87.5 K with T CDW (ε) = 94.5 – 4.72ε – 0.60ε 2 . The opposite response of T c and T CDW to the uniaxial strain suggests strong competition between these two orders. Comparison with hydrostatic pressure measurements indicate that it is the change in the c axis that is responsible for these behaviors of the CDW and superconducting transitions, and that the explicit breaking of the sixfold rotational symmetry by strain has a negligible effect. Combined with our first-principles calculations and phenomenological analysis, we conclude that the enhancement in Tc with decreasing c is caused primarily by the suppression of T CDW , rather than strain-induced modifications in the bare superconducting parameters. We propose that the sensitivity of T CDW with respect to the changes in the c axis arises from the impact of the latter on the trilinear coupling between the $M$$^{+}_{1}$ and the $L$$^{–}_{2}$ phonon modes associated with the CDW. Altogether, our paper reveals that the c-axis lattice parameter, which can be controlled by both pressure and uniaxial strain, is a powerful tuning knob for the phase diagram of CsV 3 Sb 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Prediction of double-Weyl points in the iron-based superconductor CaKFe 4 As 4

Employing a combination of symmetry analysis, low-energy modeling, and ab initio simulations, we predict the presence of magnetic-field-induced Weyl points close to the Fermi level in CaKFe 4 As 4 . Depending on the relative strengths of the magnetic field and of the spin-orbit coupling, the Weyl fermions can carry a topological charge of ±1 or ±2, making CaKFe 4 As 4 a rare realization of a double-Weyl semimetal. Here, we further predict experimental manifestations of these Weyl points, both in bulk properties, such as the anomalous Hall effect, and in surface properties, such as the emergence of prominent Fermi arcs. Because CaKFe 4 As 4 displays unconventional fully gapped superconductivity below 30 K, our findings open a route to investigate the interplay between superconductivity and Weyl fermions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Strong-coupling expansion of multi-band interacting models: Mapping onto the transverse-field J 1 - J 2 Ising model

We investigate a class of two-dimensional two-band microscopic models in which the inter-band repulsive interactions play the dominant role. We first demonstrate three different schemes of constraining the ratios between the three types of inter-band interactions – density-density, spin exchange, and pair-hopping – that render the model free of the fermionic sign-problem for any filling and, consequently, amenable to efficient Quantum Monte Carlo simulations. We then study the behavior of these sign-problem-free models in the strong-coupling regime. In the cases where spin-rotational invariance is preserved or lowered to a planar symmetry, the strong-coupling ground state is a quantum paramagnet. However, in the case where there is only a residual Ising symmetry, the strong-coupling expansion maps onto the transverse-field J 1 - J 2 Ising model, whose pseudospins are associated with local inter-band magnetic order. We show that by varying the band structure parameters within a reasonable range of values, a variety of ground states and quantum critical points can be accessed in the strong-coupling regime, some of which are not realized in the weak-coupling regime. Here, we compare these results with the case of the single-band Hubbard model, where only intra-band repulsion is present, and whose strong-coupling behavior is captured by a simple Heisenberg model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2020

Modeling Unconventional Superconductivity at the Crossover between Strong and Weak Electronic Interactions

High-temperature superconductivity emerges in many different quantum materials, often in regions of the phase diagram where the electronic kinetic energy is comparable to the electron-electron repulsion. Describing such intermediate-coupling regimes has proven challenging as standard perturbative approaches are inapplicable. In this study, we employ quantum Monte Carlo methods to solve a multiband Hubbard model that does not suffer from the sign problem and in which only repulsive interband interactions are present. In contrast to previous sign-problem-free studies, we treat magnetic, superconducting, and charge degrees of freedom on an equal footing. We find an antiferromagnetic dome accompanied by a metal-to-insulator crossover line in the intermediate-coupling regime, with a smaller superconducting dome appearing in the metallic region. Across the antiferromagnetic dome, the magnetic fluctuations change from overdamped in the metallic region to propagating in the insulating region. Our findings shed new light on the intertwining between superconductivity, magnetism, and charge correlations in quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗