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Yasir Iqbal
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Yasir Iqbal
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Yasir Iqbal
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Iqbal, Y.
Iqbal, Yasir
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8 results
Now showing 1 - 8 of 8
- PublicationSignatures of a gearwheel quantum spin liquid in a spin- 12 pyrochlore molybdate Heisenberg antiferromagnet(13-12-2017)
; ;Müller, Tobias ;Riedl, Kira ;Reuther, Johannes ;Rachel, Stephan ;ValentÃ, Roser ;Gingras, Michel J.P. ;Thomale, RonnyJeschke, Harald O.We theoretically investigate the lowerature phase of the recently synthesized Lu2Mo2O5N2 material, an extraordinarily rare realization of a S=1/2 three-dimensional pyrochlore Heisenberg antiferromagnet in which Mo5+ are the S=1/2 magnetic species. Despite a Curie-Weiss temperature (ΘCW) of -121(1) K, experiments have found no signature of magnetic ordering or spin freezing down to T∗≈0.5 K. Using density functional theory, we find that the compound is well described by a Heisenberg model with exchange parameters up to third nearest neighbors. The analysis of this model via the pseudofermion functional renormalization group method reveals paramagnetic behavior down to a temperature of at least T=|ΘCW|/100, in agreement with the experimental findings hinting at a possible three-dimensional quantum spin liquid. The spin susceptibility profile in reciprocal space shows momentum-dependent features forming a "gearwheel" pattern, characterizing what may be viewed as a molten version of a chiral noncoplanar incommensurate spiral order under the action of quantum fluctuations. Our calculated reciprocal space susceptibility maps provide benchmarks for future neutron scattering experiments on single crystals of Lu2Mo2O5N2. - PublicationMultiloop functional renormalization group approach to quantum spin systems(01-06-2022)
;Kiese, Dominik ;Müller, Tobias; ;Thomale, RonnyTrebst, SimonRenormalization group methods are well-established tools for the (numerical) investigation of the low-energy properties of correlated quantum many-body systems, allowing us to capture their scale-dependent nature. The functional renormalization group (FRG) allows us to continuously evolve a microscopic model action to an effective low-energy action as a function of decreasing energy scales via an exact functional flow equation, which is then approximated by some truncation scheme to facilitate computation. Here, we report on our implementation of multiloop FRG, an extended truncation scheme recently developed for electronic FRG calculations, within the pseudofermion functional renormalization group (pf-FRG) framework for interacting quantum spin systems. We discuss in detail the conceptual intricacies of the flow equations generated by the multiloop truncation, as well as essential refinements to the integration scheme for the resulting integrodifferential equations. To benchmark our approach, we analyze antiferromagnetic Heisenberg models on the pyrochlore, simple cubic, and face-centered cubic lattice, discussing the convergence of physical observables for higher-loop calculations and comparing with existing results where available. Combined, these methodological refinements systematically improve the pf-FRG approach to one of the numerical tools of choice when exploring frustrated quantum magnetism in higher spatial dimensions. - PublicationPinch-points to half-moons and up in the stars: The kagome skymap(01-01-2023)
;Kiese, Dominik ;Ferrari, Francesco ;Astrakhantsev, Nikita ;Niggemann, Nils ;Ghosh, Pratyay ;Müller, Tobias ;Thomale, Ronny ;Neupert, Titus ;Reuther, Johannes ;Gingras, Michel J.P. ;Trebst, SimonPinch point singularities, associated with flat band magnetic excitations, are tell-tale signatures of Coulomb spin liquids. While their properties in the presence of quantum fluctuations have been widely studied, the fate of the complementary nonanalytic features - shaped as half moons and stars - arising from adjacent shallow dispersive bands has remained unexplored. Here, we address this question for the spin S=1/2 Heisenberg antiferromagnet on the kagome lattice with second and third neighbor couplings, which allows one to tune the classical ground state characterized by flat bands to one that is governed by shallow dispersive bands for intermediate coupling strengths. Employing the complementary strengths of variational Monte Carlo, pseudofermion functional renormalization group, and density-matrix renormalization group, we establish the quantum phase diagram of the model. The U(1) Dirac spin liquid ground state of the nearest-neighbor antiferromagnet remains remarkably robust till intermediate coupling strengths when it transitions into a pinwheel valence bond crystal displaying signatures of half moons in its structure factor. Our Letter thus identifies a microscopic setting that realizes one of the proximate orders of the Dirac spin liquid identified in a recent work [Song, Wang, Vishwanath, and He, Nat. Commun. 10, 4254 (2019)2041-172310.1038/s41467-019-11727-3]. For larger couplings, we obtain a collinear magnetically ordered ground state characterized by starlike patterns. - PublicationPinwheel valence bond crystal ground state of the spin-12 Heisenberg antiferromagnet on the shuriken lattice(01-12-2021)
;Astrakhantsev, Nikita ;Ferrari, Francesco ;Niggemann, Nils ;Müller, Tobias ;Chauhan, Aishwarya ;Kshetrimayum, Augustine ;Ghosh, Pratyay ;Regnault, Nicolas ;Thomale, Ronny ;Reuther, Johannes ;Neupert, TitusWe investigate the nature of the ground state of the spin-12 Heisenberg antiferromagnet on the shuriken lattice by complementary state-of-the-art numerical techniques, such as variational Monte Carlo (VMC) with versatile Gutzwiller-projected Jastrow wave functions, unconstrained multivariable variational Monte Carlo (mVMC), and pseudofermion/pseudo-Majorana functional renormalization group (PFFRG/PMFRG) methods. We establish the presence of a quantum paramagnetic ground state and investigate its nature, by classifying symmetric and chiral quantum spin liquids, and inspecting their instabilities towards competing valence bond crystal (VBC) orders. Our VMC analysis reveals that a VBC with a pinwheel structure emerges as the lowest-energy variational ground state, and it is obtained as an instability of the U(1) Dirac spin liquid. Analogous conclusions are drawn from mVMC calculations employing accurate BCS pairing states supplemented by symmetry projectors, which confirm the presence of pinwheel VBC order by a thorough analysis of dimer-dimer correlation functions. Our work highlights the nontrivial role of quantum fluctuations via the Gutzwiller projector in resolving the subtle interplay between competing orders. - PublicationQuantum paramagnetism and helimagnetic orders in the Heisenberg model on the body centered cubic lattice(16-07-2019)
;Ghosh, Pratyay ;Müller, Tobias ;Toldin, Francesco Parisen ;Richter, Johannes; ;Thomale, Ronny ;Reuther, JohannesWe investigate the spin S=1/2 Heisenberg model on the body centered cubic lattice in the presence of ferromagnetic and antiferromagnetic nearest-neighbor J1, second-neighbor J2, and third-neighbor J3 exchange interactions. The classical ground state phase diagram obtained by a Luttinger-Tisza analysis is shown to host six different (noncollinear) helimagnetic orders in addition to ferromagnetic, Néel, stripe, and planar antiferromagnetic orders. Employing the pseudofermion functional renormalization group (PFFRG) method for quantum spins (S=1/2) we find an extended nonmagnetic region, and significant shifts to the classical phase boundaries and helimagnetic pitch vectors caused by quantum fluctuations, while no new long-range dipolar magnetic orders are stabilized. The nonmagnetic phase is found to disappear for S=1. We calculate the magnetic ordering temperatures from PFFRG and quantum Monte Carlo methods, and make comparisons to available data. - PublicationBreathing chromium spinels: a showcase for a variety of pyrochlore Heisenberg Hamiltonians(01-12-2019)
;Ghosh, Pratyay; ;Müller, Tobias ;Ponnaganti, Ravi T. ;Thomale, Ronny; ;Reuther, Johannes ;Gingras, Michel J.P.Jeschke, Harald O.We address the long-standing problem of the microscopic origin of the richly diverse phenomena in the chromium breathing pyrochlore material family. Combining electronic structure and renormalization group techniques we resolve the magnetic interactions and analyze their reciprocal-space susceptibility. We show that the physics of these materials is principally governed by long-range Heisenberg Hamiltonian interactions, a hitherto unappreciated fact. Our calculations uncover that in these isostructural compounds, the choice of chalcogen triggers a proximity of the materials to classical spin liquids featuring degenerate manifolds of wave-vectors of different dimensions: A Coulomb phase with three-dimensional degeneracy for LiInCr4O8 and LiGaCr4O8, a spiral spin liquid with two-dimensional degeneracy for CuInCr4Se8 and one-dimensional line degeneracies characteristic of the face-centered cubic antiferromagnet for LiInCr4S8, LiGaCr4S8, and CuInCr4S8. The surprisingly complex array of prototypical pyrochlore behaviors we discovered in chromium spinels may inspire studies of transition paths between different semi-classical spin liquids by doping or pressure. - PublicationStability of the spiral spin liquid in MnSc2S4(31-08-2018)
; ;Müller, Tobias ;Jeschke, Harald O. ;Thomale, RonnyReuther, JohannesWe investigate the stability of the spiral spin liquid phase in MnSc2S4 against thermal and quantum fluctuations as well as against perturbing effects of longer-range interactions. Employing ab initio density functional theory (DFT) calculations we propose a realistic Hamiltonian for MnSc2S4, featuring second (J2) and third (J3) neighbor Heisenberg interactions on the diamond lattice that are considerably larger than previously assumed. We argue that the combination of strong J2 and J3 couplings reproduces the correct magnetic Bragg peak position measured experimentally. Calculating the spin-structure factor within the pseudofermion functional-renormalization group technique, we find that close to the magnetic phase transition the sizable J3 couplings induce a strong spiral selection effect, in agreement with experiments. With increasing temperature the spiral selection becomes weaker such that in a window around three to five times the ordering temperature an approximate spiral spin liquid is realized in MnSc2S4. - PublicationNature of Unconventional Pairing in the Kagome Superconductors(22-10-2021)
;Wu, Xianxin ;Schwemmer, Tilman ;Müller, Tobias ;Consiglio, Armando ;Sangiovanni, Giorgio ;Di Sante, Domenico; ;Hanke, Werner ;Schnyder, Andreas P. ;Denner, M. Michael ;Fischer, Mark H. ;Neupert, TitusThomale, RonnyThe recent discovery of () has uncovered an intriguing arena for exotic Fermi surface instabilities in a kagome metal. Among them, superconductivity is found in the vicinity of multiple van Hove singularities, exhibiting indications of unconventional pairing. We show that the sublattice interference mechanism is central to understanding the formation of superconductivity in a kagome metal. Starting from an appropriately chosen minimal tight-binding model with multiple van Hove singularities close to the Fermi level for , we provide a random phase approximation analysis of superconducting instabilities. Nonlocal Coulomb repulsion, the sublattice profile of the van Hove bands, and the interaction strength turn out to be the crucial parameters to determine the preferred pairing symmetry. Implications for potentially topological surface states are discussed, along with a proposal for additional measurements to pin down the nature of superconductivity in .