# Browsing by Subject "cond-mat.str-el"

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Item Open Access Bold Diagrammatic Monte Carlo in the Lens of Stochastic Iterative Methods(2017-11-30) Li, Yingzhou; Lu, JianfengThis work aims at understanding of bold diagrammatic Monte Carlo (BDMC) methods for stochastic summation of Feynman diagrams from the angle of stochastic iterative methods. The convergence enhancement trick of the BDMC is investigated from the analysis of condition number and convergence of the stochastic iterative methods. Numerical experiments are carried out for model systems to compare the BDMC with related stochastic iterative approaches.Item Open Access Field-Induced Phase Transition of the Spin Liquid State in Triangular Antiferromagnet YbMgGaO4Steinhardt, William M; Shi, Zhenzhong; Samarakoon, Anjana; Dissanayake, Sachith; Graf, David; Liu, Yaohua; Zhu, Wei; Marjerrison, Casey; Batista, Cristian D; Haravifard, SaraThe triangular antiferromagnet YbMgGaO4 has emerged as a candidate for hosting quantum spin liquid state, an exotic state of matter featuring a high degree of entanglement and often characterized as without magnetic ordering in the zero-temperature limit. However, the nature of the ground state in this system has been the subject of ardent debates, as recent work has shown that chemical disorder may contribute to quantum spin liquid-like features. Here we report a field-induced phase transition observed via diffuse neutron scattering and magnetic susceptibility measurements. Comparisons to Monte Carlo simulations, using semi-classical spins and disorder, and Density Matrix Renormalization Group for the zero-temperature limit, reveal crucial information about the ground state and the roles that thermal fluctuations and chemical disorder play in the observed behavior.Item Open Access Itinerant Antiferromagnetism in RuO$_{2}$(PRL, 2017-02-23) Berlijn, T; Snijders, PC; Delaire, O; Zhou, H-D; Maier, TA; Cao, H-B; Chi, S-X; Matsuda, M; Wang, Y; Koehler, MR; Kent, PRC; Weitering, HHBulk rutile RuO$_2$ has long been considered a Pauli paramagnet. Here we report that RuO$_2$ exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately 0.05 $\mu_B$ as evidenced by polarized neutron diffraction. Density functional theory plus $U$ (DFT+$U$) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard $U$ of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutile crystal field. The combination of high N\'eel temperature and small itinerant moments make RuO$_2$ unique among ruthenate compounds and among oxide materials in general.Item Open Access Solution to new sign problems with Hamiltonian Lattice Fermions(PoS (LATTICE 2014) 058) Huffman, Emilie; Chandrasekharan, ShaileshWe present a solution to the sign problem in a class of particle-hole symmetric Hamiltonian lattice fermion models on bipartite lattices using the idea of fermion bags. The solution remains valid when the particle-hole symmetry is broken through a staggered chemical potential term. This solution allows, for the first time, simulations of some massless four-fermion models with minimal fermion doubling and with an odd number of fermion flavors using ultra-local actions. One can thus study a variety of quantum phase transitions that have remained unexplored so far due to sign problems.Item Open Access Stabilization of a Majorana Zero Mode through Quantum Frustration.(Physical Review B, 2020-07-01) Zhang, Gu; Baranger, Harold UWe analyze a system in which a topological Majorana zero mode (MZM) combines with a MZM produced by quantum frustration. At the boundary between the topological and non-topological states, a MZM does not appear. The system that we study combines two parts, a grounded topological superconducting wire that hosts two MZM at its ends, and an on-resonant quantum dot connected to two dissipative leads. The quantum dot with dissipative leads creates an effective two-channel Kondo (2CK) state in which quantum frustration yields an isolated MZM at the dot. We find that coupling the dot to one of the wire Majoranas stabilizes the MZM at the other end of the wire. In addition to providing a route to achieving an unpaired Majorana zero mode, this scheme provides a clear signature of the presence of the 2CK Majorana.Item Open Access Unveiling environmental entanglement in strongly dissipative qubits(arXiv, 2013-01-30) Bera, Soumya; Florens, Serge; Baranger, Harold; Roch, Nicolas; Nazir, Ahsan; Chin, AlexThe coupling of a qubit to a macroscopic reservoir plays a fundamental role in understanding the complex transition from the quantum to the classical world. Considering a harmonic environment, we use both intuitive arguments and numerical many-body quantum tomography to study the structure of the complete wavefunction arising in the strong-coupling regime, reached for intense qubit-environment interaction. The resulting strongly-correlated many-body ground state is built from quantum superpositions of adiabatic (polaron-like) and non-adiabatic (antipolaron-like) contributions from the bath of quantum oscillators. The emerging Schrödinger cat environmental wavefunctions can be described quantitatively via simple variational coherent states. In contrast to qubit-environment entanglement, we show that non-classicality and entanglement among the modes in the reservoir are crucial for the stabilization of qubit superpositions in regimes where standard theories predict an effectively classical spin.