157 research outputs found
Itinerant ferromagnetism in the multiorbital Hubbard model: a dynamical mean-field study
In order to resolve the long-standing issue of how the itinerant
ferromagnetism is affected by the lattice structure and Hund's coupling, we
have compared various three-dimensional lattice structures in the single- and
multiorbital Hubbard models with the dynamical mean-field theory with an
improved quantum Monte Carlo algorithm that preserves the spin-SU(2) symmetry.
The result indicates that {\it both} the lattice structure and the d-orbital
degeneracy are essential for the ferromagnetism in the parameter region
representing a transition metal. Specifically, (a) Hund's coupling, despite the
common belief, is important, which is here identified to come from
particle-hole scatterings, and (b) the ferromagnetism is a correlation effect
(outside the Stoner picture) as indicated from the band-filling dependence.Comment: 4 pages, 5 figure
Quantum Monte Carlo study for multiorbital systems with preserved spin and orbital rotational symmetries
We propose to combine the Trotter decomposition and a series expansion of the
partition function for Hund's exchange coupling in a quantum Monte Carlo (QMC)
algorithm for multiorbital systems that preserves spin and orbital rotational
symmetries. This enables us to treat the Hund's (spin-flip and pair-hopping)
terms, which is difficult in the conventional QMC method. To demonstrate this,
we first apply the algorithm to study ferromagnetism in the two-orbital Hubbard
model within the dynamical mean-field theory (DMFT). The result reveals that
the preservation of the SU(2) symmetry in Hund's exchange is important, where
the Curie temperature is grossly overestimated when the symmetry is degraded,
as is often done, to Ising (Z). We then calculate the spectral
functions of SrRuO by a three-band DMFT calculation with tight-binding
parameters taken from the local density approximation with proper rotational
symmetry.Comment: 9 pages, 9 figures. Typos corrected, some comments and references
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Control of the coupling between Kerr-cat qubits via transmon couplers
Kerr-cat qubits are a promising candidate for fault-tolerant quantum
computers owing to the biased nature of errors. The coupling between the
qubits can be utilized for a two-qubit entangling gate, but the residual
coupling causes unnecessary always-on gates and crosstalk. In order to resolve
this problem, we propose a tunable -coupling scheme using two transmon
couplers. By setting the detunings of the two couplers at opposite values, the
residual couplings via the two couplers cancel each other out. We also
apply our scheme to the gate ( rotation with angle
), one of the two-qubit entangling gates. We numerically show that the
fidelity of the gate is higher than 99.9% in a case of 16 ns
gate time and without decoherence.Comment: 8 pages, 5 figure
Polar surface engineering in ultra-thin MgO(111)/Ag(111) -- possibility of metal-insulator transition and magnetism
A recent report [Kiguchi {\it et al.}, Phys. Rev. B {\bf 68}, 115402 (2003)]
that the (111) surface of 5 MgO layers grown epitaxially on Ag(111) becomes
metallic to reduce the electric dipole moment raises a question of what will
happen when we have fewer MgO layers. Here we have revealed, first
experimentally with electron energy-loss spectroscopy, that MgO(111) remains
metallic even when one-layer thick, and theoretically with the density
functional theory that the metallization should depend on the nature of the
substrate. We further show, with a spin-density functional calculation, that a
ferromagnetic instability may be expected for thicker films.Comment: 5 pages, 7 figure
Activation Analysis of Lanthanum and Europium in Sea Water and Lake Water (Physical and Inorganic Chemistry)
Extremely large Lamb shift in a deep-strongly coupled circuit QED system with a multimode resonator
We report experimental and theoretical results on the extremely large Lamb
shift in a multimode circuit quantum electrodynamics (QED) system in the
deep-strong coupling (DSC) regime, where the qubit-resonator coupling strength
is comparable to or larger than the qubit and resonator frequencies. The system
comprises a superconducting flux qubit (FQ) and a quarter-wavelength coplanar
waveguide resonator ( CPWR) that are coupled inductively through a
shared edge that contains a Josephson junction to achieve the DSC regime.
Spectroscopy is performed around the frequency of the fundamental mode of the
CPWR, and the spectrum is fitted by the single-mode quantum Rabi Hamiltonian to
obtain the system parameters. Since the qubit is also coupled to a large number
of higher modes in the resonator, the single-mode fitting does not provide the
bare qubit energy but a value that incorporates the renormalization from all
the other modes. We derive theoretical formulas for the Lamb shift in the
multimode resonator system. As shown in previous studies, there is a cut-off
frequency for the coupling between the FQ and the modes
in the CPWR, where the coupling grows as for
and decreases as for
. Here is the frequency of the
th mode. The cut-off effect occurs because the qubit acts as an obstacle for
the current in the resonator, which suppresses the current of the modes above
at the location of the qubit and results in a reduced
coupling strength. Using our observed spectrum and theoretical formulas, we
estimate that the Lamb shift from the fundamental mode is 82.3\% and the total
Lamb shift from all the modes is 96.5\%.Comment: 16 pages, 4 figure
Case report: Mitochondrial trifunctional protein deficiency caused by HADHB gene mutation (c.1175C>T) characterized by higher brain dysfunction followed by neuropathy, presented gadolinium enhancement on brain imaging in an adult patient
Mitochondrial trifunctional protein (MTP) deficiency is an autosomal recessive disorder caused by impaired metabolism of long-chain fatty acids (LCFAs). Childhood and late-onset MTP deficiency is characterized by myopathy/rhabdomyolysis and peripheral neuropathy; however, the features are unclear. A 44-year-old woman was clinically diagnosed with Charcot-Marie-Tooth disease at 3 years of age due to gait disturbance. Her activity and voluntary speech gradually decreased in her 40s. Cognitive function was evaluated and brain imaging tests were performed. The Mini-Mental State Examination and frontal assessment battery scores were 25/30 and 10/18, respectively, suggesting higher brain dysfunction. Peripheral nerve conduction studies revealed axonal impairments. Brain computed tomography showed significant calcification. Magnetic resonance imaging revealed an increased gadolinium contrast-enhanced signal in the white matter, suggesting demyelination of the central nervous system (CNS) due to LCFAs. The diagnosis of MTP deficiency was confirmed through genetic examination. Administration of L-carnitine and a medium-chain fatty triglyceride diet was initiated, and the progression of higher brain dysfunction was retarded within 1 year. This patient's presentation was suggestive of CNS demyelination. The presence of brain calcification, higher brain dysfunction, or gadolinium enhancement in the white matter in patients with peripheral neuropathy may be suggestive of MTP deficiency
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