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B. -L. Young

Publications and source records attributed to B. -L. Young.

18 recordsLinked to original sources
arXiv2009

NMR Studies of Field Induced Magnetism in CeCoIn$_5$

Recent Nuclear Magnetic Resonance and elastic neutron scattering experiments have revealed conclusively the presence of static incommensurate magnetism in the field-induced B phase of CeCoIn5. We analyze the NMR data assuming the hyperfine coupling to the In(2) nuclei is anisotropic and simulate the spectra for several different magnetic structures. The NMR data are consistent with ordered Ce moments along the [001] direction, but are relatively insensitive to the direction of the incommensurate wavevector.

cond-mat.str-el↗
arXiv2007

Interacting Antiferromagnetic Droplets in Quantum Critical CeCoIn_5

The heavy fermion superconductor CeCoIn_5 can be tuned between superconducting and antiferromagnetic ground states by hole doping with Cd. Nuclear magnetic resonance (NMR) data indicate that these two orders coexist microscopically with an ordered moment ~0.7 μ_B. As the ground state evolves, there is no change in the low frequency spin dynamics in the disordered state. These results suggest that the magnetism emerges locally in the vicinity of the Cd dopants.

cond-mat.str-el↗
arXiv2006

Microscopic evidence for field-induced magnetism in CeCoIn$_5$

We present NMR data in the normal and superconducting states of CeCoIn$_5$ for fields close to $H_{\rm c2}(0)=11.8$ T in the $ab$ plane. Recent experiments identified a first-order transition from the normal to superconducting state for $H> 10.5$ T, and a new thermodynamic phase below 290 mK within the superconducting state. We find that the Knight shifts of the In(1), In(2) and the Co are discontinuous across the first-order transition and the magnetic linewidths increase dramatically. The broadening differs for the three sites, unlike the expectation for an Abrikosov vortex lattice, and suggests the presence of static spin moments in the vortex cores. In the low-temperature and high-field phase the broad NMR lineshapes suggest ordered local moments, rather than a long wavelength quasiparticle spin density modulation expected for an FFLO phase.

cond-mat.str-el↗
arXiv2004

Scaling in the Emergent Behavior of Heavy Electron Materials

We show that the NMR Knight shift anomaly exhibited by a large number of heavy electron materials can be understood in terms of the different hyperfine couplings of probe nuclei to localized spins and to conduction electrons. The onset of the anomaly is at a temperature T*, below which an itinerant component of the magnetic susceptibility develops. This second component characterizes the polarization of the conduction electrons by the local moments and is a signature of the emerging heavy electron state. The heavy electron component grows as log T below T*, and scales universally for all measured Ce, Yb and U based materials. Our results suggest that T* is not related to the single ion Kondo temperature, T_K, but rather represents a correlated Kondo temperature that provides a measure of the strength of the intersite coupling between the local moments. Our analysis strongly supports the two-fluid description of heavy electron materials developed by Nakatsuji, Pines and Fisk.

cond-mat.str-el↗
arXiv2002

muSR linewidth and isotropic pairing in superconducting PrOs4Sb12

Transverse-field muon spin rotation measurements in the vortex-lattice mixed state of the heavy-fermion (HF) superconductor PrOs4Sb12 yield a temperature dependence of the penetration depth indicative of an isotropic or nearly isotropic energy gap. This is not seen to date in any other HF superconductor and is a signature of isotropic pairing symmetry, possibly related to a novel nonmagnetic "quadrupolar Kondo" HF mechanism in PrOs4Sb12. The T = 0 relaxation rate σ_s(0) = 0.91(1) μs^-1 yields an estimated penetration depth λ(0) = 3440(20) Å, which is considerably shorter than in other HF superconductors.

cond-mat.str-el↗
arXiv2002

muSR in Ce_{1-x}La_xAl_3: anisotropic Kondo effect?

Zero-field muSR experiments in the heavy-fermion alloys Ce_{1-x}La_xAl_3, x = 0 and 0.2, examine a recent proposal that the system exhibits a strong anisotropic Kondo effect. We resolve a damped oscillatory component for both La concentrations, indicative of disordered antiferromagnetism. For x = 0.2 the oscillation frequency decreases smoothly with increasing temperature, and vanishes at the specific heat anomaly temperature T* \approx 2.2 K. Our results are consistent with the view that T* is due to a magnetic transition rather than anisotropic Kondo behavior.

cond-mat.str-el↗
arXiv1997

Searching for an anomalous $\bar t q γ$ coupling via single top quark production at a $γγ$ collider

We investigate the potential of a high-energy $γγ$ collider to detect an anomalous $\bar t q γ$ coupling from observation of the reaction $γγ\to t\bar q$, $\bar t q$, where $q=c$ or $u$. We find that with $b$-tagging and suitable kinematic cuts this process should be observable if the anomalous coupling $κ/Λ$ is no less than about 0.05/TeV, where $Λ$ is the scale of new physics associated with the anomalous interaction. This improves upon the bound possible from observation of top decays at the Tevatron.

hep-ph↗
arXiv1997

Single top quark production as a probe of R-parity-violating SUSY at pp and p\bar p colliders

We investigate the ability of single top quark production via qq'-> squark->tb and q \bar q'->slepton->t\bar b at the LHC and Tevatron to probe the strength of R-parity violating couplings in the minimal supersymmetric model. We found that given the existing bounds on R-parity violating couplings, single top quark production may be greatly enhanced over that predicted by the standard model, and that both colliders can either discover R-parity violating SUSY or set strong constraints on the relevant R-parity violating couplings. We further found that the LHC is much more powerful than the Tevatron in probing the squark couplings, but the two colliders have comparable sensitivity for the slepton couplings.

hep-ph↗
arXiv1997

Direct Top Quark Production at Hadron Colliders as a Probe of New Physics

We examine the effect of an anomalous flavor changing chromomagnetic moment which allows direct top quark production (two partons combining into an unaccompanied single top quark in the s-channel) at hadron colliders. We consider both t-c-g and t-u-g couplings. We find that the anomalous charm quark coupling parameter $κ_c / Λ$ can be measured down to $.06 TeV^{-1}(.009 TeV^{-1}$) at the Tevatron with the Main Injector upgrade(LHC). The anomalous up quark coupling parameter $κ_u / Λ$ can be measured to $.02 TeV^{-1}(.003 TeV^{-1}$) at the Tevatron(LHC).

hep-ph↗
arXiv1996

Implications Of A Non-Standard Light Higgs Boson

Analyses of the vacuum stability of the electroweak theory indicate that new physics occur at a scale of order of 1 TeV if a light Higgs is discovered at LEP II. In this paper, we parameterize the effects of new physics in the effective Lagrangian approach and examine its implication on the Higgs boson production at LEP II. We consider the effect of higher dimension operator on the Higgs potential and calculate the lower bound on the Higgs boson mass from the requirement of vacuum stability. We show that if a Higgs boson is seen at LEP II then under favourable conditions the deviation of the production cross section from the standard model value could be significant and therefore the presence of the new physics is detectable at LEP II.

hep-ph↗
arXiv1996

Searching for $t \to c g$ at the Fermilab Tevatron

We examine the experimental observability of the decay mode $t \rightarrow c g$ at the Fermilab Tevatron via the flavor-changing neutral current vertex $t \bar c g$. We find that with the existing data, one should be able to probe the $t \bar c g$ coupling to a value smaller than indirect limits previously obtained from $b\rightarrow s γ$ and the measured branching fraction for $t\rightarrow bW$, reaching $BF(t \rightarrow cg) \sim$ 15\% - 28\%. A data sample of 1~fb$^{-1}$ (10~fb$^{-1}$) at $\sqrt{s}=2$~TeV may probe the $t \rightarrow c g$ branching fraction to a level of 2\% (0.5\%).

hep-ph↗
arXiv1996

Top-Quark Decay Via the Anomalous Coupling $\bar t c γ$ at Hadron Colliders

We determine the constraints on the anomalous top-quark coupling associated with the flavor-changing neutral current vertex $\bar t c γ$ from the limits on the $b$-quark rare decay $b\rightarrow s γ$ and non-standard top-quark decays. Based on these constraints, we discuss the experimental observability of the rare decay mode $t \rightarrow c γ$, both at the Fermilab Tevatron with a luminosity-upgrade and at the LHC.

hep-ph↗
arXiv1995

Electroweak Sphaleron For Effective Theory In The Limit Of Large Higgs Boson Mass

Theoretical arguments suggest that the Higgs sector of the standard model is an effective theory. We parametrize the new physics by means of an effective Lagrangian technique and study its effect on the energy of the electroweak sphaleron. We found that in the presence of a certain class of higher dimension operators the sphaleron energy becomes arbitrarily large as the Higgs boson mass increases. The physical meaning of this result and its implications to the electroweak baryogenesis in the dynamical models are discussed.

hep-ph↗
arXiv1995

Non-Universal Correction To $Z \to b {\bar{b}}$ And Flavor Changing Neutral Current Couplings

A non-universal interaction associated with top quark induces flavor changing neutral currents (FCNC) among light fermions. The size of the FCNC effect depends crucially on the dynamics of the fermion mass generation. In this paper, we study the effect of a non-universal interaction on $Z b b$, $Z b s$ {\it etc}, by using an effective lagrangian technique and assuming the quark mass matrices in the form of a generalized Fritzsch ansatz. We point out that if fitting $R_b$ to the LEP data within $1 σ$, the induced FCNC couplings are very close to the experimental limits.

hep-ph↗
arXiv1994

Phenomenology of a non-standard top quark Yukawa coupling

There are theoretical speculations that the top quark may have different properties from that predicted by the standard model. We use an effective Lagrangian technique to model such a non-standard top quark scenario and study its effects on the electroweak baryogenesis, low and high energy physics.

hep-ph↗
arXiv1993

Effective Lagrangian Approach to Electroweak Baryogenesis: Higgs mass limit and Electric dipole moments of fermion

A natural solution to the hierachy problem of the standard model is to assume new physics to appear at the TeV scale. We parametrize the effects of this new physics in terms of an effective lagrangian and examine its impacts on electroweak baryogenesis. We point out that with such an effective lagrangian successful electroweak baryogenesis implies: i) Higgs boson lies within the reach of LEP II; ii)electric dipole moments of electron and neutron are detectable in the near future.

hep-ph↗
arXiv1993

Right-handed Neutral Currents, Families and the LEP $l^+l^-γγ$ Events

We explore the possibility of having new physics which could account for the $l^+l^-γγ$ events with $M_{γγ}\simeq 60\ $GeV recently reported by LEP. We consider models which contain an extra neutral gauge boson ($Z'$) that couples only to right-handed fermions. The models naturally require at least three quark and lepton families from anomaly cancellation. We find simple realizations of such models that have $\ell^+\ell^-γγ$ events at a rate similar to that observed at LEP.

hep-ph↗